Merge branch 'main' into update-config-at-assignment

Cargo.lock

@@ -2900,6 +2900,7 @@ dependencies = [
  "openssl",
  "pretty_assertions",
  "reedline",
+ "regex",
  "rstest",
  "serde_json",
  "serial_test",
@@ -3151,6 +3152,7 @@ dependencies = [
 name = "nu-engine"
 version = "0.95.1"
 dependencies = [
+ "log",
  "nu-glob",
  "nu-path",
  "nu-protocol",
@@ -3341,6 +3343,7 @@ dependencies = [
  "convert_case",
  "fancy-regex",
  "indexmap",
+ "log",
  "lru",
  "miette",
  "nix",

Cargo.toml

@@ -232,6 +232,7 @@ assert_cmd = "2.0"
 dirs-next = { workspace = true }
 tango-bench = "0.5"
 pretty_assertions = { workspace = true }
+regex = { workspace = true }
 rstest = { workspace = true, default-features = false }
 serial_test = "3.1"
 tempfile = { workspace = true }
@@ -310,4 +311,4 @@ reedline = { git = "https://github.com/nushell/reedline", branch = "main" }
 # Run individual benchmarks like `cargo bench -- <regex>` e.g. `cargo bench -- parse`
 [[bench]]
 name = "benchmarks"
-harness = false
+harness = false

benches/benchmarks.rs

@@ -45,6 +45,10 @@ fn setup_stack_and_engine_from_command(command: &str) -> (Stack, EngineState) {
     };
 
     let mut stack = Stack::new();
+
+    // Support running benchmarks with IR mode
+    stack.use_ir = std::env::var_os("NU_USE_IR").is_some();
+
     evaluate_commands(
         &commands,
         &mut engine,

crates/nu-cli/src/commands/keybindings_list.rs

@@ -49,22 +49,24 @@ impl Command for KeybindingsList {
 
     fn run(
         &self,
-        _engine_state: &EngineState,
-        _stack: &mut Stack,
+        engine_state: &EngineState,
+        stack: &mut Stack,
         call: &Call,
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
-        let records = if call.named_len() == 0 {
-            let all_options = ["modifiers", "keycodes", "edits", "modes", "events"];
-            all_options
-                .iter()
-                .flat_map(|argument| get_records(argument, call.head))
-                .collect()
-        } else {
-            call.named_iter()
-                .flat_map(|(argument, _, _)| get_records(argument.item.as_str(), call.head))
-                .collect()
-        };
+        let all_options = ["modifiers", "keycodes", "edits", "modes", "events"];
+
+        let presence = all_options
+            .iter()
+            .map(|option| call.has_flag(engine_state, stack, option))
+            .collect::<Result<Vec<_>, ShellError>>()?;
+
+        let records = all_options
+            .iter()
+            .zip(presence)
+            .filter(|(_, present)| *present)
+            .flat_map(|(option, _)| get_records(option, call.head))
+            .collect();
 
         Ok(Value::list(records, call.head).into_pipeline_data())
     }

crates/nu-cli/src/eval_cmds.rs

@@ -69,6 +69,11 @@ pub fn evaluate_commands(
             std::process::exit(1);
         }
 
+        if let Some(err) = working_set.compile_errors.first() {
+            report_error(&working_set, err);
+            // Not a fatal error, for now
+        }
+
         (output, working_set.render())
     };
 

crates/nu-cli/src/repl.rs

@@ -268,6 +268,9 @@ fn loop_iteration(ctx: LoopContext) -> (bool, Stack, Reedline) {
     if let Err(err) = engine_state.merge_env(&mut stack, cwd) {
         report_error_new(engine_state, &err);
     }
+    // Check whether $env.NU_USE_IR is set, so that the user can change it in the REPL
+    // Temporary while IR eval is optional
+    stack.use_ir = stack.has_env_var(engine_state, "NU_USE_IR");
     perf!("merge env", start_time, use_color);
 
     start_time = std::time::Instant::now();

crates/nu-cli/src/util.rs

@@ -262,6 +262,11 @@ fn evaluate_source(
             return Ok(Some(1));
         }
 
+        if let Some(err) = working_set.compile_errors.first() {
+            report_error(&working_set, err);
+            // Not a fatal error, for now
+        }
+
         (output, working_set.render())
     };
 

crates/nu-cmd-lang/src/core_commands/const_.rs

@@ -46,6 +46,9 @@ impl Command for Const {
         call: &Call,
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        // This is compiled specially by the IR compiler. The code here is never used when
+        // running in IR mode.
+        let call = call.assert_ast_call()?;
         let var_id = if let Some(id) = call.positional_nth(0).and_then(|pos| pos.as_var()) {
             id
         } else {

crates/nu-cmd-lang/src/core_commands/do_.rs

@@ -81,6 +81,10 @@ impl Command for Do {
 
         bind_args_to(&mut callee_stack, &block.signature, rest, head)?;
         let eval_block_with_early_return = get_eval_block_with_early_return(engine_state);
+
+        // Applies to all block evaluation once set true
+        callee_stack.use_ir = caller_stack.has_env_var(engine_state, "NU_USE_IR");
+
         let result = eval_block_with_early_return(engine_state, &mut callee_stack, block, input);
 
         if has_env {

crates/nu-cmd-lang/src/core_commands/for_.rs

@@ -48,6 +48,9 @@ impl Command for For {
         call: &Call,
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        // This is compiled specially by the IR compiler. The code here is never used when
+        // running in IR mode.
+        let call = call.assert_ast_call()?;
         let head = call.head;
         let var_id = call
             .positional_nth(0)

crates/nu-cmd-lang/src/core_commands/if_.rs

@@ -60,6 +60,9 @@ impl Command for If {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        // This is compiled specially by the IR compiler. The code here is never used when
+        // running in IR mode.
+        let call = call.assert_ast_call()?;
         let cond = call.positional_nth(0).expect("checked through parser");
         let then_block = call
             .positional_nth(1)
@@ -99,6 +102,9 @@ impl Command for If {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        // This is compiled specially by the IR compiler. The code here is never used when
+        // running in IR mode.
+        let call = call.assert_ast_call()?;
         let cond = call.positional_nth(0).expect("checked through parser");
         let then_block = call
             .positional_nth(1)

crates/nu-cmd-lang/src/core_commands/let_.rs

@@ -46,6 +46,9 @@ impl Command for Let {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        // This is compiled specially by the IR compiler. The code here is never used when
+        // running in IR mode.
+        let call = call.assert_ast_call()?;
         let var_id = call
             .positional_nth(0)
             .expect("checked through parser")

crates/nu-cmd-lang/src/core_commands/loop_.rs

@@ -37,6 +37,9 @@ impl Command for Loop {
         call: &Call,
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        // This is compiled specially by the IR compiler. The code here is never used when
+        // running in IR mode.
+        let call = call.assert_ast_call()?;
         let head = call.head;
         let block_id = call
             .positional_nth(0)

crates/nu-cmd-lang/src/core_commands/match_.rs

@@ -43,6 +43,9 @@ impl Command for Match {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        // This is compiled specially by the IR compiler. The code here is never used when
+        // running in IR mode.
+        let call = call.assert_ast_call()?;
         let value: Value = call.req(engine_state, stack, 0)?;
         let matches = call
             .positional_nth(1)

crates/nu-cmd-lang/src/core_commands/mut_.rs

@@ -46,6 +46,9 @@ impl Command for Mut {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        // This is compiled specially by the IR compiler. The code here is never used when
+        // running in IR mode.
+        let call = call.assert_ast_call()?;
         let var_id = call
             .positional_nth(0)
             .expect("checked through parser")

crates/nu-cmd-lang/src/core_commands/overlay/use_.rs

@@ -65,9 +65,9 @@ impl Command for OverlayUse {
         name_arg.item = trim_quotes_str(&name_arg.item).to_string();
 
         let maybe_origin_module_id =
-            if let Some(overlay_expr) = call.get_parser_info("overlay_expr") {
+            if let Some(overlay_expr) = call.get_parser_info(caller_stack, "overlay_expr") {
                 if let Expr::Overlay(module_id) = &overlay_expr.expr {
-                    module_id
+                    *module_id
                 } else {
                     return Err(ShellError::NushellFailedSpanned {
                         msg: "Not an overlay".to_string(),
@@ -110,7 +110,7 @@ impl Command for OverlayUse {
             // a) adding a new overlay
             // b) refreshing an active overlay (the origin module changed)
 
-            let module = engine_state.get_module(*module_id);
+            let module = engine_state.get_module(module_id);
 
             // Evaluate the export-env block (if any) and keep its environment
             if let Some(block_id) = module.env_block {
@@ -118,7 +118,7 @@ impl Command for OverlayUse {
                     &name_arg.item,
                     engine_state,
                     caller_stack,
-                    get_dirs_var_from_call(call),
+                    get_dirs_var_from_call(caller_stack, call),
                 )?;
 
                 let block = engine_state.get_block(block_id);

crates/nu-cmd-lang/src/core_commands/try_.rs

@@ -47,6 +47,9 @@ impl Command for Try {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        // This is compiled specially by the IR compiler. The code here is never used when
+        // running in IR mode.
+        let call = call.assert_ast_call()?;
         let try_block = call
             .positional_nth(0)
             .expect("checked through parser")

crates/nu-cmd-lang/src/core_commands/use_.rs

@@ -57,7 +57,7 @@ This command is a parser keyword. For details, check:
         let Some(Expression {
             expr: Expr::ImportPattern(import_pattern),
             ..
-        }) = call.get_parser_info("import_pattern")
+        }) = call.get_parser_info(caller_stack, "import_pattern")
         else {
             return Err(ShellError::GenericError {
                 error: "Unexpected import".into(),
@@ -68,6 +68,9 @@ This command is a parser keyword. For details, check:
             });
         };
 
+        // Necessary so that we can modify the stack.
+        let import_pattern = import_pattern.clone();
+
         if let Some(module_id) = import_pattern.head.id {
             // Add constants
             for var_id in &import_pattern.constants {
@@ -99,7 +102,7 @@ This command is a parser keyword. For details, check:
                     &module_arg_str,
                     engine_state,
                     caller_stack,
-                    get_dirs_var_from_call(call),
+                    get_dirs_var_from_call(caller_stack, call),
                 )?;
                 let maybe_parent = maybe_file_path
                     .as_ref()

crates/nu-cmd-lang/src/core_commands/while_.rs

@@ -46,6 +46,9 @@ impl Command for While {
         call: &Call,
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        // This is compiled specially by the IR compiler. The code here is never used when
+        // running in IR mode.
+        let call = call.assert_ast_call()?;
         let head = call.head;
         let cond = call.positional_nth(0).expect("checked through parser");
         let block_id = call

crates/nu-command/src/bytes/build_.rs

@@ -49,10 +49,8 @@ impl Command for BytesBuild {
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
         let mut output = vec![];
-        for val in call.rest_iter_flattened(0, |expr| {
-            let eval_expression = get_eval_expression(engine_state);
-            eval_expression(engine_state, stack, expr)
-        })? {
+        let eval_expression = get_eval_expression(engine_state);
+        for val in call.rest_iter_flattened(engine_state, stack, eval_expression, 0)? {
             let val_span = val.span();
             match val {
                 Value::Binary { mut val, .. } => output.append(&mut val),

crates/nu-command/src/debug/explain.rs

@@ -1,6 +1,6 @@
 use nu_engine::{command_prelude::*, get_eval_expression};
 use nu_protocol::{
-    ast::{Argument, Block, Expr, Expression},
+    ast::{self, Argument, Block, Expr, Expression},
     engine::Closure,
 };
 
@@ -106,7 +106,7 @@ pub fn get_pipeline_elements(
 fn get_arguments(
     engine_state: &EngineState,
     stack: &mut Stack,
-    call: &Call,
+    call: &ast::Call,
     eval_expression_fn: fn(&EngineState, &mut Stack, &Expression) -> Result<Value, ShellError>,
 ) -> Vec<Value> {
     let mut arg_value = vec![];

crates/nu-command/src/debug/metadata.rs

@@ -28,6 +28,10 @@ impl Command for Metadata {
             .category(Category::Debug)
     }
 
+    fn requires_ast_for_arguments(&self) -> bool {
+        true
+    }
+
     fn run(
         &self,
         engine_state: &EngineState,
@@ -35,7 +39,7 @@ impl Command for Metadata {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
-        let arg = call.positional_nth(0);
+        let arg = call.positional_nth(stack, 0);
         let head = call.head;
 
         match arg {

crates/nu-command/src/debug/mod.rs

@@ -10,6 +10,7 @@ mod profile;
 mod timeit;
 mod view;
 mod view_files;
+mod view_ir;
 mod view_source;
 mod view_span;
 
@@ -25,5 +26,6 @@ pub use profile::DebugProfile;
 pub use timeit::TimeIt;
 pub use view::View;
 pub use view_files::ViewFiles;
+pub use view_ir::ViewIr;
 pub use view_source::ViewSource;
 pub use view_span::ViewSpan;

crates/nu-command/src/debug/timeit.rs

@@ -32,6 +32,10 @@ impl Command for TimeIt {
         vec!["timing", "timer", "benchmark", "measure"]
     }
 
+    fn requires_ast_for_arguments(&self) -> bool {
+        true
+    }
+
     fn run(
         &self,
         engine_state: &EngineState,
@@ -39,13 +43,14 @@ impl Command for TimeIt {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
-        let command_to_run = call.positional_nth(0);
+        // reset outdest, so the command can write to stdout and stderr.
+        let stack = &mut stack.push_redirection(None, None);
+
+        let command_to_run = call.positional_nth(stack, 0);
 
         // Get the start time after all other computation has been done.
         let start_time = Instant::now();
 
-        // reset outdest, so the command can write to stdout and stderr.
-        let stack = &mut stack.push_redirection(None, None);
         if let Some(command_to_run) = command_to_run {
             if let Some(block_id) = command_to_run.as_block() {
                 let eval_block = get_eval_block(engine_state);
@@ -53,7 +58,8 @@ impl Command for TimeIt {
                 eval_block(engine_state, stack, block, input)?
             } else {
                 let eval_expression_with_input = get_eval_expression_with_input(engine_state);
-                eval_expression_with_input(engine_state, stack, command_to_run, input)?.0
+                let expression = &command_to_run.clone();
+                eval_expression_with_input(engine_state, stack, expression, input)?.0
             }
         } else {
             PipelineData::empty()

crates/nu-command/src/debug/view_ir.rs

@@ -0,0 +1,83 @@
+use nu_engine::command_prelude::*;
+use nu_protocol::engine::Closure;
+
+#[derive(Clone)]
+pub struct ViewIr;
+
+impl Command for ViewIr {
+    fn name(&self) -> &str {
+        "view ir"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::new(self.name())
+            .required(
+                "closure",
+                SyntaxShape::Closure(None),
+                "The closure to see compiled code for.",
+            )
+            .switch(
+                "json",
+                "Dump the raw block data as JSON (unstable).",
+                Some('j'),
+            )
+            .input_output_type(Type::Nothing, Type::String)
+    }
+
+    fn usage(&self) -> &str {
+        "View the compiled IR code for a block of code."
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let closure: Closure = call.req(engine_state, stack, 0)?;
+        let json = call.has_flag(engine_state, stack, "json")?;
+
+        let block = engine_state.get_block(closure.block_id);
+        let ir_block = block
+            .ir_block
+            .as_ref()
+            .ok_or_else(|| ShellError::GenericError {
+                error: "Can't view IR for this block".into(),
+                msg: "block is missing compiled representation".into(),
+                span: block.span,
+                help: Some("the IrBlock is probably missing due to a compilation error".into()),
+                inner: vec![],
+            })?;
+
+        let formatted = if json {
+            let formatted_instructions = ir_block
+                .instructions
+                .iter()
+                .map(|instruction| {
+                    instruction
+                        .display(engine_state, &ir_block.data)
+                        .to_string()
+                })
+                .collect::<Vec<_>>();
+
+            serde_json::to_string_pretty(&serde_json::json!({
+                "block_id": closure.block_id,
+                "span": block.span,
+                "ir_block": ir_block,
+                "formatted_instructions": formatted_instructions,
+            }))
+            .map_err(|err| ShellError::GenericError {
+                error: "JSON serialization failed".into(),
+                msg: err.to_string(),
+                span: Some(call.head),
+                help: None,
+                inner: vec![],
+            })?
+        } else {
+            format!("{}", ir_block.display(engine_state))
+        };
+
+        Ok(Value::string(formatted, call.head).into_pipeline_data())
+    }
+}

crates/nu-command/src/default_context.rs

@@ -154,6 +154,7 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
             TimeIt,
             View,
             ViewFiles,
+            ViewIr,
             ViewSource,
             ViewSpan,
         };

crates/nu-command/src/env/export_env.rs

@@ -33,6 +33,10 @@ impl Command for ExportEnv {
         CommandType::Keyword
     }
 
+    fn requires_ast_for_arguments(&self) -> bool {
+        true
+    }
+
     fn run(
         &self,
         engine_state: &EngineState,
@@ -41,7 +45,7 @@ impl Command for ExportEnv {
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
         let block_id = call
-            .positional_nth(0)
+            .positional_nth(caller_stack, 0)
             .expect("checked through parser")
             .as_block()
             .expect("internal error: missing block");

crates/nu-command/src/env/source_env.rs

@@ -56,7 +56,7 @@ impl Command for SourceEnv {
             &source_filename.item,
             engine_state,
             caller_stack,
-            get_dirs_var_from_call(call),
+            get_dirs_var_from_call(caller_stack, call),
         )? {
             PathBuf::from(&path)
         } else {

crates/nu-command/src/filesystem/du.rs

@@ -102,7 +102,7 @@ impl Command for Du {
         let current_dir = current_dir(engine_state, stack)?;
 
         let paths = get_rest_for_glob_pattern(engine_state, stack, call, 0)?;
-        let paths = if call.rest_iter(0).count() == 0 {
+        let paths = if !call.has_positional_args(stack, 0) {
             None
         } else {
             Some(paths)

crates/nu-command/src/filesystem/ls.rs

@@ -108,7 +108,7 @@ impl Command for Ls {
         };
 
         let pattern_arg = get_rest_for_glob_pattern(engine_state, stack, call, 0)?;
-        let input_pattern_arg = if call.rest_iter(0).count() == 0 {
+        let input_pattern_arg = if !call.has_positional_args(stack, 0) {
             None
         } else {
             Some(pattern_arg)

crates/nu-command/src/filesystem/open.rs

@@ -1,7 +1,7 @@
 use super::util::get_rest_for_glob_pattern;
 #[allow(deprecated)]
 use nu_engine::{command_prelude::*, current_dir, get_eval_block};
-use nu_protocol::{ByteStream, DataSource, NuGlob, PipelineMetadata};
+use nu_protocol::{ast, ByteStream, DataSource, NuGlob, PipelineMetadata};
 use std::path::Path;
 
 #[cfg(feature = "sqlite")]
@@ -56,7 +56,7 @@ impl Command for Open {
         let mut paths = get_rest_for_glob_pattern(engine_state, stack, call, 0)?;
         let eval_block = get_eval_block(engine_state);
 
-        if paths.is_empty() && call.rest_iter(0).next().is_none() {
+        if paths.is_empty() && !call.has_positional_args(stack, 0) {
             // try to use path from pipeline input if there were no positional or spread args
             let (filename, span) = match input {
                 PipelineData::Value(val, ..) => {
@@ -180,7 +180,8 @@ impl Command for Open {
                                 let block = engine_state.get_block(block_id);
                                 eval_block(engine_state, stack, block, stream)
                             } else {
-                                decl.run(engine_state, stack, &Call::new(call_span), stream)
+                                let call = ast::Call::new(call_span);
+                                decl.run(engine_state, stack, &(&call).into(), stream)
                             };
                             output.push(command_output.map_err(|inner| {
                                     ShellError::GenericError{

crates/nu-command/src/filesystem/save.rs

@@ -4,10 +4,8 @@ use nu_engine::get_eval_block;
 use nu_engine::{command_prelude::*, current_dir};
 use nu_path::expand_path_with;
 use nu_protocol::{
-    ast::{Expr, Expression},
-    byte_stream::copy_with_signals,
-    process::ChildPipe,
-    ByteStreamSource, DataSource, OutDest, PipelineMetadata, Signals,
+    ast, byte_stream::copy_with_signals, process::ChildPipe, ByteStreamSource, DataSource, OutDest,
+    PipelineMetadata, Signals,
 };
 use std::{
     fs::File,
@@ -69,24 +67,6 @@ impl Command for Save {
         let append = call.has_flag(engine_state, stack, "append")?;
         let force = call.has_flag(engine_state, stack, "force")?;
         let progress = call.has_flag(engine_state, stack, "progress")?;
-        let out_append = if let Some(Expression {
-            expr: Expr::Bool(out_append),
-            ..
-        }) = call.get_parser_info("out-append")
-        {
-            *out_append
-        } else {
-            false
-        };
-        let err_append = if let Some(Expression {
-            expr: Expr::Bool(err_append),
-            ..
-        }) = call.get_parser_info("err-append")
-        {
-            *err_append
-        } else {
-            false
-        };
 
         let span = call.head;
         #[allow(deprecated)]
@@ -109,14 +89,7 @@ impl Command for Save {
             PipelineData::ByteStream(stream, metadata) => {
                 check_saving_to_source_file(metadata.as_ref(), &path, stderr_path.as_ref())?;
 
-                let (file, stderr_file) = get_files(
-                    &path,
-                    stderr_path.as_ref(),
-                    append,
-                    out_append,
-                    err_append,
-                    force,
-                )?;
+                let (file, stderr_file) = get_files(&path, stderr_path.as_ref(), append, force)?;
 
                 let size = stream.known_size();
                 let signals = engine_state.signals();
@@ -221,14 +194,7 @@ impl Command for Save {
                     stderr_path.as_ref(),
                 )?;
 
-                let (mut file, _) = get_files(
-                    &path,
-                    stderr_path.as_ref(),
-                    append,
-                    out_append,
-                    err_append,
-                    force,
-                )?;
+                let (mut file, _) = get_files(&path, stderr_path.as_ref(), append, force)?;
                 for val in ls {
                     file.write_all(&value_to_bytes(val)?)
                         .map_err(|err| ShellError::IOError {
@@ -258,14 +224,7 @@ impl Command for Save {
                     input_to_bytes(input, Path::new(&path.item), raw, engine_state, stack, span)?;
 
                 // Only open file after successful conversion
-                let (mut file, _) = get_files(
-                    &path,
-                    stderr_path.as_ref(),
-                    append,
-                    out_append,
-                    err_append,
-                    force,
-                )?;
+                let (mut file, _) = get_files(&path, stderr_path.as_ref(), append, force)?;
 
                 file.write_all(&bytes).map_err(|err| ShellError::IOError {
                     msg: err.to_string(),
@@ -397,7 +356,8 @@ fn convert_to_extension(
             let eval_block = get_eval_block(engine_state);
             eval_block(engine_state, stack, block, input)
         } else {
-            decl.run(engine_state, stack, &Call::new(span), input)
+            let call = ast::Call::new(span);
+            decl.run(engine_state, stack, &(&call).into(), input)
         }
     } else {
         Ok(input)
@@ -473,19 +433,17 @@ fn get_files(
     path: &Spanned<PathBuf>,
     stderr_path: Option<&Spanned<PathBuf>>,
     append: bool,
-    out_append: bool,
-    err_append: bool,
     force: bool,
 ) -> Result<(File, Option<File>), ShellError> {
     // First check both paths
-    let (path, path_span) = prepare_path(path, append || out_append, force)?;
+    let (path, path_span) = prepare_path(path, append, force)?;
     let stderr_path_and_span = stderr_path
         .as_ref()
-        .map(|stderr_path| prepare_path(stderr_path, append || err_append, force))
+        .map(|stderr_path| prepare_path(stderr_path, append, force))
         .transpose()?;
 
     // Only if both files can be used open and possibly truncate them
-    let file = open_file(path, path_span, append || out_append)?;
+    let file = open_file(path, path_span, append)?;
 
     let stderr_file = stderr_path_and_span
         .map(|(stderr_path, stderr_path_span)| {
@@ -498,7 +456,7 @@ fn get_files(
                     inner: vec![],
                 })
             } else {
-                open_file(stderr_path, stderr_path_span, append || err_append)
+                open_file(stderr_path, stderr_path_span, append)
             }
         })
         .transpose()?;

crates/nu-command/src/filesystem/util.rs

@@ -1,6 +1,6 @@
 use dialoguer::Input;
 use nu_engine::{command_prelude::*, get_eval_expression};
-use nu_protocol::{ast::Expr, FromValue, NuGlob};
+use nu_protocol::{FromValue, NuGlob};
 use std::{
     error::Error,
     path::{Path, PathBuf},
@@ -92,42 +92,19 @@ pub fn is_older(src: &Path, dst: &Path) -> Option<bool> {
 
 /// Get rest arguments from given `call`, starts with `starting_pos`.
 ///
-/// It's similar to `call.rest`, except that it always returns NuGlob.  And if input argument has
-/// Type::Glob, the NuGlob is unquoted, which means it's required to expand.
+/// It's similar to `call.rest`, except that it always returns NuGlob.
 pub fn get_rest_for_glob_pattern(
     engine_state: &EngineState,
     stack: &mut Stack,
     call: &Call,
     starting_pos: usize,
 ) -> Result<Vec<Spanned<NuGlob>>, ShellError> {
-    let mut output = vec![];
     let eval_expression = get_eval_expression(engine_state);
 
-    for result in call.rest_iter_flattened(starting_pos, |expr| {
-        let result = eval_expression(engine_state, stack, expr);
-        match result {
-            Err(e) => Err(e),
-            Ok(result) => {
-                let span = result.span();
-                // convert from string to quoted string if expr is a variable
-                // or string interpolation
-                match result {
-                    Value::String { val, .. }
-                        if matches!(
-                            &expr.expr,
-                            Expr::FullCellPath(_) | Expr::StringInterpolation(_)
-                        ) =>
-                    {
-                        // should not expand if given input type is not glob.
-                        Ok(Value::glob(val, expr.ty != Type::Glob, span))
-                    }
-                    other => Ok(other),
-                }
-            }
-        }
-    })? {
-        output.push(FromValue::from_value(result)?);
-    }
-
-    Ok(output)
+    call.rest_iter_flattened(engine_state, stack, eval_expression, starting_pos)?
+        .into_iter()
+        // This used to be much more complex, but I think `FromValue` should be able to handle the
+        // nuance here.
+        .map(FromValue::from_value)
+        .collect()
 }

crates/nu-command/src/filters/transpose.rs

@@ -149,27 +149,27 @@ pub fn transpose(
     if !args.rest.is_empty() && args.header_row {
         return Err(ShellError::IncompatibleParametersSingle {
             msg: "Can not provide header names and use `--header-row`".into(),
-            span: call.get_named_arg("header-row").expect("has flag").span,
+            span: call.get_flag_span(stack, "header-row").expect("has flag"),
         });
     }
     if !args.header_row && args.keep_all {
         return Err(ShellError::IncompatibleParametersSingle {
             msg: "Can only be used with `--header-row`(`-r`)".into(),
-            span: call.get_named_arg("keep-all").expect("has flag").span,
+            span: call.get_flag_span(stack, "keep-all").expect("has flag"),
         });
     }
     if !args.header_row && args.keep_last {
         return Err(ShellError::IncompatibleParametersSingle {
             msg: "Can only be used with `--header-row`(`-r`)".into(),
-            span: call.get_named_arg("keep-last").expect("has flag").span,
+            span: call.get_flag_span(stack, "keep-last").expect("has flag"),
         });
     }
     if args.keep_all && args.keep_last {
         return Err(ShellError::IncompatibleParameters {
             left_message: "can't use `--keep-last` at the same time".into(),
-            left_span: call.get_named_arg("keep-last").expect("has flag").span,
+            left_span: call.get_flag_span(stack, "keep-last").expect("has flag"),
             right_message: "because of `--keep-all`".into(),
-            right_span: call.get_named_arg("keep-all").expect("has flag").span,
+            right_span: call.get_flag_span(stack, "keep-all").expect("has flag"),
         });
     }
 

crates/nu-command/src/filters/utils.rs

@@ -1,7 +1,6 @@
 use nu_engine::{CallExt, ClosureEval};
 use nu_protocol::{
-    ast::Call,
-    engine::{Closure, EngineState, Stack},
+    engine::{Call, Closure, EngineState, Stack},
     IntoPipelineData, PipelineData, ShellError, Span, Value,
 };
 

crates/nu-command/src/generators/cal.rs

@@ -1,7 +1,7 @@
 use chrono::{Datelike, Local, NaiveDate};
 use nu_color_config::StyleComputer;
 use nu_engine::command_prelude::*;
-use nu_protocol::ast::{Expr, Expression};
+use nu_protocol::ast::{self, Expr, Expression};
 
 use std::collections::VecDeque;
 
@@ -143,7 +143,7 @@ pub fn cal(
         style_computer,
     )?;
 
-    let mut table_no_index = Call::new(Span::unknown());
+    let mut table_no_index = ast::Call::new(Span::unknown());
     table_no_index.add_named((
         Spanned {
             item: "index".to_string(),
@@ -160,7 +160,12 @@ pub fn cal(
     let cal_table_output =
         Value::list(calendar_vec_deque.into_iter().collect(), tag).into_pipeline_data();
     if !arguments.as_table {
-        crate::Table.run(engine_state, stack, &table_no_index, cal_table_output)
+        crate::Table.run(
+            engine_state,
+            stack,
+            &(&table_no_index).into(),
+            cal_table_output,
+        )
     } else {
         Ok(cal_table_output)
     }

crates/nu-command/src/math/utils.rs

@@ -1,6 +1,6 @@
 use core::slice;
 use indexmap::IndexMap;
-use nu_protocol::{ast::Call, IntoPipelineData, PipelineData, ShellError, Signals, Span, Value};
+use nu_protocol::{engine::Call, IntoPipelineData, PipelineData, ShellError, Signals, Span, Value};
 
 pub fn run_with_function(
     call: &Call,

crates/nu-command/src/platform/ansi/ansi_.rs

@@ -676,7 +676,7 @@ Operating system commands:
             }
         };
 
-        let output = heavy_lifting(code, escape, osc, call)?;
+        let output = heavy_lifting(code, escape, osc, stack, call)?;
 
         Ok(Value::string(output, call.head).into_pipeline_data())
     }
@@ -713,26 +713,30 @@ Operating system commands:
             }
         };
 
-        let output = heavy_lifting(code, escape, osc, call)?;
+        let output = heavy_lifting(code, escape, osc, &Stack::new(), call)?;
 
         Ok(Value::string(output, call.head).into_pipeline_data())
     }
 }
 
-fn heavy_lifting(code: Value, escape: bool, osc: bool, call: &Call) -> Result<String, ShellError> {
+fn heavy_lifting(
+    code: Value,
+    escape: bool,
+    osc: bool,
+    stack: &Stack,
+    call: &Call,
+) -> Result<String, ShellError> {
     let param_is_string = matches!(code, Value::String { .. });
     if escape && osc {
         return Err(ShellError::IncompatibleParameters {
             left_message: "escape".into(),
             left_span: call
-                .get_named_arg("escape")
-                .expect("Unexpected missing argument")
-                .span,
+                .get_flag_span(stack, "escape")
+                .expect("Unexpected missing argument"),
             right_message: "osc".into(),
             right_span: call
-                .get_named_arg("osc")
-                .expect("Unexpected missing argument")
-                .span,
+                .get_flag_span(stack, "osc")
+                .expect("Unexpected missing argument"),
         });
     }
     let code_string = if param_is_string {
@@ -744,10 +748,7 @@ fn heavy_lifting(code: Value, escape: bool, osc: bool, call: &Call) -> Result<St
     if (escape || osc) && (param_is_valid_string) {
         let code_vec: Vec<char> = code_string.chars().collect();
         if code_vec[0] == '\\' {
-            let span = match call.get_flag_expr("escape") {
-                Some(expr) => expr.span,
-                None => call.head,
-            };
+            let span = call.get_flag_span(stack, "escape").unwrap_or(call.head);
 
             return Err(ShellError::TypeMismatch {
                 err_message: "no need for escape characters".into(),

crates/nu-command/src/platform/is_terminal.rs

@@ -58,7 +58,7 @@ impl Command for IsTerminal {
             _ => {
                 return Err(ShellError::IncompatibleParametersSingle {
                     msg: "Only one stream may be checked".into(),
-                    span: Span::merge_many(call.arguments.iter().map(|arg| arg.span())),
+                    span: call.arguments_span(),
                 });
             }
         };

crates/nu-command/src/platform/kill.rs

@@ -84,27 +84,26 @@ impl Command for Kill {
                 {
                     return Err(ShellError::IncompatibleParameters {
                         left_message: "force".to_string(),
-                        left_span: call
-                            .get_named_arg("force")
-                            .ok_or_else(|| ShellError::GenericError {
+                        left_span: call.get_flag_span(stack, "force").ok_or_else(|| {
+                            ShellError::GenericError {
                                 error: "Flag error".into(),
                                 msg: "flag force not found".into(),
                                 span: Some(call.head),
                                 help: None,
                                 inner: vec![],
-                            })?
-                            .span,
+                            }
+                        })?,
                         right_message: "signal".to_string(),
                         right_span: Span::merge(
-                            call.get_named_arg("signal")
-                                .ok_or_else(|| ShellError::GenericError {
+                            call.get_flag_span(stack, "signal").ok_or_else(|| {
+                                ShellError::GenericError {
                                     error: "Flag error".into(),
                                     msg: "flag signal not found".into(),
                                     span: Some(call.head),
                                     help: None,
                                     inner: vec![],
-                                })?
-                                .span,
+                                }
+                            })?,
                             signal_span,
                         ),
                     });

crates/nu-command/src/strings/encode_decode/base64.rs

@@ -8,8 +8,8 @@ use base64::{
 };
 use nu_cmd_base::input_handler::{operate as general_operate, CmdArgument};
 use nu_protocol::{
-    ast::{Call, CellPath},
-    engine::EngineState,
+    ast::CellPath,
+    engine::{Call, EngineState},
     PipelineData, ShellError, Span, Spanned, Value,
 };
 

crates/nu-command/src/strings/mod.rs

@@ -17,8 +17,7 @@ pub use str_::*;
 
 use nu_engine::CallExt;
 use nu_protocol::{
-    ast::Call,
-    engine::{EngineState, Stack, StateWorkingSet},
+    engine::{Call, EngineState, Stack, StateWorkingSet},
     ShellError,
 };
 

crates/nu-command/src/system/nu_check.rs

@@ -87,7 +87,7 @@ impl Command for NuCheck {
                         &path_str.item,
                         engine_state,
                         stack,
-                        get_dirs_var_from_call(call),
+                        get_dirs_var_from_call(stack, call),
                     ) {
                         Ok(path) => {
                             if let Some(path) = path {

crates/nu-command/src/system/run_external.rs

@@ -1,9 +1,7 @@
 use nu_cmd_base::hook::eval_hook;
 use nu_engine::{command_prelude::*, env_to_strings, get_eval_expression};
 use nu_path::{dots::expand_ndots, expand_tilde};
-use nu_protocol::{
-    ast::Expression, did_you_mean, process::ChildProcess, ByteStream, NuGlob, OutDest, Signals,
-};
+use nu_protocol::{did_you_mean, process::ChildProcess, ByteStream, NuGlob, OutDest, Signals};
 use nu_system::ForegroundChild;
 use nu_utils::IgnoreCaseExt;
 use pathdiff::diff_paths;
@@ -222,20 +220,21 @@ pub fn eval_arguments_from_call(
     call: &Call,
 ) -> Result<Vec<Spanned<OsString>>, ShellError> {
     let cwd = engine_state.cwd(Some(stack))?;
-    let mut args: Vec<Spanned<OsString>> = vec![];
-    for (expr, spread) in call.rest_iter(1) {
-        for arg in eval_argument(engine_state, stack, expr, spread)? {
-            match arg {
-                // Expand globs passed to run-external
-                Value::Glob { val, no_expand, .. } if !no_expand => args.extend(
-                    expand_glob(&val, cwd.as_ref(), expr.span, engine_state.signals())?
-                        .into_iter()
-                        .map(|s| s.into_spanned(expr.span)),
-                ),
-                other => {
-                    args.push(OsString::from(coerce_into_string(other)?).into_spanned(expr.span))
-                }
-            }
+    let eval_expression = get_eval_expression(engine_state);
+    let call_args = call.rest_iter_flattened(engine_state, stack, eval_expression, 1)?;
+    let mut args: Vec<Spanned<OsString>> = Vec::with_capacity(call_args.len());
+
+    for arg in call_args {
+        let span = arg.span();
+        match arg {
+            // Expand globs passed to run-external
+            Value::Glob { val, no_expand, .. } if !no_expand => args.extend(
+                expand_glob(&val, cwd.as_std_path(), span, engine_state.signals())?
+                    .into_iter()
+                    .map(|s| s.into_spanned(span)),
+            ),
+            other => args
+                .push(OsString::from(coerce_into_string(engine_state, other)?).into_spanned(span)),
         }
     }
     Ok(args)
@@ -243,42 +242,17 @@ pub fn eval_arguments_from_call(
 
 /// Custom `coerce_into_string()`, including globs, since those are often args to `run-external`
 /// as well
-fn coerce_into_string(val: Value) -> Result<String, ShellError> {
+fn coerce_into_string(engine_state: &EngineState, val: Value) -> Result<String, ShellError> {
     match val {
+        Value::List { .. } => Err(ShellError::CannotPassListToExternal {
+            arg: String::from_utf8_lossy(engine_state.get_span_contents(val.span())).into_owned(),
+            span: val.span(),
+        }),
         Value::Glob { val, .. } => Ok(val),
         _ => val.coerce_into_string(),
     }
 }
 
-/// Evaluate an argument, returning more than one value if it was a list to be spread.
-fn eval_argument(
-    engine_state: &EngineState,
-    stack: &mut Stack,
-    expr: &Expression,
-    spread: bool,
-) -> Result<Vec<Value>, ShellError> {
-    let eval = get_eval_expression(engine_state);
-    match eval(engine_state, stack, expr)? {
-        Value::List { vals, .. } => {
-            if spread {
-                Ok(vals)
-            } else {
-                Err(ShellError::CannotPassListToExternal {
-                    arg: String::from_utf8_lossy(engine_state.get_span_contents(expr.span)).into(),
-                    span: expr.span,
-                })
-            }
-        }
-        value => {
-            if spread {
-                Err(ShellError::CannotSpreadAsList { span: expr.span })
-            } else {
-                Ok(vec![value])
-            }
-        }
-    }
-}
-
 /// Performs glob expansion on `arg`. If the expansion found no matches or the pattern
 /// is not a valid glob, then this returns the original string as the expansion result.
 ///

crates/nu-command/src/system/uname.rs

@@ -1,10 +1,5 @@
-use nu_protocol::record;
-use nu_protocol::Value;
-use nu_protocol::{
-    ast::Call,
-    engine::{Command, EngineState, Stack},
-    Category, Example, PipelineData, ShellError, Signature, Type,
-};
+use nu_engine::command_prelude::*;
+use nu_protocol::{record, Value};
 
 #[derive(Clone)]
 pub struct UName;

crates/nu-command/src/viewers/table.rs

@@ -344,7 +344,7 @@ fn get_theme_flag(
 struct CmdInput<'a> {
     engine_state: &'a EngineState,
     stack: &'a mut Stack,
-    call: &'a Call,
+    call: &'a Call<'a>,
     data: PipelineData,
 }
 
@@ -352,7 +352,7 @@ impl<'a> CmdInput<'a> {
     fn new(
         engine_state: &'a EngineState,
         stack: &'a mut Stack,
-        call: &'a Call,
+        call: &'a Call<'a>,
         data: PipelineData,
     ) -> Self {
         Self {

crates/nu-engine/Cargo.toml

@@ -15,6 +15,7 @@ nu-protocol = { path = "../nu-protocol", features = ["plugin"], version = "0.95.
 nu-path = { path = "../nu-path", version = "0.95.1" }
 nu-glob = { path = "../nu-glob", version = "0.95.1" }
 nu-utils = { path = "../nu-utils", version = "0.95.1" }
+log = { workspace = true }
 
 [features]
-plugin = []
+plugin = []

crates/nu-engine/src/call_ext.rs

@@ -1,10 +1,10 @@
 use crate::eval_expression;
 use nu_protocol::{
-    ast::Call,
+    ast,
     debugger::WithoutDebug,
-    engine::{EngineState, Stack, StateWorkingSet},
+    engine::{self, EngineState, Stack, StateWorkingSet},
     eval_const::eval_constant,
-    FromValue, ShellError, Value,
+    ir, FromValue, ShellError, Span, Value,
 };
 
 pub trait CallExt {
@@ -23,6 +23,9 @@ pub trait CallExt {
         name: &str,
     ) -> Result<Option<T>, ShellError>;
 
+    /// Efficiently get the span of a flag argument
+    fn get_flag_span(&self, stack: &Stack, name: &str) -> Option<Span>;
+
     fn rest<T: FromValue>(
         &self,
         engine_state: &EngineState,
@@ -56,9 +59,12 @@ pub trait CallExt {
         stack: &mut Stack,
         name: &str,
     ) -> Result<T, ShellError>;
+
+    /// True if the command has any positional or rest arguments, excluding before the given index.
+    fn has_positional_args(&self, stack: &Stack, starting_pos: usize) -> bool;
 }
 
-impl CallExt for Call {
+impl CallExt for ast::Call {
     fn has_flag(
         &self,
         engine_state: &EngineState,
@@ -104,6 +110,10 @@ impl CallExt for Call {
         }
     }
 
+    fn get_flag_span(&self, _stack: &Stack, name: &str) -> Option<Span> {
+        self.get_named_arg(name).map(|arg| arg.span)
+    }
+
     fn rest<T: FromValue>(
         &self,
         engine_state: &EngineState,
@@ -189,4 +199,205 @@ impl CallExt for Call {
             })
         }
     }
+
+    fn has_positional_args(&self, _stack: &Stack, starting_pos: usize) -> bool {
+        self.rest_iter(starting_pos).next().is_some()
+    }
+}
+
+impl CallExt for ir::Call {
+    fn has_flag(
+        &self,
+        _engine_state: &EngineState,
+        stack: &mut Stack,
+        flag_name: &str,
+    ) -> Result<bool, ShellError> {
+        Ok(self
+            .named_iter(stack)
+            .find(|(name, _)| name.item == flag_name)
+            .is_some_and(|(_, value)| {
+                // Handle --flag=false
+                !matches!(value, Some(Value::Bool { val: false, .. }))
+            }))
+    }
+
+    fn get_flag<T: FromValue>(
+        &self,
+        _engine_state: &EngineState,
+        stack: &mut Stack,
+        name: &str,
+    ) -> Result<Option<T>, ShellError> {
+        if let Some(val) = self.get_named_arg(stack, name) {
+            T::from_value(val.clone()).map(Some)
+        } else {
+            Ok(None)
+        }
+    }
+
+    fn get_flag_span(&self, stack: &Stack, name: &str) -> Option<Span> {
+        self.named_iter(stack)
+            .find_map(|(i_name, _)| (i_name.item == name).then_some(i_name.span))
+    }
+
+    fn rest<T: FromValue>(
+        &self,
+        _engine_state: &EngineState,
+        stack: &mut Stack,
+        starting_pos: usize,
+    ) -> Result<Vec<T>, ShellError> {
+        self.rest_iter_flattened(stack, starting_pos)?
+            .into_iter()
+            .map(T::from_value)
+            .collect()
+    }
+
+    fn opt<T: FromValue>(
+        &self,
+        _engine_state: &EngineState,
+        stack: &mut Stack,
+        pos: usize,
+    ) -> Result<Option<T>, ShellError> {
+        self.positional_iter(stack)
+            .nth(pos)
+            .cloned()
+            .map(T::from_value)
+            .transpose()
+    }
+
+    fn opt_const<T: FromValue>(
+        &self,
+        _working_set: &StateWorkingSet,
+        _pos: usize,
+    ) -> Result<Option<T>, ShellError> {
+        Err(ShellError::IrEvalError {
+            msg: "const evaluation is not yet implemented on ir::Call".into(),
+            span: Some(self.head),
+        })
+    }
+
+    fn req<T: FromValue>(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        pos: usize,
+    ) -> Result<T, ShellError> {
+        if let Some(val) = self.opt(engine_state, stack, pos)? {
+            Ok(val)
+        } else if self.positional_len(stack) == 0 {
+            Err(ShellError::AccessEmptyContent { span: self.head })
+        } else {
+            Err(ShellError::AccessBeyondEnd {
+                max_idx: self.positional_len(stack) - 1,
+                span: self.head,
+            })
+        }
+    }
+
+    fn req_parser_info<T: FromValue>(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        name: &str,
+    ) -> Result<T, ShellError> {
+        // FIXME: this depends on the AST evaluator. We can fix this by making the parser info an
+        // enum rather than using expressions. It's not clear that evaluation of this is ever really
+        // needed.
+        if let Some(expr) = self.get_parser_info(stack, name) {
+            let expr = expr.clone();
+            let stack = &mut stack.use_call_arg_out_dest();
+            let result = eval_expression::<WithoutDebug>(engine_state, stack, &expr)?;
+            FromValue::from_value(result)
+        } else {
+            Err(ShellError::CantFindColumn {
+                col_name: name.into(),
+                span: None,
+                src_span: self.head,
+            })
+        }
+    }
+
+    fn has_positional_args(&self, stack: &Stack, starting_pos: usize) -> bool {
+        self.rest_iter(stack, starting_pos).next().is_some()
+    }
+}
+
+macro_rules! proxy {
+    ($self:ident . $method:ident ($($param:expr),*)) => (match &$self.inner {
+        engine::CallImpl::AstRef(call) => call.$method($($param),*),
+        engine::CallImpl::AstBox(call) => call.$method($($param),*),
+        engine::CallImpl::IrRef(call) => call.$method($($param),*),
+        engine::CallImpl::IrBox(call) => call.$method($($param),*),
+    })
+}
+
+impl CallExt for engine::Call<'_> {
+    fn has_flag(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        flag_name: &str,
+    ) -> Result<bool, ShellError> {
+        proxy!(self.has_flag(engine_state, stack, flag_name))
+    }
+
+    fn get_flag<T: FromValue>(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        name: &str,
+    ) -> Result<Option<T>, ShellError> {
+        proxy!(self.get_flag(engine_state, stack, name))
+    }
+
+    fn get_flag_span(&self, stack: &Stack, name: &str) -> Option<Span> {
+        proxy!(self.get_flag_span(stack, name))
+    }
+
+    fn rest<T: FromValue>(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        starting_pos: usize,
+    ) -> Result<Vec<T>, ShellError> {
+        proxy!(self.rest(engine_state, stack, starting_pos))
+    }
+
+    fn opt<T: FromValue>(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        pos: usize,
+    ) -> Result<Option<T>, ShellError> {
+        proxy!(self.opt(engine_state, stack, pos))
+    }
+
+    fn opt_const<T: FromValue>(
+        &self,
+        working_set: &StateWorkingSet,
+        pos: usize,
+    ) -> Result<Option<T>, ShellError> {
+        proxy!(self.opt_const(working_set, pos))
+    }
+
+    fn req<T: FromValue>(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        pos: usize,
+    ) -> Result<T, ShellError> {
+        proxy!(self.req(engine_state, stack, pos))
+    }
+
+    fn req_parser_info<T: FromValue>(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        name: &str,
+    ) -> Result<T, ShellError> {
+        proxy!(self.req_parser_info(engine_state, stack, name))
+    }
+
+    fn has_positional_args(&self, stack: &Stack, starting_pos: usize) -> bool {
+        proxy!(self.has_positional_args(stack, starting_pos))
+    }
 }

crates/nu-engine/src/closure_eval.rs

@@ -62,8 +62,8 @@ pub struct ClosureEval {
     stack: Stack,
     block: Arc<Block>,
     arg_index: usize,
-    env_vars: Vec<EnvVars>,
-    env_hidden: HashMap<String, HashSet<String>>,
+    env_vars: Vec<Arc<EnvVars>>,
+    env_hidden: Arc<HashMap<String, HashSet<String>>>,
     eval: EvalBlockWithEarlyReturnFn,
 }
 

crates/nu-engine/src/command_prelude.rs

@@ -1,7 +1,7 @@
 pub use crate::CallExt;
 pub use nu_protocol::{
-    ast::{Call, CellPath},
-    engine::{Command, EngineState, Stack, StateWorkingSet},
+    ast::CellPath,
+    engine::{Call, Command, EngineState, Stack, StateWorkingSet},
     record, ByteStream, ByteStreamType, Category, ErrSpan, Example, IntoInterruptiblePipelineData,
     IntoPipelineData, IntoSpanned, PipelineData, Record, ShellError, Signature, Span, Spanned,
     SyntaxShape, Type, Value,

crates/nu-engine/src/compile/builder.rs

@@ -0,0 +1,575 @@
+use nu_protocol::{
+    ir::{DataSlice, Instruction, IrAstRef, IrBlock, Literal},
+    CompileError, IntoSpanned, RegId, Span, Spanned,
+};
+
+/// A label identifier. Only exists while building code. Replaced with the actual target.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct LabelId(pub usize);
+
+/// Builds [`IrBlock`]s progressively by consuming instructions and handles register allocation.
+#[derive(Debug)]
+pub(crate) struct BlockBuilder {
+    pub(crate) block_span: Option<Span>,
+    pub(crate) instructions: Vec<Instruction>,
+    pub(crate) spans: Vec<Span>,
+    /// The actual instruction index that a label refers to. While building IR, branch targets are
+    /// specified as indices into this array rather than the true instruction index. This makes it
+    /// easier to make modifications to code, as just this array needs to be changed, and it's also
+    /// less error prone as during `finish()` we check to make sure all of the used labels have had
+    /// an index actually set.
+    pub(crate) labels: Vec<Option<usize>>,
+    pub(crate) data: Vec<u8>,
+    pub(crate) ast: Vec<Option<IrAstRef>>,
+    pub(crate) comments: Vec<String>,
+    pub(crate) register_allocation_state: Vec<bool>,
+    pub(crate) file_count: u32,
+    pub(crate) loop_stack: Vec<Loop>,
+}
+
+impl BlockBuilder {
+    /// Starts a new block, with the first register (`%0`) allocated as input.
+    pub(crate) fn new(block_span: Option<Span>) -> Self {
+        BlockBuilder {
+            block_span,
+            instructions: vec![],
+            spans: vec![],
+            labels: vec![],
+            data: vec![],
+            ast: vec![],
+            comments: vec![],
+            register_allocation_state: vec![true],
+            file_count: 0,
+            loop_stack: vec![],
+        }
+    }
+
+    /// Get the next unused register for code generation.
+    pub(crate) fn next_register(&mut self) -> Result<RegId, CompileError> {
+        if let Some(index) = self
+            .register_allocation_state
+            .iter_mut()
+            .position(|is_allocated| {
+                if !*is_allocated {
+                    *is_allocated = true;
+                    true
+                } else {
+                    false
+                }
+            })
+        {
+            Ok(RegId(index as u32))
+        } else if self.register_allocation_state.len() < (u32::MAX as usize - 2) {
+            let reg_id = RegId(self.register_allocation_state.len() as u32);
+            self.register_allocation_state.push(true);
+            Ok(reg_id)
+        } else {
+            Err(CompileError::RegisterOverflow {
+                block_span: self.block_span,
+            })
+        }
+    }
+
+    /// Check if a register is initialized with a value.
+    pub(crate) fn is_allocated(&self, reg_id: RegId) -> bool {
+        self.register_allocation_state
+            .get(reg_id.0 as usize)
+            .is_some_and(|state| *state)
+    }
+
+    /// Mark a register as initialized.
+    pub(crate) fn mark_register(&mut self, reg_id: RegId) -> Result<(), CompileError> {
+        if let Some(is_allocated) = self.register_allocation_state.get_mut(reg_id.0 as usize) {
+            *is_allocated = true;
+            Ok(())
+        } else {
+            Err(CompileError::RegisterOverflow {
+                block_span: self.block_span,
+            })
+        }
+    }
+
+    /// Mark a register as empty, so that it can be used again by something else.
+    #[track_caller]
+    pub(crate) fn free_register(&mut self, reg_id: RegId) -> Result<(), CompileError> {
+        let index = reg_id.0 as usize;
+
+        if self
+            .register_allocation_state
+            .get(index)
+            .is_some_and(|is_allocated| *is_allocated)
+        {
+            self.register_allocation_state[index] = false;
+            Ok(())
+        } else {
+            log::warn!("register {reg_id} uninitialized, builder = {self:#?}");
+            Err(CompileError::RegisterUninitialized {
+                reg_id,
+                caller: std::panic::Location::caller().to_string(),
+            })
+        }
+    }
+
+    /// Define a label, which can be used by branch instructions. The target can optionally be
+    /// specified now.
+    pub(crate) fn label(&mut self, target_index: Option<usize>) -> LabelId {
+        let label_id = self.labels.len();
+        self.labels.push(target_index);
+        LabelId(label_id)
+    }
+
+    /// Change the target of a label.
+    pub(crate) fn set_label(
+        &mut self,
+        label_id: LabelId,
+        target_index: usize,
+    ) -> Result<(), CompileError> {
+        *self
+            .labels
+            .get_mut(label_id.0)
+            .ok_or(CompileError::UndefinedLabel {
+                label_id: label_id.0,
+                span: None,
+            })? = Some(target_index);
+        Ok(())
+    }
+
+    /// Insert an instruction into the block, automatically marking any registers populated by
+    /// the instruction, and freeing any registers consumed by the instruction.
+    #[track_caller]
+    pub(crate) fn push(&mut self, instruction: Spanned<Instruction>) -> Result<(), CompileError> {
+        // Free read registers, and mark write registers.
+        //
+        // If a register is both read and written, it should be on both sides, so that we can verify
+        // that the register was in the right state beforehand.
+        let mut allocate = |read: &[RegId], write: &[RegId]| -> Result<(), CompileError> {
+            for reg in read {
+                self.free_register(*reg)?;
+            }
+            for reg in write {
+                self.mark_register(*reg)?;
+            }
+            Ok(())
+        };
+
+        let allocate_result = match &instruction.item {
+            Instruction::Unreachable => Ok(()),
+            Instruction::LoadLiteral { dst, lit } => {
+                allocate(&[], &[*dst]).and(
+                    // Free any registers on the literal
+                    match lit {
+                        Literal::Range {
+                            start,
+                            step,
+                            end,
+                            inclusion: _,
+                        } => allocate(&[*start, *step, *end], &[]),
+                        Literal::Bool(_)
+                        | Literal::Int(_)
+                        | Literal::Float(_)
+                        | Literal::Filesize(_)
+                        | Literal::Duration(_)
+                        | Literal::Binary(_)
+                        | Literal::Block(_)
+                        | Literal::Closure(_)
+                        | Literal::RowCondition(_)
+                        | Literal::List { capacity: _ }
+                        | Literal::Record { capacity: _ }
+                        | Literal::Filepath {
+                            val: _,
+                            no_expand: _,
+                        }
+                        | Literal::Directory {
+                            val: _,
+                            no_expand: _,
+                        }
+                        | Literal::GlobPattern {
+                            val: _,
+                            no_expand: _,
+                        }
+                        | Literal::String(_)
+                        | Literal::RawString(_)
+                        | Literal::CellPath(_)
+                        | Literal::Date(_)
+                        | Literal::Nothing => Ok(()),
+                    },
+                )
+            }
+            Instruction::LoadValue { dst, val: _ } => allocate(&[], &[*dst]),
+            Instruction::Move { dst, src } => allocate(&[*src], &[*dst]),
+            Instruction::Clone { dst, src } => allocate(&[*src], &[*dst, *src]),
+            Instruction::Collect { src_dst } => allocate(&[*src_dst], &[*src_dst]),
+            Instruction::Span { src_dst } => allocate(&[*src_dst], &[*src_dst]),
+            Instruction::Drop { src } => allocate(&[*src], &[]),
+            Instruction::Drain { src } => allocate(&[*src], &[]),
+            Instruction::LoadVariable { dst, var_id: _ } => allocate(&[], &[*dst]),
+            Instruction::StoreVariable { var_id: _, src } => allocate(&[*src], &[]),
+            Instruction::LoadEnv { dst, key: _ } => allocate(&[], &[*dst]),
+            Instruction::LoadEnvOpt { dst, key: _ } => allocate(&[], &[*dst]),
+            Instruction::StoreEnv { key: _, src } => allocate(&[*src], &[]),
+            Instruction::PushPositional { src } => allocate(&[*src], &[]),
+            Instruction::AppendRest { src } => allocate(&[*src], &[]),
+            Instruction::PushFlag { name: _ } => Ok(()),
+            Instruction::PushShortFlag { short: _ } => Ok(()),
+            Instruction::PushNamed { name: _, src } => allocate(&[*src], &[]),
+            Instruction::PushShortNamed { short: _, src } => allocate(&[*src], &[]),
+            Instruction::PushParserInfo { name: _, info: _ } => Ok(()),
+            Instruction::RedirectOut { mode: _ } => Ok(()),
+            Instruction::RedirectErr { mode: _ } => Ok(()),
+            Instruction::CheckErrRedirected { src } => allocate(&[*src], &[*src]),
+            Instruction::OpenFile {
+                file_num: _,
+                path,
+                append: _,
+            } => allocate(&[*path], &[]),
+            Instruction::WriteFile { file_num: _, src } => allocate(&[*src], &[]),
+            Instruction::CloseFile { file_num: _ } => Ok(()),
+            Instruction::Call {
+                decl_id: _,
+                src_dst,
+            } => allocate(&[*src_dst], &[*src_dst]),
+            Instruction::StringAppend { src_dst, val } => allocate(&[*src_dst, *val], &[*src_dst]),
+            Instruction::GlobFrom {
+                src_dst,
+                no_expand: _,
+            } => allocate(&[*src_dst], &[*src_dst]),
+            Instruction::ListPush { src_dst, item } => allocate(&[*src_dst, *item], &[*src_dst]),
+            Instruction::ListSpread { src_dst, items } => {
+                allocate(&[*src_dst, *items], &[*src_dst])
+            }
+            Instruction::RecordInsert { src_dst, key, val } => {
+                allocate(&[*src_dst, *key, *val], &[*src_dst])
+            }
+            Instruction::RecordSpread { src_dst, items } => {
+                allocate(&[*src_dst, *items], &[*src_dst])
+            }
+            Instruction::Not { src_dst } => allocate(&[*src_dst], &[*src_dst]),
+            Instruction::BinaryOp {
+                lhs_dst,
+                op: _,
+                rhs,
+            } => allocate(&[*lhs_dst, *rhs], &[*lhs_dst]),
+            Instruction::FollowCellPath { src_dst, path } => {
+                allocate(&[*src_dst, *path], &[*src_dst])
+            }
+            Instruction::CloneCellPath { dst, src, path } => {
+                allocate(&[*src, *path], &[*src, *dst])
+            }
+            Instruction::UpsertCellPath {
+                src_dst,
+                path,
+                new_value,
+            } => allocate(&[*src_dst, *path, *new_value], &[*src_dst]),
+            Instruction::Jump { index: _ } => Ok(()),
+            Instruction::BranchIf { cond, index: _ } => allocate(&[*cond], &[]),
+            Instruction::BranchIfEmpty { src, index: _ } => allocate(&[*src], &[*src]),
+            Instruction::Match {
+                pattern: _,
+                src,
+                index: _,
+            } => allocate(&[*src], &[*src]),
+            Instruction::CheckMatchGuard { src } => allocate(&[*src], &[*src]),
+            Instruction::Iterate {
+                dst,
+                stream,
+                end_index: _,
+            } => allocate(&[*stream], &[*dst, *stream]),
+            Instruction::OnError { index: _ } => Ok(()),
+            Instruction::OnErrorInto { index: _, dst } => allocate(&[], &[*dst]),
+            Instruction::PopErrorHandler => Ok(()),
+            Instruction::CheckExternalFailed { dst, src } => allocate(&[*src], &[*dst, *src]),
+            Instruction::ReturnEarly { src } => allocate(&[*src], &[]),
+            Instruction::Return { src } => allocate(&[*src], &[]),
+        };
+
+        // Add more context to the error
+        match allocate_result {
+            Ok(()) => (),
+            Err(CompileError::RegisterUninitialized { reg_id, caller }) => {
+                return Err(CompileError::RegisterUninitializedWhilePushingInstruction {
+                    reg_id,
+                    caller,
+                    instruction: format!("{:?}", instruction.item),
+                    span: instruction.span,
+                });
+            }
+            Err(err) => return Err(err),
+        }
+
+        self.instructions.push(instruction.item);
+        self.spans.push(instruction.span);
+        self.ast.push(None);
+        self.comments.push(String::new());
+        Ok(())
+    }
+
+    /// Set the AST of the last instruction. Separate method because it's rarely used.
+    pub(crate) fn set_last_ast(&mut self, ast_ref: Option<IrAstRef>) {
+        *self.ast.last_mut().expect("no last instruction") = ast_ref;
+    }
+
+    /// Add a comment to the last instruction.
+    pub(crate) fn add_comment(&mut self, comment: impl std::fmt::Display) {
+        add_comment(
+            self.comments.last_mut().expect("no last instruction"),
+            comment,
+        )
+    }
+
+    /// Load a register with a literal.
+    pub(crate) fn load_literal(
+        &mut self,
+        reg_id: RegId,
+        literal: Spanned<Literal>,
+    ) -> Result<(), CompileError> {
+        self.push(
+            Instruction::LoadLiteral {
+                dst: reg_id,
+                lit: literal.item,
+            }
+            .into_spanned(literal.span),
+        )?;
+        Ok(())
+    }
+
+    /// Allocate a new register and load a literal into it.
+    pub(crate) fn literal(&mut self, literal: Spanned<Literal>) -> Result<RegId, CompileError> {
+        let reg_id = self.next_register()?;
+        self.load_literal(reg_id, literal)?;
+        Ok(reg_id)
+    }
+
+    /// Deallocate a register and set it to `Empty`, if it is allocated
+    pub(crate) fn drop_reg(&mut self, reg_id: RegId) -> Result<(), CompileError> {
+        if self.is_allocated(reg_id) {
+            self.push(Instruction::Drop { src: reg_id }.into_spanned(Span::unknown()))?;
+        }
+        Ok(())
+    }
+
+    /// Set a register to `Empty`, but mark it as in-use, e.g. for input
+    pub(crate) fn load_empty(&mut self, reg_id: RegId) -> Result<(), CompileError> {
+        self.drop_reg(reg_id)?;
+        self.mark_register(reg_id)
+    }
+
+    /// Drain the stream in a register (fully consuming it)
+    pub(crate) fn drain(&mut self, src: RegId, span: Span) -> Result<(), CompileError> {
+        self.push(Instruction::Drain { src }.into_spanned(span))
+    }
+
+    /// Add data to the `data` array and return a [`DataSlice`] referencing it.
+    pub(crate) fn data(&mut self, data: impl AsRef<[u8]>) -> Result<DataSlice, CompileError> {
+        let data = data.as_ref();
+        let start = self.data.len();
+        if data.is_empty() {
+            Ok(DataSlice::empty())
+        } else if start + data.len() < u32::MAX as usize {
+            let slice = DataSlice {
+                start: start as u32,
+                len: data.len() as u32,
+            };
+            self.data.extend_from_slice(data);
+            Ok(slice)
+        } else {
+            Err(CompileError::DataOverflow {
+                block_span: self.block_span,
+            })
+        }
+    }
+
+    /// Clone a register with a `clone` instruction.
+    pub(crate) fn clone_reg(&mut self, src: RegId, span: Span) -> Result<RegId, CompileError> {
+        let dst = self.next_register()?;
+        self.push(Instruction::Clone { dst, src }.into_spanned(span))?;
+        Ok(dst)
+    }
+
+    /// Add a `branch-if` instruction
+    pub(crate) fn branch_if(
+        &mut self,
+        cond: RegId,
+        label_id: LabelId,
+        span: Span,
+    ) -> Result<(), CompileError> {
+        self.push(
+            Instruction::BranchIf {
+                cond,
+                index: label_id.0,
+            }
+            .into_spanned(span),
+        )
+    }
+
+    /// Add a `branch-if-empty` instruction
+    pub(crate) fn branch_if_empty(
+        &mut self,
+        src: RegId,
+        label_id: LabelId,
+        span: Span,
+    ) -> Result<(), CompileError> {
+        self.push(
+            Instruction::BranchIfEmpty {
+                src,
+                index: label_id.0,
+            }
+            .into_spanned(span),
+        )
+    }
+
+    /// Add a `jump` instruction
+    pub(crate) fn jump(&mut self, label_id: LabelId, span: Span) -> Result<(), CompileError> {
+        self.push(Instruction::Jump { index: label_id.0 }.into_spanned(span))
+    }
+
+    /// The index that the next instruction [`.push()`]ed will have.
+    pub(crate) fn here(&self) -> usize {
+        self.instructions.len()
+    }
+
+    /// Allocate a new file number, for redirection.
+    pub(crate) fn next_file_num(&mut self) -> Result<u32, CompileError> {
+        let next = self.file_count;
+        self.file_count = self
+            .file_count
+            .checked_add(1)
+            .ok_or(CompileError::FileOverflow {
+                block_span: self.block_span,
+            })?;
+        Ok(next)
+    }
+
+    /// Push a new loop state onto the builder. Creates new labels that must be set.
+    pub(crate) fn begin_loop(&mut self) -> Loop {
+        let loop_ = Loop {
+            break_label: self.label(None),
+            continue_label: self.label(None),
+        };
+        self.loop_stack.push(loop_);
+        loop_
+    }
+
+    /// True if we are currently in a loop.
+    pub(crate) fn is_in_loop(&self) -> bool {
+        !self.loop_stack.is_empty()
+    }
+
+    /// Add a loop breaking jump instruction.
+    pub(crate) fn push_break(&mut self, span: Span) -> Result<(), CompileError> {
+        let loop_ = self
+            .loop_stack
+            .last()
+            .ok_or_else(|| CompileError::NotInALoop {
+                msg: "`break` called from outside of a loop".into(),
+                span: Some(span),
+            })?;
+        self.jump(loop_.break_label, span)
+    }
+
+    /// Add a loop continuing jump instruction.
+    pub(crate) fn push_continue(&mut self, span: Span) -> Result<(), CompileError> {
+        let loop_ = self
+            .loop_stack
+            .last()
+            .ok_or_else(|| CompileError::NotInALoop {
+                msg: "`continue` called from outside of a loop".into(),
+                span: Some(span),
+            })?;
+        self.jump(loop_.continue_label, span)
+    }
+
+    /// Pop the loop state. Checks that the loop being ended is the same one that was expected.
+    pub(crate) fn end_loop(&mut self, loop_: Loop) -> Result<(), CompileError> {
+        let ended_loop = self
+            .loop_stack
+            .pop()
+            .ok_or_else(|| CompileError::NotInALoop {
+                msg: "end_loop() called outside of a loop".into(),
+                span: None,
+            })?;
+
+        if ended_loop == loop_ {
+            Ok(())
+        } else {
+            Err(CompileError::IncoherentLoopState {
+                block_span: self.block_span,
+            })
+        }
+    }
+
+    /// Mark an unreachable code path. Produces an error at runtime if executed.
+    pub(crate) fn unreachable(&mut self, span: Span) -> Result<(), CompileError> {
+        self.push(Instruction::Unreachable.into_spanned(span))
+    }
+
+    /// Consume the builder and produce the final [`IrBlock`].
+    pub(crate) fn finish(mut self) -> Result<IrBlock, CompileError> {
+        // Add comments to label targets
+        for (index, label_target) in self.labels.iter().enumerate() {
+            if let Some(label_target) = label_target {
+                add_comment(
+                    &mut self.comments[*label_target],
+                    format_args!("label({index})"),
+                );
+            }
+        }
+
+        // Populate the actual target indices of labels into the instructions
+        for ((index, instruction), span) in
+            self.instructions.iter_mut().enumerate().zip(&self.spans)
+        {
+            if let Some(label_id) = instruction.branch_target() {
+                let target_index = self.labels.get(label_id).cloned().flatten().ok_or(
+                    CompileError::UndefinedLabel {
+                        label_id,
+                        span: Some(*span),
+                    },
+                )?;
+                // Add a comment to the target index that we come from here
+                add_comment(
+                    &mut self.comments[target_index],
+                    format_args!("from({index}:)"),
+                );
+                instruction.set_branch_target(target_index).map_err(|_| {
+                    CompileError::SetBranchTargetOfNonBranchInstruction {
+                        instruction: format!("{:?}", instruction),
+                        span: *span,
+                    }
+                })?;
+            }
+        }
+
+        Ok(IrBlock {
+            instructions: self.instructions,
+            spans: self.spans,
+            data: self.data.into(),
+            ast: self.ast,
+            comments: self.comments.into_iter().map(|s| s.into()).collect(),
+            register_count: self
+                .register_allocation_state
+                .len()
+                .try_into()
+                .expect("register count overflowed in finish() despite previous checks"),
+            file_count: self.file_count,
+        })
+    }
+}
+
+/// Keeps track of the `break` and `continue` target labels for a loop.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct Loop {
+    pub(crate) break_label: LabelId,
+    pub(crate) continue_label: LabelId,
+}
+
+/// Add a new comment to an existing one
+fn add_comment(comment: &mut String, new_comment: impl std::fmt::Display) {
+    use std::fmt::Write;
+    write!(
+        comment,
+        "{}{}",
+        if comment.is_empty() { "" } else { ", " },
+        new_comment
+    )
+    .expect("formatting failed");
+}

crates/nu-engine/src/compile/call.rs

@@ -0,0 +1,270 @@
+use std::sync::Arc;
+
+use nu_protocol::{
+    ast::{Argument, Call, Expression, ExternalArgument},
+    engine::StateWorkingSet,
+    ir::{Instruction, IrAstRef, Literal},
+    IntoSpanned, RegId, Span, Spanned,
+};
+
+use super::{compile_expression, keyword::*, BlockBuilder, CompileError, RedirectModes};
+
+pub(crate) fn compile_call(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    call: &Call,
+    redirect_modes: RedirectModes,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    let decl = working_set.get_decl(call.decl_id);
+
+    // Check if this call has --help - if so, just redirect to `help`
+    if call.named_iter().any(|(name, _, _)| name.item == "help") {
+        return compile_help(
+            working_set,
+            builder,
+            decl.name().into_spanned(call.head),
+            io_reg,
+        );
+    }
+
+    // Try to figure out if this is a keyword call like `if`, and handle those specially
+    if decl.is_keyword() {
+        match decl.name() {
+            "if" => {
+                return compile_if(working_set, builder, call, redirect_modes, io_reg);
+            }
+            "match" => {
+                return compile_match(working_set, builder, call, redirect_modes, io_reg);
+            }
+            "const" => {
+                // This differs from the behavior of the const command, which adds the const value
+                // to the stack. Since `load-variable` also checks `engine_state` for the variable
+                // and will get a const value though, is it really necessary to do that?
+                return builder.load_empty(io_reg);
+            }
+            "alias" => {
+                // Alias does nothing
+                return builder.load_empty(io_reg);
+            }
+            "let" | "mut" => {
+                return compile_let(working_set, builder, call, redirect_modes, io_reg);
+            }
+            "try" => {
+                return compile_try(working_set, builder, call, redirect_modes, io_reg);
+            }
+            "loop" => {
+                return compile_loop(working_set, builder, call, redirect_modes, io_reg);
+            }
+            "while" => {
+                return compile_while(working_set, builder, call, redirect_modes, io_reg);
+            }
+            "for" => {
+                return compile_for(working_set, builder, call, redirect_modes, io_reg);
+            }
+            "break" => {
+                return compile_break(working_set, builder, call, redirect_modes, io_reg);
+            }
+            "continue" => {
+                return compile_continue(working_set, builder, call, redirect_modes, io_reg);
+            }
+            "return" => {
+                return compile_return(working_set, builder, call, redirect_modes, io_reg);
+            }
+            _ => (),
+        }
+    }
+
+    // Keep AST if the decl needs it.
+    let requires_ast = decl.requires_ast_for_arguments();
+
+    // It's important that we evaluate the args first before trying to set up the argument
+    // state for the call.
+    //
+    // We could technically compile anything that isn't another call safely without worrying about
+    // the argument state, but we'd have to check all of that first and it just isn't really worth
+    // it.
+    enum CompiledArg<'a> {
+        Positional(RegId, Span, Option<IrAstRef>),
+        Named(
+            &'a str,
+            Option<&'a str>,
+            Option<RegId>,
+            Span,
+            Option<IrAstRef>,
+        ),
+        Spread(RegId, Span, Option<IrAstRef>),
+    }
+
+    let mut compiled_args = vec![];
+
+    for arg in &call.arguments {
+        let arg_reg = arg
+            .expr()
+            .map(|expr| {
+                let arg_reg = builder.next_register()?;
+
+                compile_expression(
+                    working_set,
+                    builder,
+                    expr,
+                    RedirectModes::capture_out(arg.span()),
+                    None,
+                    arg_reg,
+                )?;
+
+                Ok(arg_reg)
+            })
+            .transpose()?;
+
+        let ast_ref = arg
+            .expr()
+            .filter(|_| requires_ast)
+            .map(|expr| IrAstRef(Arc::new(expr.clone())));
+
+        match arg {
+            Argument::Positional(_) | Argument::Unknown(_) => {
+                compiled_args.push(CompiledArg::Positional(
+                    arg_reg.expect("expr() None in non-Named"),
+                    arg.span(),
+                    ast_ref,
+                ))
+            }
+            Argument::Named((name, short, _)) => compiled_args.push(CompiledArg::Named(
+                &name.item,
+                short.as_ref().map(|spanned| spanned.item.as_str()),
+                arg_reg,
+                arg.span(),
+                ast_ref,
+            )),
+            Argument::Spread(_) => compiled_args.push(CompiledArg::Spread(
+                arg_reg.expect("expr() None in non-Named"),
+                arg.span(),
+                ast_ref,
+            )),
+        }
+    }
+
+    // Now that the args are all compiled, set up the call state (argument stack and redirections)
+    for arg in compiled_args {
+        match arg {
+            CompiledArg::Positional(reg, span, ast_ref) => {
+                builder.push(Instruction::PushPositional { src: reg }.into_spanned(span))?;
+                builder.set_last_ast(ast_ref);
+            }
+            CompiledArg::Named(name, short, Some(reg), span, ast_ref) => {
+                if !name.is_empty() {
+                    let name = builder.data(name)?;
+                    builder.push(Instruction::PushNamed { name, src: reg }.into_spanned(span))?;
+                } else {
+                    let short = builder.data(short.unwrap_or(""))?;
+                    builder
+                        .push(Instruction::PushShortNamed { short, src: reg }.into_spanned(span))?;
+                }
+                builder.set_last_ast(ast_ref);
+            }
+            CompiledArg::Named(name, short, None, span, ast_ref) => {
+                if !name.is_empty() {
+                    let name = builder.data(name)?;
+                    builder.push(Instruction::PushFlag { name }.into_spanned(span))?;
+                } else {
+                    let short = builder.data(short.unwrap_or(""))?;
+                    builder.push(Instruction::PushShortFlag { short }.into_spanned(span))?;
+                }
+                builder.set_last_ast(ast_ref);
+            }
+            CompiledArg::Spread(reg, span, ast_ref) => {
+                builder.push(Instruction::AppendRest { src: reg }.into_spanned(span))?;
+                builder.set_last_ast(ast_ref);
+            }
+        }
+    }
+
+    // Add any parser info from the call
+    for (name, info) in &call.parser_info {
+        let name = builder.data(name)?;
+        let info = Box::new(info.clone());
+        builder.push(Instruction::PushParserInfo { name, info }.into_spanned(call.head))?;
+    }
+
+    if let Some(mode) = redirect_modes.out {
+        builder.push(mode.map(|mode| Instruction::RedirectOut { mode }))?;
+    }
+
+    if let Some(mode) = redirect_modes.err {
+        builder.push(mode.map(|mode| Instruction::RedirectErr { mode }))?;
+    }
+
+    // The state is set up, so we can do the call into io_reg
+    builder.push(
+        Instruction::Call {
+            decl_id: call.decl_id,
+            src_dst: io_reg,
+        }
+        .into_spanned(call.head),
+    )?;
+
+    Ok(())
+}
+
+pub(crate) fn compile_help(
+    working_set: &StateWorkingSet<'_>,
+    builder: &mut BlockBuilder,
+    decl_name: Spanned<&str>,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    let help_command_id =
+        working_set
+            .find_decl(b"help")
+            .ok_or_else(|| CompileError::MissingRequiredDeclaration {
+                decl_name: "help".into(),
+                span: decl_name.span,
+            })?;
+
+    let name_data = builder.data(decl_name.item)?;
+    let name_literal = builder.literal(decl_name.map(|_| Literal::String(name_data)))?;
+
+    builder.push(Instruction::PushPositional { src: name_literal }.into_spanned(decl_name.span))?;
+
+    builder.push(
+        Instruction::Call {
+            decl_id: help_command_id,
+            src_dst: io_reg,
+        }
+        .into_spanned(decl_name.span),
+    )?;
+
+    Ok(())
+}
+
+pub(crate) fn compile_external_call(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    head: &Expression,
+    args: &[ExternalArgument],
+    redirect_modes: RedirectModes,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    // Pass everything to run-external
+    let run_external_id = working_set
+        .find_decl(b"run-external")
+        .ok_or(CompileError::RunExternalNotFound { span: head.span })?;
+
+    let mut call = Call::new(head.span);
+    call.decl_id = run_external_id;
+
+    call.arguments.push(Argument::Positional(head.clone()));
+
+    for arg in args {
+        match arg {
+            ExternalArgument::Regular(expr) => {
+                call.arguments.push(Argument::Positional(expr.clone()));
+            }
+            ExternalArgument::Spread(expr) => {
+                call.arguments.push(Argument::Spread(expr.clone()));
+            }
+        }
+    }
+
+    compile_call(working_set, builder, &call, redirect_modes, io_reg)
+}

crates/nu-engine/src/compile/expression.rs

@@ -0,0 +1,535 @@
+use super::{
+    compile_binary_op, compile_block, compile_call, compile_external_call, compile_load_env,
+    BlockBuilder, CompileError, RedirectModes,
+};
+
+use nu_protocol::{
+    ast::{CellPath, Expr, Expression, ListItem, RecordItem, ValueWithUnit},
+    engine::StateWorkingSet,
+    ir::{DataSlice, Instruction, Literal},
+    IntoSpanned, RegId, Span, Value, ENV_VARIABLE_ID,
+};
+
+pub(crate) fn compile_expression(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    expr: &Expression,
+    redirect_modes: RedirectModes,
+    in_reg: Option<RegId>,
+    out_reg: RegId,
+) -> Result<(), CompileError> {
+    let drop_input = |builder: &mut BlockBuilder| {
+        if let Some(in_reg) = in_reg {
+            if in_reg != out_reg {
+                builder.drop_reg(in_reg)?;
+            }
+        }
+        Ok(())
+    };
+
+    let lit = |builder: &mut BlockBuilder, literal: Literal| {
+        drop_input(builder)?;
+
+        builder
+            .push(
+                Instruction::LoadLiteral {
+                    dst: out_reg,
+                    lit: literal,
+                }
+                .into_spanned(expr.span),
+            )
+            .map(|_| ())
+    };
+
+    let ignore = |builder: &mut BlockBuilder| {
+        drop_input(builder)?;
+        builder.load_empty(out_reg)
+    };
+
+    let unexpected = |expr_name: &str| CompileError::UnexpectedExpression {
+        expr_name: expr_name.into(),
+        span: expr.span,
+    };
+
+    let move_in_reg_to_out_reg = |builder: &mut BlockBuilder| {
+        // Ensure that out_reg contains the input value, because a call only uses one register
+        if let Some(in_reg) = in_reg {
+            if in_reg != out_reg {
+                // Have to move in_reg to out_reg so it can be used
+                builder.push(
+                    Instruction::Move {
+                        dst: out_reg,
+                        src: in_reg,
+                    }
+                    .into_spanned(expr.span),
+                )?;
+            }
+        } else {
+            // Will have to initialize out_reg with Empty first
+            builder.load_empty(out_reg)?;
+        }
+        Ok(())
+    };
+
+    match &expr.expr {
+        Expr::Bool(b) => lit(builder, Literal::Bool(*b)),
+        Expr::Int(i) => lit(builder, Literal::Int(*i)),
+        Expr::Float(f) => lit(builder, Literal::Float(*f)),
+        Expr::Binary(bin) => {
+            let data_slice = builder.data(bin)?;
+            lit(builder, Literal::Binary(data_slice))
+        }
+        Expr::Range(range) => {
+            // Compile the subexpressions of the range
+            let compile_part = |builder: &mut BlockBuilder,
+                                part_expr: Option<&Expression>|
+             -> Result<RegId, CompileError> {
+                let reg = builder.next_register()?;
+                if let Some(part_expr) = part_expr {
+                    compile_expression(
+                        working_set,
+                        builder,
+                        part_expr,
+                        RedirectModes::capture_out(part_expr.span),
+                        None,
+                        reg,
+                    )?;
+                } else {
+                    builder.load_literal(reg, Literal::Nothing.into_spanned(expr.span))?;
+                }
+                Ok(reg)
+            };
+
+            drop_input(builder)?;
+
+            let start = compile_part(builder, range.from.as_ref())?;
+            let step = compile_part(builder, range.next.as_ref())?;
+            let end = compile_part(builder, range.to.as_ref())?;
+
+            // Assemble the range
+            builder.load_literal(
+                out_reg,
+                Literal::Range {
+                    start,
+                    step,
+                    end,
+                    inclusion: range.operator.inclusion,
+                }
+                .into_spanned(expr.span),
+            )
+        }
+        Expr::Var(var_id) => {
+            drop_input(builder)?;
+            builder.push(
+                Instruction::LoadVariable {
+                    dst: out_reg,
+                    var_id: *var_id,
+                }
+                .into_spanned(expr.span),
+            )?;
+            Ok(())
+        }
+        Expr::VarDecl(_) => Err(unexpected("VarDecl")),
+        Expr::Call(call) => {
+            move_in_reg_to_out_reg(builder)?;
+
+            compile_call(working_set, builder, call, redirect_modes, out_reg)
+        }
+        Expr::ExternalCall(head, args) => {
+            move_in_reg_to_out_reg(builder)?;
+
+            compile_external_call(working_set, builder, head, args, redirect_modes, out_reg)
+        }
+        Expr::Operator(_) => Err(unexpected("Operator")),
+        Expr::RowCondition(block_id) => lit(builder, Literal::RowCondition(*block_id)),
+        Expr::UnaryNot(subexpr) => {
+            drop_input(builder)?;
+            compile_expression(
+                working_set,
+                builder,
+                subexpr,
+                RedirectModes::capture_out(subexpr.span),
+                None,
+                out_reg,
+            )?;
+            builder.push(Instruction::Not { src_dst: out_reg }.into_spanned(expr.span))?;
+            Ok(())
+        }
+        Expr::BinaryOp(lhs, op, rhs) => {
+            if let Expr::Operator(ref operator) = op.expr {
+                drop_input(builder)?;
+                compile_binary_op(
+                    working_set,
+                    builder,
+                    lhs,
+                    operator.clone().into_spanned(op.span),
+                    rhs,
+                    expr.span,
+                    out_reg,
+                )
+            } else {
+                Err(CompileError::UnsupportedOperatorExpression { span: op.span })
+            }
+        }
+        Expr::Subexpression(block_id) => {
+            let block = working_set.get_block(*block_id);
+            compile_block(working_set, builder, block, redirect_modes, in_reg, out_reg)
+        }
+        Expr::Block(block_id) => lit(builder, Literal::Block(*block_id)),
+        Expr::Closure(block_id) => lit(builder, Literal::Closure(*block_id)),
+        Expr::MatchBlock(_) => Err(unexpected("MatchBlock")), // only for `match` keyword
+        Expr::List(items) => {
+            // Guess capacity based on items (does not consider spread as more than 1)
+            lit(
+                builder,
+                Literal::List {
+                    capacity: items.len(),
+                },
+            )?;
+            for item in items {
+                // Compile the expression of the item / spread
+                let reg = builder.next_register()?;
+                let expr = match item {
+                    ListItem::Item(expr) | ListItem::Spread(_, expr) => expr,
+                };
+                compile_expression(
+                    working_set,
+                    builder,
+                    expr,
+                    RedirectModes::capture_out(expr.span),
+                    None,
+                    reg,
+                )?;
+
+                match item {
+                    ListItem::Item(_) => {
+                        // Add each item using list-push
+                        builder.push(
+                            Instruction::ListPush {
+                                src_dst: out_reg,
+                                item: reg,
+                            }
+                            .into_spanned(expr.span),
+                        )?;
+                    }
+                    ListItem::Spread(spread_span, _) => {
+                        // Spread the list using list-spread
+                        builder.push(
+                            Instruction::ListSpread {
+                                src_dst: out_reg,
+                                items: reg,
+                            }
+                            .into_spanned(*spread_span),
+                        )?;
+                    }
+                }
+            }
+            Ok(())
+        }
+        Expr::Table(table) => {
+            lit(
+                builder,
+                Literal::List {
+                    capacity: table.rows.len(),
+                },
+            )?;
+
+            // Evaluate the columns
+            let column_registers = table
+                .columns
+                .iter()
+                .map(|column| {
+                    let reg = builder.next_register()?;
+                    compile_expression(
+                        working_set,
+                        builder,
+                        column,
+                        RedirectModes::capture_out(column.span),
+                        None,
+                        reg,
+                    )?;
+                    Ok(reg)
+                })
+                .collect::<Result<Vec<RegId>, CompileError>>()?;
+
+            // Build records for each row
+            for row in table.rows.iter() {
+                let row_reg = builder.next_register()?;
+                builder.load_literal(
+                    row_reg,
+                    Literal::Record {
+                        capacity: table.columns.len(),
+                    }
+                    .into_spanned(expr.span),
+                )?;
+                for (column_reg, item) in column_registers.iter().zip(row.iter()) {
+                    let column_reg = builder.clone_reg(*column_reg, item.span)?;
+                    let item_reg = builder.next_register()?;
+                    compile_expression(
+                        working_set,
+                        builder,
+                        item,
+                        RedirectModes::capture_out(item.span),
+                        None,
+                        item_reg,
+                    )?;
+                    builder.push(
+                        Instruction::RecordInsert {
+                            src_dst: row_reg,
+                            key: column_reg,
+                            val: item_reg,
+                        }
+                        .into_spanned(item.span),
+                    )?;
+                }
+                builder.push(
+                    Instruction::ListPush {
+                        src_dst: out_reg,
+                        item: row_reg,
+                    }
+                    .into_spanned(expr.span),
+                )?;
+            }
+
+            // Free the column registers, since they aren't needed anymore
+            for reg in column_registers {
+                builder.drop_reg(reg)?;
+            }
+
+            Ok(())
+        }
+        Expr::Record(items) => {
+            lit(
+                builder,
+                Literal::Record {
+                    capacity: items.len(),
+                },
+            )?;
+
+            for item in items {
+                match item {
+                    RecordItem::Pair(key, val) => {
+                        // Add each item using record-insert
+                        let key_reg = builder.next_register()?;
+                        let val_reg = builder.next_register()?;
+                        compile_expression(
+                            working_set,
+                            builder,
+                            key,
+                            RedirectModes::capture_out(key.span),
+                            None,
+                            key_reg,
+                        )?;
+                        compile_expression(
+                            working_set,
+                            builder,
+                            val,
+                            RedirectModes::capture_out(val.span),
+                            None,
+                            val_reg,
+                        )?;
+                        builder.push(
+                            Instruction::RecordInsert {
+                                src_dst: out_reg,
+                                key: key_reg,
+                                val: val_reg,
+                            }
+                            .into_spanned(expr.span),
+                        )?;
+                    }
+                    RecordItem::Spread(spread_span, expr) => {
+                        // Spread the expression using record-spread
+                        let reg = builder.next_register()?;
+                        compile_expression(
+                            working_set,
+                            builder,
+                            expr,
+                            RedirectModes::capture_out(expr.span),
+                            None,
+                            reg,
+                        )?;
+                        builder.push(
+                            Instruction::RecordSpread {
+                                src_dst: out_reg,
+                                items: reg,
+                            }
+                            .into_spanned(*spread_span),
+                        )?;
+                    }
+                }
+            }
+            Ok(())
+        }
+        Expr::Keyword(kw) => {
+            // keyword: just pass through expr, since commands that use it and are not being
+            // specially handled already are often just positional anyway
+            compile_expression(
+                working_set,
+                builder,
+                &kw.expr,
+                redirect_modes,
+                in_reg,
+                out_reg,
+            )
+        }
+        Expr::ValueWithUnit(value_with_unit) => {
+            lit(builder, literal_from_value_with_unit(value_with_unit)?)
+        }
+        Expr::DateTime(dt) => lit(builder, Literal::Date(Box::new(*dt))),
+        Expr::Filepath(path, no_expand) => {
+            let val = builder.data(path)?;
+            lit(
+                builder,
+                Literal::Filepath {
+                    val,
+                    no_expand: *no_expand,
+                },
+            )
+        }
+        Expr::Directory(path, no_expand) => {
+            let val = builder.data(path)?;
+            lit(
+                builder,
+                Literal::Directory {
+                    val,
+                    no_expand: *no_expand,
+                },
+            )
+        }
+        Expr::GlobPattern(path, no_expand) => {
+            let val = builder.data(path)?;
+            lit(
+                builder,
+                Literal::GlobPattern {
+                    val,
+                    no_expand: *no_expand,
+                },
+            )
+        }
+        Expr::String(s) => {
+            let data_slice = builder.data(s)?;
+            lit(builder, Literal::String(data_slice))
+        }
+        Expr::RawString(rs) => {
+            let data_slice = builder.data(rs)?;
+            lit(builder, Literal::RawString(data_slice))
+        }
+        Expr::CellPath(path) => lit(builder, Literal::CellPath(Box::new(path.clone()))),
+        Expr::FullCellPath(full_cell_path) => {
+            if matches!(full_cell_path.head.expr, Expr::Var(ENV_VARIABLE_ID)) {
+                compile_load_env(builder, expr.span, &full_cell_path.tail, out_reg)
+            } else {
+                compile_expression(
+                    working_set,
+                    builder,
+                    &full_cell_path.head,
+                    RedirectModes::capture_out(expr.span),
+                    in_reg,
+                    out_reg,
+                )?;
+                // Only do the follow if this is actually needed
+                if !full_cell_path.tail.is_empty() {
+                    let cell_path_reg = builder.literal(
+                        Literal::CellPath(Box::new(CellPath {
+                            members: full_cell_path.tail.clone(),
+                        }))
+                        .into_spanned(expr.span),
+                    )?;
+                    builder.push(
+                        Instruction::FollowCellPath {
+                            src_dst: out_reg,
+                            path: cell_path_reg,
+                        }
+                        .into_spanned(expr.span),
+                    )?;
+                }
+                Ok(())
+            }
+        }
+        Expr::ImportPattern(_) => Err(unexpected("ImportPattern")),
+        Expr::Overlay(_) => Err(unexpected("Overlay")),
+        Expr::Signature(_) => ignore(builder), // no effect
+        Expr::StringInterpolation(exprs) | Expr::GlobInterpolation(exprs, _) => {
+            let mut exprs_iter = exprs.iter().peekable();
+
+            if exprs_iter
+                .peek()
+                .is_some_and(|e| matches!(e.expr, Expr::String(..) | Expr::RawString(..)))
+            {
+                // If the first expression is a string or raw string literal, just take it and build
+                // from that
+                compile_expression(
+                    working_set,
+                    builder,
+                    exprs_iter.next().expect("peek() was Some"),
+                    RedirectModes::capture_out(expr.span),
+                    None,
+                    out_reg,
+                )?;
+            } else {
+                // Start with an empty string
+                lit(builder, Literal::String(DataSlice::empty()))?;
+            }
+
+            // Compile each expression and append to out_reg
+            for expr in exprs_iter {
+                let scratch_reg = builder.next_register()?;
+                compile_expression(
+                    working_set,
+                    builder,
+                    expr,
+                    RedirectModes::capture_out(expr.span),
+                    None,
+                    scratch_reg,
+                )?;
+                builder.push(
+                    Instruction::StringAppend {
+                        src_dst: out_reg,
+                        val: scratch_reg,
+                    }
+                    .into_spanned(expr.span),
+                )?;
+            }
+
+            // If it's a glob interpolation, change it to a glob
+            if let Expr::GlobInterpolation(_, no_expand) = expr.expr {
+                builder.push(
+                    Instruction::GlobFrom {
+                        src_dst: out_reg,
+                        no_expand,
+                    }
+                    .into_spanned(expr.span),
+                )?;
+            }
+
+            Ok(())
+        }
+        Expr::Nothing => lit(builder, Literal::Nothing),
+        Expr::Garbage => Err(CompileError::Garbage { span: expr.span }),
+    }
+}
+
+fn literal_from_value_with_unit(value_with_unit: &ValueWithUnit) -> Result<Literal, CompileError> {
+    let Expr::Int(int_value) = value_with_unit.expr.expr else {
+        return Err(CompileError::UnexpectedExpression {
+            expr_name: format!("{:?}", value_with_unit.expr),
+            span: value_with_unit.expr.span,
+        });
+    };
+
+    match value_with_unit
+        .unit
+        .item
+        .build_value(int_value, Span::unknown())
+        .map_err(|err| CompileError::InvalidLiteral {
+            msg: err.to_string(),
+            span: value_with_unit.expr.span,
+        })? {
+        Value::Filesize { val, .. } => Ok(Literal::Filesize(val)),
+        Value::Duration { val, .. } => Ok(Literal::Duration(val)),
+        other => Err(CompileError::InvalidLiteral {
+            msg: format!("bad value returned by Unit::build_value(): {other:?}"),
+            span: value_with_unit.unit.span,
+        }),
+    }
+}

crates/nu-engine/src/compile/keyword.rs

@@ -0,0 +1,902 @@
+use nu_protocol::{
+    ast::{Block, Call, Expr, Expression},
+    engine::StateWorkingSet,
+    ir::Instruction,
+    IntoSpanned, RegId, Type, VarId,
+};
+
+use super::{compile_block, compile_expression, BlockBuilder, CompileError, RedirectModes};
+
+/// Compile a call to `if` as a branch-if
+pub(crate) fn compile_if(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    call: &Call,
+    redirect_modes: RedirectModes,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    // Pseudocode:
+    //
+    //        %io_reg <- <condition>
+    //        not %io_reg
+    //        branch-if %io_reg, FALSE
+    // TRUE:  ...<true_block>...
+    //        jump END
+    // FALSE: ...<else_expr>... OR drop %io_reg
+    // END:
+    let invalid = || CompileError::InvalidKeywordCall {
+        keyword: "if".into(),
+        span: call.head,
+    };
+
+    let condition = call.positional_nth(0).ok_or_else(invalid)?;
+    let true_block_arg = call.positional_nth(1).ok_or_else(invalid)?;
+    let else_arg = call.positional_nth(2);
+
+    let true_block_id = true_block_arg.as_block().ok_or_else(invalid)?;
+    let true_block = working_set.get_block(true_block_id);
+
+    let true_label = builder.label(None);
+    let false_label = builder.label(None);
+    let end_label = builder.label(None);
+
+    let not_condition_reg = {
+        // Compile the condition first
+        let condition_reg = builder.next_register()?;
+        compile_expression(
+            working_set,
+            builder,
+            condition,
+            RedirectModes::capture_out(condition.span),
+            None,
+            condition_reg,
+        )?;
+
+        // Negate the condition - we basically only want to jump if the condition is false
+        builder.push(
+            Instruction::Not {
+                src_dst: condition_reg,
+            }
+            .into_spanned(call.head),
+        )?;
+
+        condition_reg
+    };
+
+    // Set up a branch if the condition is false.
+    builder.branch_if(not_condition_reg, false_label, call.head)?;
+    builder.add_comment("if false");
+
+    // Compile the true case
+    builder.set_label(true_label, builder.here())?;
+    compile_block(
+        working_set,
+        builder,
+        true_block,
+        redirect_modes.clone(),
+        Some(io_reg),
+        io_reg,
+    )?;
+
+    // Add a jump over the false case
+    builder.jump(end_label, else_arg.map(|e| e.span).unwrap_or(call.head))?;
+    builder.add_comment("end if");
+
+    // On the else side now, assert that io_reg is still valid
+    builder.set_label(false_label, builder.here())?;
+    builder.mark_register(io_reg)?;
+
+    if let Some(else_arg) = else_arg {
+        let Expression {
+            expr: Expr::Keyword(else_keyword),
+            ..
+        } = else_arg
+        else {
+            return Err(invalid());
+        };
+
+        if else_keyword.keyword.as_ref() != b"else" {
+            return Err(invalid());
+        }
+
+        let else_expr = &else_keyword.expr;
+
+        match &else_expr.expr {
+            Expr::Block(block_id) => {
+                let false_block = working_set.get_block(*block_id);
+                compile_block(
+                    working_set,
+                    builder,
+                    false_block,
+                    redirect_modes,
+                    Some(io_reg),
+                    io_reg,
+                )?;
+            }
+            _ => {
+                // The else case supports bare expressions too, not only blocks
+                compile_expression(
+                    working_set,
+                    builder,
+                    else_expr,
+                    redirect_modes,
+                    Some(io_reg),
+                    io_reg,
+                )?;
+            }
+        }
+    } else {
+        // We don't have an else expression/block, so just set io_reg = Empty
+        builder.load_empty(io_reg)?;
+    }
+
+    // Set the end label
+    builder.set_label(end_label, builder.here())?;
+
+    Ok(())
+}
+
+/// Compile a call to `match`
+pub(crate) fn compile_match(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    call: &Call,
+    redirect_modes: RedirectModes,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    // Pseudocode:
+    //
+    //         %match_reg <- <match_expr>
+    //         collect %match_reg
+    //         match (pat1), %match_reg, PAT1
+    // MATCH2: match (pat2), %match_reg, PAT2
+    // FAIL:   drop %io_reg
+    //         drop %match_reg
+    //         jump END
+    // PAT1:   %guard_reg <- <guard_expr>
+    //         check-match-guard %guard_reg
+    //         not %guard_reg
+    //         branch-if %guard_reg, MATCH2
+    //         drop %match_reg
+    //         <...expr...>
+    //         jump END
+    // PAT2:   drop %match_reg
+    //         <...expr...>
+    //         jump END
+    // END:
+    let invalid = || CompileError::InvalidKeywordCall {
+        keyword: "match".into(),
+        span: call.head,
+    };
+
+    let match_expr = call.positional_nth(0).ok_or_else(invalid)?;
+
+    let match_block_arg = call.positional_nth(1).ok_or_else(invalid)?;
+    let match_block = match_block_arg.as_match_block().ok_or_else(invalid)?;
+
+    let match_reg = builder.next_register()?;
+
+    // Evaluate the match expression (patterns will be checked against this).
+    compile_expression(
+        working_set,
+        builder,
+        match_expr,
+        RedirectModes::capture_out(match_expr.span),
+        None,
+        match_reg,
+    )?;
+
+    // Important to collect it first
+    builder.push(Instruction::Collect { src_dst: match_reg }.into_spanned(match_expr.span))?;
+
+    // Generate the `match` instructions. Guards are not used at this stage.
+    let mut match_labels = Vec::with_capacity(match_block.len());
+    let mut next_labels = Vec::with_capacity(match_block.len());
+    let end_label = builder.label(None);
+
+    for (pattern, _) in match_block {
+        let match_label = builder.label(None);
+        match_labels.push(match_label);
+        builder.push(
+            Instruction::Match {
+                pattern: Box::new(pattern.pattern.clone()),
+                src: match_reg,
+                index: match_label.0,
+            }
+            .into_spanned(pattern.span),
+        )?;
+        // Also add a label for the next match instruction or failure case
+        next_labels.push(builder.label(Some(builder.here())));
+    }
+
+    // Match fall-through to jump to the end, if no match
+    builder.load_empty(io_reg)?;
+    builder.drop_reg(match_reg)?;
+    builder.jump(end_label, call.head)?;
+
+    // Generate each of the match expressions. Handle guards here, if present.
+    for (index, (pattern, expr)) in match_block.iter().enumerate() {
+        let match_label = match_labels[index];
+        let next_label = next_labels[index];
+
+        // `io_reg` and `match_reg` are still valid at each of these branch targets
+        builder.mark_register(io_reg)?;
+        builder.mark_register(match_reg)?;
+
+        // Set the original match instruction target here
+        builder.set_label(match_label, builder.here())?;
+
+        // Handle guard, if present
+        if let Some(guard) = &pattern.guard {
+            let guard_reg = builder.next_register()?;
+            compile_expression(
+                working_set,
+                builder,
+                guard,
+                RedirectModes::capture_out(guard.span),
+                None,
+                guard_reg,
+            )?;
+            builder
+                .push(Instruction::CheckMatchGuard { src: guard_reg }.into_spanned(guard.span))?;
+            builder.push(Instruction::Not { src_dst: guard_reg }.into_spanned(guard.span))?;
+            // Branch to the next match instruction if the branch fails to match
+            builder.branch_if(
+                guard_reg,
+                next_label,
+                // Span the branch with the next pattern, or the head if this is the end
+                match_block
+                    .get(index + 1)
+                    .map(|b| b.0.span)
+                    .unwrap_or(call.head),
+            )?;
+            builder.add_comment("if match guard false");
+        }
+
+        // match_reg no longer needed, successful match
+        builder.drop_reg(match_reg)?;
+
+        // Execute match right hand side expression
+        if let Some(block_id) = expr.as_block() {
+            let block = working_set.get_block(block_id);
+            compile_block(
+                working_set,
+                builder,
+                block,
+                redirect_modes.clone(),
+                Some(io_reg),
+                io_reg,
+            )?;
+        } else {
+            compile_expression(
+                working_set,
+                builder,
+                expr,
+                redirect_modes.clone(),
+                Some(io_reg),
+                io_reg,
+            )?;
+        }
+
+        // Jump to the end after the match logic is done
+        builder.jump(end_label, call.head)?;
+        builder.add_comment("end match");
+    }
+
+    // Set the end destination
+    builder.set_label(end_label, builder.here())?;
+
+    Ok(())
+}
+
+/// Compile a call to `let` or `mut` (just do store-variable)
+pub(crate) fn compile_let(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    call: &Call,
+    _redirect_modes: RedirectModes,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    // Pseudocode:
+    //
+    // %io_reg <- ...<block>... <- %io_reg
+    // store-variable $var, %io_reg
+    let invalid = || CompileError::InvalidKeywordCall {
+        keyword: "let".into(),
+        span: call.head,
+    };
+
+    let var_decl_arg = call.positional_nth(0).ok_or_else(invalid)?;
+    let block_arg = call.positional_nth(1).ok_or_else(invalid)?;
+
+    let var_id = var_decl_arg.as_var().ok_or_else(invalid)?;
+    let block_id = block_arg.as_block().ok_or_else(invalid)?;
+    let block = working_set.get_block(block_id);
+
+    let variable = working_set.get_variable(var_id);
+
+    compile_block(
+        working_set,
+        builder,
+        block,
+        RedirectModes::capture_out(call.head),
+        Some(io_reg),
+        io_reg,
+    )?;
+
+    // If the variable is a glob type variable, we should cast it with GlobFrom
+    if variable.ty == Type::Glob {
+        builder.push(
+            Instruction::GlobFrom {
+                src_dst: io_reg,
+                no_expand: true,
+            }
+            .into_spanned(call.head),
+        )?;
+    }
+
+    builder.push(
+        Instruction::StoreVariable {
+            var_id,
+            src: io_reg,
+        }
+        .into_spanned(call.head),
+    )?;
+    builder.add_comment("let");
+
+    // Don't forget to set io_reg to Empty afterward, as that's the result of an assignment
+    builder.load_empty(io_reg)?;
+
+    Ok(())
+}
+
+/// Compile a call to `try`, setting an error handler over the evaluated block
+pub(crate) fn compile_try(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    call: &Call,
+    redirect_modes: RedirectModes,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    // Pseudocode (literal block):
+    //
+    //       on-error-into ERR, %io_reg           // or without
+    //       %io_reg <- <...block...> <- %io_reg
+    //       check-external-failed %failed_reg, %io_reg
+    //       branch-if %failed_reg, FAIL
+    //       pop-error-handler
+    //       jump END
+    // FAIL: drain %io_reg
+    //       unreachable
+    // ERR:  clone %err_reg, %io_reg
+    //       store-variable $err_var, %err_reg         // or without
+    //       %io_reg <- <...catch block...> <- %io_reg // set to empty if no catch block
+    // END:
+    //
+    // with expression that can't be inlined:
+    //
+    //       %closure_reg <- <catch_expr>
+    //       on-error-into ERR, %io_reg
+    //       %io_reg <- <...block...> <- %io_reg
+    //       check-external-failed %failed_reg, %io_reg
+    //       branch-if %failed_reg, FAIL
+    //       pop-error-handler
+    //       jump END
+    // FAIL: drain %io_reg
+    //       unreachable
+    // ERR:  clone %err_reg, %io_reg
+    //       push-positional %closure_reg
+    //       push-positional %err_reg
+    //       call "do", %io_reg
+    // END:
+    let invalid = || CompileError::InvalidKeywordCall {
+        keyword: "try".into(),
+        span: call.head,
+    };
+
+    let block_arg = call.positional_nth(0).ok_or_else(invalid)?;
+    let block_id = block_arg.as_block().ok_or_else(invalid)?;
+    let block = working_set.get_block(block_id);
+
+    let catch_expr = match call.positional_nth(1) {
+        Some(kw_expr) => Some(kw_expr.as_keyword().ok_or_else(invalid)?),
+        None => None,
+    };
+    let catch_span = catch_expr.map(|e| e.span).unwrap_or(call.head);
+
+    let err_label = builder.label(None);
+    let failed_label = builder.label(None);
+    let end_label = builder.label(None);
+
+    // We have two ways of executing `catch`: if it was provided as a literal, we can inline it.
+    // Otherwise, we have to evaluate the expression and keep it as a register, and then call `do`.
+    enum CatchType<'a> {
+        Block {
+            block: &'a Block,
+            var_id: Option<VarId>,
+        },
+        Closure {
+            closure_reg: RegId,
+        },
+    }
+
+    let catch_type = catch_expr
+        .map(|catch_expr| match catch_expr.as_block() {
+            Some(block_id) => {
+                let block = working_set.get_block(block_id);
+                let var_id = block.signature.get_positional(0).and_then(|v| v.var_id);
+                Ok(CatchType::Block { block, var_id })
+            }
+            None => {
+                // We have to compile the catch_expr and use it as a closure
+                let closure_reg = builder.next_register()?;
+                compile_expression(
+                    working_set,
+                    builder,
+                    catch_expr,
+                    RedirectModes::capture_out(catch_expr.span),
+                    None,
+                    closure_reg,
+                )?;
+                Ok(CatchType::Closure { closure_reg })
+            }
+        })
+        .transpose()?;
+
+    // Put the error handler instruction. If we have a catch expression then we should capture the
+    // error.
+    if catch_type.is_some() {
+        builder.push(
+            Instruction::OnErrorInto {
+                index: err_label.0,
+                dst: io_reg,
+            }
+            .into_spanned(call.head),
+        )?
+    } else {
+        // Otherwise, we don't need the error value.
+        builder.push(Instruction::OnError { index: err_label.0 }.into_spanned(call.head))?
+    };
+
+    builder.add_comment("try");
+
+    // Compile the block
+    compile_block(
+        working_set,
+        builder,
+        block,
+        redirect_modes.clone(),
+        Some(io_reg),
+        io_reg,
+    )?;
+
+    // Check for external command exit code failure, and also redirect that to the catch handler
+    let failed_reg = builder.next_register()?;
+    builder.push(
+        Instruction::CheckExternalFailed {
+            dst: failed_reg,
+            src: io_reg,
+        }
+        .into_spanned(catch_span),
+    )?;
+    builder.branch_if(failed_reg, failed_label, catch_span)?;
+
+    // Successful case: pop the error handler
+    builder.push(Instruction::PopErrorHandler.into_spanned(call.head))?;
+
+    // Jump over the failure case
+    builder.jump(end_label, catch_span)?;
+
+    // Set up an error handler preamble for failed external.
+    // Draining the %io_reg results in the error handler being called with Empty, and sets
+    // $env.LAST_EXIT_CODE
+    builder.set_label(failed_label, builder.here())?;
+    builder.drain(io_reg, catch_span)?;
+    builder.add_comment("branches to err");
+    builder.unreachable(catch_span)?;
+
+    // This is the real error handler
+    builder.set_label(err_label, builder.here())?;
+
+    // Mark out register as likely not clean - state in error handler is not well defined
+    builder.mark_register(io_reg)?;
+
+    // Now compile whatever is necessary for the error handler
+    match catch_type {
+        Some(CatchType::Block { block, var_id }) => {
+            // Error will be in io_reg
+            builder.mark_register(io_reg)?;
+            if let Some(var_id) = var_id {
+                // Take a copy of the error as $err, since it will also be input
+                let err_reg = builder.next_register()?;
+                builder.push(
+                    Instruction::Clone {
+                        dst: err_reg,
+                        src: io_reg,
+                    }
+                    .into_spanned(catch_span),
+                )?;
+                builder.push(
+                    Instruction::StoreVariable {
+                        var_id,
+                        src: err_reg,
+                    }
+                    .into_spanned(catch_span),
+                )?;
+            }
+            // Compile the block, now that the variable is set
+            compile_block(
+                working_set,
+                builder,
+                block,
+                redirect_modes,
+                Some(io_reg),
+                io_reg,
+            )?;
+        }
+        Some(CatchType::Closure { closure_reg }) => {
+            // We should call `do`. Error will be in io_reg
+            let do_decl_id = working_set.find_decl(b"do").ok_or_else(|| {
+                CompileError::MissingRequiredDeclaration {
+                    decl_name: "do".into(),
+                    span: call.head,
+                }
+            })?;
+
+            // Take a copy of io_reg, because we pass it both as an argument and input
+            builder.mark_register(io_reg)?;
+            let err_reg = builder.next_register()?;
+            builder.push(
+                Instruction::Clone {
+                    dst: err_reg,
+                    src: io_reg,
+                }
+                .into_spanned(catch_span),
+            )?;
+
+            // Push the closure and the error
+            builder
+                .push(Instruction::PushPositional { src: closure_reg }.into_spanned(catch_span))?;
+            builder.push(Instruction::PushPositional { src: err_reg }.into_spanned(catch_span))?;
+
+            // Call `$err | do $closure $err`
+            builder.push(
+                Instruction::Call {
+                    decl_id: do_decl_id,
+                    src_dst: io_reg,
+                }
+                .into_spanned(catch_span),
+            )?;
+        }
+        None => {
+            // Just set out to empty.
+            builder.load_empty(io_reg)?;
+        }
+    }
+
+    // This is the end - if we succeeded, should jump here
+    builder.set_label(end_label, builder.here())?;
+
+    Ok(())
+}
+
+/// Compile a call to `loop` (via `jump`)
+pub(crate) fn compile_loop(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    call: &Call,
+    _redirect_modes: RedirectModes,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    // Pseudocode:
+    //
+    //       drop %io_reg
+    // LOOP: %io_reg <- ...<block>...
+    //       drain %io_reg
+    //       jump %LOOP
+    // END:  drop %io_reg
+    let invalid = || CompileError::InvalidKeywordCall {
+        keyword: "loop".into(),
+        span: call.head,
+    };
+
+    let block_arg = call.positional_nth(0).ok_or_else(invalid)?;
+    let block_id = block_arg.as_block().ok_or_else(invalid)?;
+    let block = working_set.get_block(block_id);
+
+    let loop_ = builder.begin_loop();
+    builder.load_empty(io_reg)?;
+
+    builder.set_label(loop_.continue_label, builder.here())?;
+
+    compile_block(
+        working_set,
+        builder,
+        block,
+        RedirectModes::default(),
+        None,
+        io_reg,
+    )?;
+
+    // Drain the output, just like for a semicolon
+    builder.drain(io_reg, call.head)?;
+
+    builder.jump(loop_.continue_label, call.head)?;
+    builder.add_comment("loop");
+
+    builder.set_label(loop_.break_label, builder.here())?;
+    builder.end_loop(loop_)?;
+
+    // State of %io_reg is not necessarily well defined here due to control flow, so make sure it's
+    // empty.
+    builder.mark_register(io_reg)?;
+    builder.load_empty(io_reg)?;
+
+    Ok(())
+}
+
+/// Compile a call to `while`, via branch instructions
+pub(crate) fn compile_while(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    call: &Call,
+    _redirect_modes: RedirectModes,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    // Pseudocode:
+    //
+    // LOOP:  %io_reg <- <condition>
+    //        branch-if %io_reg, TRUE
+    //        jump FALSE
+    // TRUE:  %io_reg <- ...<block>...
+    //        drain %io_reg
+    //        jump LOOP
+    // FALSE: drop %io_reg
+    let invalid = || CompileError::InvalidKeywordCall {
+        keyword: "while".into(),
+        span: call.head,
+    };
+
+    let cond_arg = call.positional_nth(0).ok_or_else(invalid)?;
+    let block_arg = call.positional_nth(1).ok_or_else(invalid)?;
+    let block_id = block_arg.as_block().ok_or_else(invalid)?;
+    let block = working_set.get_block(block_id);
+
+    let loop_ = builder.begin_loop();
+    builder.set_label(loop_.continue_label, builder.here())?;
+
+    let true_label = builder.label(None);
+
+    compile_expression(
+        working_set,
+        builder,
+        cond_arg,
+        RedirectModes::capture_out(call.head),
+        None,
+        io_reg,
+    )?;
+
+    builder.branch_if(io_reg, true_label, call.head)?;
+    builder.add_comment("while");
+    builder.jump(loop_.break_label, call.head)?;
+    builder.add_comment("end while");
+
+    builder.set_label(true_label, builder.here())?;
+
+    compile_block(
+        working_set,
+        builder,
+        block,
+        RedirectModes::default(),
+        None,
+        io_reg,
+    )?;
+
+    // Drain the result, just like for a semicolon
+    builder.drain(io_reg, call.head)?;
+
+    builder.jump(loop_.continue_label, call.head)?;
+    builder.add_comment("while");
+
+    builder.set_label(loop_.break_label, builder.here())?;
+    builder.end_loop(loop_)?;
+
+    // State of %io_reg is not necessarily well defined here due to control flow, so make sure it's
+    // empty.
+    builder.mark_register(io_reg)?;
+    builder.load_empty(io_reg)?;
+
+    Ok(())
+}
+
+/// Compile a call to `for` (via `iterate`)
+pub(crate) fn compile_for(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    call: &Call,
+    _redirect_modes: RedirectModes,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    // Pseudocode:
+    //
+    //       %stream_reg <- <in_expr>
+    // LOOP: iterate %io_reg, %stream_reg, END
+    //       store-variable $var, %io_reg
+    //       %io_reg <- <...block...>
+    //       drain %io_reg
+    //       jump LOOP
+    // END:  drop %io_reg
+    let invalid = || CompileError::InvalidKeywordCall {
+        keyword: "for".into(),
+        span: call.head,
+    };
+
+    if call.get_named_arg("numbered").is_some() {
+        // This is deprecated and we don't support it.
+        return Err(invalid());
+    }
+
+    let var_decl_arg = call.positional_nth(0).ok_or_else(invalid)?;
+    let var_id = var_decl_arg.as_var().ok_or_else(invalid)?;
+
+    let in_arg = call.positional_nth(1).ok_or_else(invalid)?;
+    let in_expr = in_arg.as_keyword().ok_or_else(invalid)?;
+
+    let block_arg = call.positional_nth(2).ok_or_else(invalid)?;
+    let block_id = block_arg.as_block().ok_or_else(invalid)?;
+    let block = working_set.get_block(block_id);
+
+    // Ensure io_reg is marked so we don't use it
+    builder.mark_register(io_reg)?;
+
+    let stream_reg = builder.next_register()?;
+
+    compile_expression(
+        working_set,
+        builder,
+        in_expr,
+        RedirectModes::capture_out(in_expr.span),
+        None,
+        stream_reg,
+    )?;
+
+    // Set up loop state
+    let loop_ = builder.begin_loop();
+    builder.set_label(loop_.continue_label, builder.here())?;
+
+    // This gets a value from the stream each time it's executed
+    // io_reg basically will act as our scratch register here
+    builder.push(
+        Instruction::Iterate {
+            dst: io_reg,
+            stream: stream_reg,
+            end_index: loop_.break_label.0,
+        }
+        .into_spanned(call.head),
+    )?;
+    builder.add_comment("for");
+
+    // Put the received value in the variable
+    builder.push(
+        Instruction::StoreVariable {
+            var_id,
+            src: io_reg,
+        }
+        .into_spanned(var_decl_arg.span),
+    )?;
+
+    // Do the body of the block
+    compile_block(
+        working_set,
+        builder,
+        block,
+        RedirectModes::default(),
+        None,
+        io_reg,
+    )?;
+
+    // Drain the output, just like for a semicolon
+    builder.drain(io_reg, call.head)?;
+
+    // Loop back to iterate to get the next value
+    builder.jump(loop_.continue_label, call.head)?;
+
+    // Set the end of the loop
+    builder.set_label(loop_.break_label, builder.here())?;
+    builder.end_loop(loop_)?;
+
+    // We don't need stream_reg anymore, after the loop
+    // io_reg may or may not be empty, so be sure it is
+    builder.free_register(stream_reg)?;
+    builder.mark_register(io_reg)?;
+    builder.load_empty(io_reg)?;
+
+    Ok(())
+}
+
+/// Compile a call to `break`.
+pub(crate) fn compile_break(
+    _working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    call: &Call,
+    _redirect_modes: RedirectModes,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    if builder.is_in_loop() {
+        builder.load_empty(io_reg)?;
+        builder.push_break(call.head)?;
+        builder.add_comment("break");
+    } else {
+        // Fall back to calling the command if we can't find the loop target statically
+        builder.push(
+            Instruction::Call {
+                decl_id: call.decl_id,
+                src_dst: io_reg,
+            }
+            .into_spanned(call.head),
+        )?;
+    }
+    Ok(())
+}
+
+/// Compile a call to `continue`.
+pub(crate) fn compile_continue(
+    _working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    call: &Call,
+    _redirect_modes: RedirectModes,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    if builder.is_in_loop() {
+        builder.load_empty(io_reg)?;
+        builder.push_continue(call.head)?;
+        builder.add_comment("continue");
+    } else {
+        // Fall back to calling the command if we can't find the loop target statically
+        builder.push(
+            Instruction::Call {
+                decl_id: call.decl_id,
+                src_dst: io_reg,
+            }
+            .into_spanned(call.head),
+        )?;
+    }
+    Ok(())
+}
+
+/// Compile a call to `return` as a `return-early` instruction.
+///
+/// This is not strictly necessary, but it is more efficient.
+pub(crate) fn compile_return(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    call: &Call,
+    _redirect_modes: RedirectModes,
+    io_reg: RegId,
+) -> Result<(), CompileError> {
+    // Pseudocode:
+    //
+    // %io_reg <- <arg_expr>
+    // return-early %io_reg
+    if let Some(arg_expr) = call.positional_nth(0) {
+        compile_expression(
+            working_set,
+            builder,
+            arg_expr,
+            RedirectModes::capture_out(arg_expr.span),
+            None,
+            io_reg,
+        )?;
+    } else {
+        builder.load_empty(io_reg)?;
+    }
+
+    // TODO: It would be nice if this could be `return` instead, but there is a little bit of
+    // behaviour remaining that still depends on `ShellError::Return`
+    builder.push(Instruction::ReturnEarly { src: io_reg }.into_spanned(call.head))?;
+
+    // io_reg is supposed to remain allocated
+    builder.load_empty(io_reg)?;
+
+    Ok(())
+}

crates/nu-engine/src/compile/mod.rs

@@ -0,0 +1,204 @@
+use nu_protocol::{
+    ast::{Block, Pipeline, PipelineRedirection, RedirectionSource, RedirectionTarget},
+    engine::StateWorkingSet,
+    ir::{Instruction, IrBlock, RedirectMode},
+    CompileError, IntoSpanned, RegId, Span,
+};
+
+mod builder;
+mod call;
+mod expression;
+mod keyword;
+mod operator;
+mod redirect;
+
+use builder::BlockBuilder;
+use call::*;
+use expression::compile_expression;
+use operator::*;
+use redirect::*;
+
+const BLOCK_INPUT: RegId = RegId(0);
+
+/// Compile Nushell pipeline abstract syntax tree (AST) to internal representation (IR) instructions
+/// for evaluation.
+pub fn compile(working_set: &StateWorkingSet, block: &Block) -> Result<IrBlock, CompileError> {
+    let mut builder = BlockBuilder::new(block.span);
+
+    let span = block.span.unwrap_or(Span::unknown());
+
+    compile_block(
+        working_set,
+        &mut builder,
+        block,
+        RedirectModes::caller(span),
+        Some(BLOCK_INPUT),
+        BLOCK_INPUT,
+    )?;
+
+    // A complete block has to end with a `return`
+    builder.push(Instruction::Return { src: BLOCK_INPUT }.into_spanned(span))?;
+
+    builder.finish()
+}
+
+/// Compiles a [`Block`] in-place into an IR block. This can be used in a nested manner, for example
+/// by [`compile_if()`], where the instructions for the blocks for the if/else are inlined into the
+/// top-level IR block.
+fn compile_block(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    block: &Block,
+    redirect_modes: RedirectModes,
+    in_reg: Option<RegId>,
+    out_reg: RegId,
+) -> Result<(), CompileError> {
+    let span = block.span.unwrap_or(Span::unknown());
+    let mut redirect_modes = Some(redirect_modes);
+    if !block.pipelines.is_empty() {
+        let last_index = block.pipelines.len() - 1;
+        for (index, pipeline) in block.pipelines.iter().enumerate() {
+            compile_pipeline(
+                working_set,
+                builder,
+                pipeline,
+                span,
+                // the redirect mode only applies to the last pipeline.
+                if index == last_index {
+                    redirect_modes
+                        .take()
+                        .expect("should only take redirect_modes once")
+                } else {
+                    RedirectModes::default()
+                },
+                // input is only passed to the first pipeline.
+                if index == 0 { in_reg } else { None },
+                out_reg,
+            )?;
+
+            if index != last_index {
+                // Explicitly drain the out reg after each non-final pipeline, because that's how
+                // the semicolon functions.
+                if builder.is_allocated(out_reg) {
+                    builder.push(Instruction::Drain { src: out_reg }.into_spanned(span))?;
+                }
+                builder.load_empty(out_reg)?;
+            }
+        }
+        Ok(())
+    } else if in_reg.is_none() {
+        builder.load_empty(out_reg)
+    } else {
+        Ok(())
+    }
+}
+
+fn compile_pipeline(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    pipeline: &Pipeline,
+    fallback_span: Span,
+    redirect_modes: RedirectModes,
+    in_reg: Option<RegId>,
+    out_reg: RegId,
+) -> Result<(), CompileError> {
+    let mut iter = pipeline.elements.iter().peekable();
+    let mut in_reg = in_reg;
+    let mut redirect_modes = Some(redirect_modes);
+    while let Some(element) = iter.next() {
+        let span = element.pipe.unwrap_or(fallback_span);
+
+        // We have to get the redirection mode from either the explicit redirection in the pipeline
+        // element, or from the next expression if it's specified there. If this is the last
+        // element, then it's from whatever is passed in as the mode to use.
+
+        let next_redirect_modes = if let Some(next_element) = iter.peek() {
+            let mut modes = redirect_modes_of_expression(working_set, &next_element.expr, span)?;
+
+            // If there's a next element with no inherent redirection we always pipe out *unless*
+            // this is a single redirection of stderr to pipe (e>|)
+            if modes.out.is_none()
+                && !matches!(
+                    element.redirection,
+                    Some(PipelineRedirection::Single {
+                        source: RedirectionSource::Stderr,
+                        target: RedirectionTarget::Pipe { .. }
+                    })
+                )
+            {
+                let pipe_span = next_element.pipe.unwrap_or(next_element.expr.span);
+                modes.out = Some(RedirectMode::Pipe.into_spanned(pipe_span));
+            }
+
+            modes
+        } else {
+            redirect_modes
+                .take()
+                .expect("should only take redirect_modes once")
+        };
+
+        let spec_redirect_modes = match &element.redirection {
+            Some(PipelineRedirection::Single { source, target }) => {
+                let mode = redirection_target_to_mode(working_set, builder, target)?;
+                match source {
+                    RedirectionSource::Stdout => RedirectModes {
+                        out: Some(mode),
+                        err: None,
+                    },
+                    RedirectionSource::Stderr => RedirectModes {
+                        out: None,
+                        err: Some(mode),
+                    },
+                    RedirectionSource::StdoutAndStderr => RedirectModes {
+                        out: Some(mode),
+                        err: Some(mode),
+                    },
+                }
+            }
+            Some(PipelineRedirection::Separate { out, err }) => {
+                // In this case, out and err must not both be Pipe
+                assert!(
+                    !matches!(
+                        (out, err),
+                        (
+                            RedirectionTarget::Pipe { .. },
+                            RedirectionTarget::Pipe { .. }
+                        )
+                    ),
+                    "for Separate redirection, out and err targets must not both be Pipe"
+                );
+                let out = redirection_target_to_mode(working_set, builder, out)?;
+                let err = redirection_target_to_mode(working_set, builder, err)?;
+                RedirectModes {
+                    out: Some(out),
+                    err: Some(err),
+                }
+            }
+            None => RedirectModes {
+                out: None,
+                err: None,
+            },
+        };
+
+        let redirect_modes = RedirectModes {
+            out: spec_redirect_modes.out.or(next_redirect_modes.out),
+            err: spec_redirect_modes.err.or(next_redirect_modes.err),
+        };
+
+        compile_expression(
+            working_set,
+            builder,
+            &element.expr,
+            redirect_modes.clone(),
+            in_reg,
+            out_reg,
+        )?;
+
+        // Clean up the redirection
+        finish_redirection(builder, redirect_modes, out_reg)?;
+
+        // The next pipeline element takes input from this output
+        in_reg = Some(out_reg);
+    }
+    Ok(())
+}

crates/nu-engine/src/compile/operator.rs

@@ -0,0 +1,378 @@
+use nu_protocol::{
+    ast::{Assignment, Boolean, CellPath, Expr, Expression, Math, Operator, PathMember},
+    engine::StateWorkingSet,
+    ir::{Instruction, Literal},
+    IntoSpanned, RegId, Span, Spanned, ENV_VARIABLE_ID,
+};
+use nu_utils::IgnoreCaseExt;
+
+use super::{compile_expression, BlockBuilder, CompileError, RedirectModes};
+
+pub(crate) fn compile_binary_op(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    lhs: &Expression,
+    op: Spanned<Operator>,
+    rhs: &Expression,
+    span: Span,
+    out_reg: RegId,
+) -> Result<(), CompileError> {
+    if let Operator::Assignment(assign_op) = op.item {
+        if let Some(decomposed_op) = decompose_assignment(assign_op) {
+            // Compiling an assignment that uses a binary op with the existing value
+            compile_binary_op(
+                working_set,
+                builder,
+                lhs,
+                decomposed_op.into_spanned(op.span),
+                rhs,
+                span,
+                out_reg,
+            )?;
+        } else {
+            // Compiling a plain assignment, where the current left-hand side value doesn't matter
+            compile_expression(
+                working_set,
+                builder,
+                rhs,
+                RedirectModes::capture_out(rhs.span),
+                None,
+                out_reg,
+            )?;
+        }
+
+        compile_assignment(working_set, builder, lhs, op.span, out_reg)?;
+
+        // Load out_reg with Nothing, as that's the result of an assignment
+        builder.load_literal(out_reg, Literal::Nothing.into_spanned(op.span))
+    } else {
+        // Not an assignment: just do the binary op
+        let lhs_reg = out_reg;
+
+        compile_expression(
+            working_set,
+            builder,
+            lhs,
+            RedirectModes::capture_out(lhs.span),
+            None,
+            lhs_reg,
+        )?;
+
+        match op.item {
+            // `and` / `or` are short-circuiting, and we can get by with one register and a branch
+            Operator::Boolean(Boolean::And) => {
+                let true_label = builder.label(None);
+                builder.branch_if(lhs_reg, true_label, op.span)?;
+
+                // If the branch was not taken it's false, so short circuit to load false
+                let false_label = builder.label(None);
+                builder.jump(false_label, op.span)?;
+
+                builder.set_label(true_label, builder.here())?;
+                compile_expression(
+                    working_set,
+                    builder,
+                    rhs,
+                    RedirectModes::capture_out(rhs.span),
+                    None,
+                    lhs_reg,
+                )?;
+
+                let end_label = builder.label(None);
+                builder.jump(end_label, op.span)?;
+
+                // Consumed by `branch-if`, so we have to set it false again
+                builder.set_label(false_label, builder.here())?;
+                builder.load_literal(lhs_reg, Literal::Bool(false).into_spanned(lhs.span))?;
+
+                builder.set_label(end_label, builder.here())?;
+            }
+            Operator::Boolean(Boolean::Or) => {
+                let true_label = builder.label(None);
+                builder.branch_if(lhs_reg, true_label, op.span)?;
+
+                // If the branch was not taken it's false, so do the right-side expression
+                compile_expression(
+                    working_set,
+                    builder,
+                    rhs,
+                    RedirectModes::capture_out(rhs.span),
+                    None,
+                    lhs_reg,
+                )?;
+
+                let end_label = builder.label(None);
+                builder.jump(end_label, op.span)?;
+
+                // Consumed by `branch-if`, so we have to set it true again
+                builder.set_label(true_label, builder.here())?;
+                builder.load_literal(lhs_reg, Literal::Bool(true).into_spanned(lhs.span))?;
+
+                builder.set_label(end_label, builder.here())?;
+            }
+            _ => {
+                // Any other operator, via `binary-op`
+                let rhs_reg = builder.next_register()?;
+
+                compile_expression(
+                    working_set,
+                    builder,
+                    rhs,
+                    RedirectModes::capture_out(rhs.span),
+                    None,
+                    rhs_reg,
+                )?;
+
+                builder.push(
+                    Instruction::BinaryOp {
+                        lhs_dst: lhs_reg,
+                        op: op.item,
+                        rhs: rhs_reg,
+                    }
+                    .into_spanned(op.span),
+                )?;
+            }
+        }
+
+        if lhs_reg != out_reg {
+            builder.push(
+                Instruction::Move {
+                    dst: out_reg,
+                    src: lhs_reg,
+                }
+                .into_spanned(op.span),
+            )?;
+        }
+
+        builder.push(Instruction::Span { src_dst: out_reg }.into_spanned(span))?;
+
+        Ok(())
+    }
+}
+
+/// The equivalent plain operator to use for an assignment, if any
+pub(crate) fn decompose_assignment(assignment: Assignment) -> Option<Operator> {
+    match assignment {
+        Assignment::Assign => None,
+        Assignment::PlusAssign => Some(Operator::Math(Math::Plus)),
+        Assignment::AppendAssign => Some(Operator::Math(Math::Append)),
+        Assignment::MinusAssign => Some(Operator::Math(Math::Minus)),
+        Assignment::MultiplyAssign => Some(Operator::Math(Math::Multiply)),
+        Assignment::DivideAssign => Some(Operator::Math(Math::Divide)),
+    }
+}
+
+/// Compile assignment of the value in a register to a left-hand expression
+pub(crate) fn compile_assignment(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    lhs: &Expression,
+    assignment_span: Span,
+    rhs_reg: RegId,
+) -> Result<(), CompileError> {
+    match lhs.expr {
+        Expr::Var(var_id) => {
+            // Double check that the variable is supposed to be mutable
+            if !working_set.get_variable(var_id).mutable {
+                return Err(CompileError::AssignmentRequiresMutableVar { span: lhs.span });
+            }
+
+            builder.push(
+                Instruction::StoreVariable {
+                    var_id,
+                    src: rhs_reg,
+                }
+                .into_spanned(assignment_span),
+            )?;
+            Ok(())
+        }
+        Expr::FullCellPath(ref path) => match (&path.head, &path.tail) {
+            (
+                Expression {
+                    expr: Expr::Var(var_id),
+                    ..
+                },
+                _,
+            ) if *var_id == ENV_VARIABLE_ID => {
+                // This will be an assignment to an environment variable.
+                let Some(PathMember::String { val: key, .. }) = path.tail.first() else {
+                    return Err(CompileError::CannotReplaceEnv { span: lhs.span });
+                };
+
+                // Some env vars can't be set by Nushell code.
+                const AUTOMATIC_NAMES: &[&str] = &["PWD", "FILE_PWD", "CURRENT_FILE"];
+                if AUTOMATIC_NAMES.iter().any(|name| key.eq_ignore_case(name)) {
+                    return Err(CompileError::AutomaticEnvVarSetManually {
+                        envvar_name: "PWD".into(),
+                        span: lhs.span,
+                    });
+                }
+
+                let key_data = builder.data(key)?;
+
+                let val_reg = if path.tail.len() > 1 {
+                    // Get the current value of the head and first tail of the path, from env
+                    let head_reg = builder.next_register()?;
+
+                    // We could use compile_load_env, but this shares the key data...
+                    // Always use optional, because it doesn't matter if it's already there
+                    builder.push(
+                        Instruction::LoadEnvOpt {
+                            dst: head_reg,
+                            key: key_data,
+                        }
+                        .into_spanned(lhs.span),
+                    )?;
+
+                    // Default to empty record so we can do further upserts
+                    let default_label = builder.label(None);
+                    let upsert_label = builder.label(None);
+                    builder.branch_if_empty(head_reg, default_label, assignment_span)?;
+                    builder.jump(upsert_label, assignment_span)?;
+
+                    builder.set_label(default_label, builder.here())?;
+                    builder.load_literal(
+                        head_reg,
+                        Literal::Record { capacity: 0 }.into_spanned(lhs.span),
+                    )?;
+
+                    // Do the upsert on the current value to incorporate rhs
+                    builder.set_label(upsert_label, builder.here())?;
+                    compile_upsert_cell_path(
+                        builder,
+                        (&path.tail[1..]).into_spanned(lhs.span),
+                        head_reg,
+                        rhs_reg,
+                        assignment_span,
+                    )?;
+
+                    head_reg
+                } else {
+                    // Path has only one tail, so we don't need the current value to do an upsert,
+                    // just set it directly to rhs
+                    rhs_reg
+                };
+
+                // Finally, store the modified env variable
+                builder.push(
+                    Instruction::StoreEnv {
+                        key: key_data,
+                        src: val_reg,
+                    }
+                    .into_spanned(assignment_span),
+                )?;
+                Ok(())
+            }
+            (_, tail) if tail.is_empty() => {
+                // If the path tail is empty, we can really just treat this as if it were an
+                // assignment to the head
+                compile_assignment(working_set, builder, &path.head, assignment_span, rhs_reg)
+            }
+            _ => {
+                // Just a normal assignment to some path
+                let head_reg = builder.next_register()?;
+
+                // Compile getting current value of the head expression
+                compile_expression(
+                    working_set,
+                    builder,
+                    &path.head,
+                    RedirectModes::capture_out(path.head.span),
+                    None,
+                    head_reg,
+                )?;
+
+                // Upsert the tail of the path into the old value of the head expression
+                compile_upsert_cell_path(
+                    builder,
+                    path.tail.as_slice().into_spanned(lhs.span),
+                    head_reg,
+                    rhs_reg,
+                    assignment_span,
+                )?;
+
+                // Now compile the assignment of the updated value to the head
+                compile_assignment(working_set, builder, &path.head, assignment_span, head_reg)
+            }
+        },
+        Expr::Garbage => Err(CompileError::Garbage { span: lhs.span }),
+        _ => Err(CompileError::AssignmentRequiresVar { span: lhs.span }),
+    }
+}
+
+/// Compile an upsert-cell-path instruction, with known literal members
+pub(crate) fn compile_upsert_cell_path(
+    builder: &mut BlockBuilder,
+    members: Spanned<&[PathMember]>,
+    src_dst: RegId,
+    new_value: RegId,
+    span: Span,
+) -> Result<(), CompileError> {
+    let path_reg = builder.literal(
+        Literal::CellPath(
+            CellPath {
+                members: members.item.to_vec(),
+            }
+            .into(),
+        )
+        .into_spanned(members.span),
+    )?;
+    builder.push(
+        Instruction::UpsertCellPath {
+            src_dst,
+            path: path_reg,
+            new_value,
+        }
+        .into_spanned(span),
+    )?;
+    Ok(())
+}
+
+/// Compile the correct sequence to get an environment variable + follow a path on it
+pub(crate) fn compile_load_env(
+    builder: &mut BlockBuilder,
+    span: Span,
+    path: &[PathMember],
+    out_reg: RegId,
+) -> Result<(), CompileError> {
+    if path.is_empty() {
+        builder.push(
+            Instruction::LoadVariable {
+                dst: out_reg,
+                var_id: ENV_VARIABLE_ID,
+            }
+            .into_spanned(span),
+        )?;
+    } else {
+        let (key, optional) = match &path[0] {
+            PathMember::String { val, optional, .. } => (builder.data(val)?, *optional),
+            PathMember::Int { span, .. } => {
+                return Err(CompileError::AccessEnvByInt { span: *span })
+            }
+        };
+        let tail = &path[1..];
+
+        if optional {
+            builder.push(Instruction::LoadEnvOpt { dst: out_reg, key }.into_spanned(span))?;
+        } else {
+            builder.push(Instruction::LoadEnv { dst: out_reg, key }.into_spanned(span))?;
+        }
+
+        if !tail.is_empty() {
+            let path = builder.literal(
+                Literal::CellPath(Box::new(CellPath {
+                    members: tail.to_vec(),
+                }))
+                .into_spanned(span),
+            )?;
+            builder.push(
+                Instruction::FollowCellPath {
+                    src_dst: out_reg,
+                    path,
+                }
+                .into_spanned(span),
+            )?;
+        }
+    }
+    Ok(())
+}

crates/nu-engine/src/compile/redirect.rs

@@ -0,0 +1,157 @@
+use nu_protocol::{
+    ast::{Expression, RedirectionTarget},
+    engine::StateWorkingSet,
+    ir::{Instruction, RedirectMode},
+    IntoSpanned, OutDest, RegId, Span, Spanned,
+};
+
+use super::{compile_expression, BlockBuilder, CompileError};
+
+#[derive(Default, Clone)]
+pub(crate) struct RedirectModes {
+    pub(crate) out: Option<Spanned<RedirectMode>>,
+    pub(crate) err: Option<Spanned<RedirectMode>>,
+}
+
+impl RedirectModes {
+    pub(crate) fn capture_out(span: Span) -> Self {
+        RedirectModes {
+            out: Some(RedirectMode::Capture.into_spanned(span)),
+            err: None,
+        }
+    }
+
+    pub(crate) fn caller(span: Span) -> RedirectModes {
+        RedirectModes {
+            out: Some(RedirectMode::Caller.into_spanned(span)),
+            err: Some(RedirectMode::Caller.into_spanned(span)),
+        }
+    }
+}
+
+pub(crate) fn redirection_target_to_mode(
+    working_set: &StateWorkingSet,
+    builder: &mut BlockBuilder,
+    target: &RedirectionTarget,
+) -> Result<Spanned<RedirectMode>, CompileError> {
+    Ok(match target {
+        RedirectionTarget::File {
+            expr,
+            append,
+            span: redir_span,
+        } => {
+            let file_num = builder.next_file_num()?;
+            let path_reg = builder.next_register()?;
+            compile_expression(
+                working_set,
+                builder,
+                expr,
+                RedirectModes::capture_out(*redir_span),
+                None,
+                path_reg,
+            )?;
+            builder.push(
+                Instruction::OpenFile {
+                    file_num,
+                    path: path_reg,
+                    append: *append,
+                }
+                .into_spanned(*redir_span),
+            )?;
+            RedirectMode::File { file_num }.into_spanned(*redir_span)
+        }
+        RedirectionTarget::Pipe { span } => RedirectMode::Pipe.into_spanned(*span),
+    })
+}
+
+pub(crate) fn redirect_modes_of_expression(
+    working_set: &StateWorkingSet,
+    expression: &Expression,
+    redir_span: Span,
+) -> Result<RedirectModes, CompileError> {
+    let (out, err) = expression.expr.pipe_redirection(working_set);
+    Ok(RedirectModes {
+        out: out
+            .map(|r| r.into_spanned(redir_span))
+            .map(out_dest_to_redirect_mode)
+            .transpose()?,
+        err: err
+            .map(|r| r.into_spanned(redir_span))
+            .map(out_dest_to_redirect_mode)
+            .transpose()?,
+    })
+}
+
+/// Finish the redirection for an expression, writing to and closing files as necessary
+pub(crate) fn finish_redirection(
+    builder: &mut BlockBuilder,
+    modes: RedirectModes,
+    out_reg: RegId,
+) -> Result<(), CompileError> {
+    if let Some(Spanned {
+        item: RedirectMode::File { file_num },
+        span,
+    }) = modes.out
+    {
+        // If out is a file and err is a pipe, we must not consume the expression result -
+        // that is actually the err, in that case.
+        if !matches!(
+            modes.err,
+            Some(Spanned {
+                item: RedirectMode::Pipe { .. },
+                ..
+            })
+        ) {
+            builder.push(
+                Instruction::WriteFile {
+                    file_num,
+                    src: out_reg,
+                }
+                .into_spanned(span),
+            )?;
+            builder.load_empty(out_reg)?;
+        }
+        builder.push(Instruction::CloseFile { file_num }.into_spanned(span))?;
+    }
+
+    match modes.err {
+        Some(Spanned {
+            item: RedirectMode::File { file_num },
+            span,
+        }) => {
+            // Close the file, unless it's the same as out (in which case it was already closed)
+            if !modes.out.is_some_and(|out_mode| match out_mode.item {
+                RedirectMode::File {
+                    file_num: out_file_num,
+                } => file_num == out_file_num,
+                _ => false,
+            }) {
+                builder.push(Instruction::CloseFile { file_num }.into_spanned(span))?;
+            }
+        }
+        Some(Spanned {
+            item: RedirectMode::Pipe,
+            span,
+        }) => {
+            builder.push(Instruction::CheckErrRedirected { src: out_reg }.into_spanned(span))?;
+        }
+        _ => (),
+    }
+
+    Ok(())
+}
+
+pub(crate) fn out_dest_to_redirect_mode(
+    out_dest: Spanned<OutDest>,
+) -> Result<Spanned<RedirectMode>, CompileError> {
+    let span = out_dest.span;
+    out_dest
+        .map(|out_dest| match out_dest {
+            OutDest::Pipe => Ok(RedirectMode::Pipe),
+            OutDest::Capture => Ok(RedirectMode::Capture),
+            OutDest::Null => Ok(RedirectMode::Null),
+            OutDest::Inherit => Ok(RedirectMode::Inherit),
+            OutDest::File(_) => Err(CompileError::InvalidRedirectMode { span }),
+        })
+        .transpose()
+}

crates/nu-engine/src/documentation.rs

@@ -45,10 +45,12 @@ fn nu_highlight_string(code_string: &str, engine_state: &EngineState, stack: &mu
     if let Some(highlighter) = engine_state.find_decl(b"nu-highlight", &[]) {
         let decl = engine_state.get_decl(highlighter);
 
+        let call = Call::new(Span::unknown());
+
         if let Ok(output) = decl.run(
             engine_state,
             stack,
-            &Call::new(Span::unknown()),
+            &(&call).into(),
             Value::string(code_string, Span::unknown()).into_pipeline_data(),
         ) {
             let result = output.into_value(Span::unknown());
@@ -282,11 +284,12 @@ fn get_documentation(
             let _ = write!(long_desc, "\n  > {}\n", example.example);
         } else if let Some(highlighter) = engine_state.find_decl(b"nu-highlight", &[]) {
             let decl = engine_state.get_decl(highlighter);
+            let call = Call::new(Span::unknown());
 
             match decl.run(
                 engine_state,
                 stack,
-                &Call::new(Span::unknown()),
+                &(&call).into(),
                 Value::string(example.example, Span::unknown()).into_pipeline_data(),
             ) {
                 Ok(output) => {
@@ -339,7 +342,7 @@ fn get_documentation(
                         .run(
                             engine_state,
                             stack,
-                            &table_call,
+                            &(&table_call).into(),
                             PipelineData::Value(result.clone(), None),
                         )
                         .ok()

crates/nu-engine/src/env.rs

@@ -1,8 +1,8 @@
 use crate::ClosureEvalOnce;
 use nu_path::canonicalize_with;
 use nu_protocol::{
-    ast::{Call, Expr},
-    engine::{EngineState, Stack, StateWorkingSet},
+    ast::Expr,
+    engine::{Call, EngineState, Stack, StateWorkingSet},
     ShellError, Span, Value, VarId,
 };
 use std::{
@@ -244,14 +244,15 @@ pub fn path_str(
 }
 
 pub const DIR_VAR_PARSER_INFO: &str = "dirs_var";
-pub fn get_dirs_var_from_call(call: &Call) -> Option<VarId> {
-    call.get_parser_info(DIR_VAR_PARSER_INFO).and_then(|x| {
-        if let Expr::Var(id) = x.expr {
-            Some(id)
-        } else {
-            None
-        }
-    })
+pub fn get_dirs_var_from_call(stack: &Stack, call: &Call) -> Option<VarId> {
+    call.get_parser_info(stack, DIR_VAR_PARSER_INFO)
+        .and_then(|x| {
+            if let Expr::Var(id) = x.expr {
+                Some(id)
+            } else {
+                None
+            }
+        })
 }
 
 /// This helper function is used to find files during eval

crates/nu-engine/src/eval.rs

@@ -1,3 +1,4 @@
+use crate::eval_ir_block;
 #[allow(deprecated)]
 use crate::{current_dir, get_full_help};
 use nu_path::{expand_path_with, AbsolutePathBuf};
@@ -7,7 +8,7 @@ use nu_protocol::{
         PipelineRedirection, RedirectionSource, RedirectionTarget,
     },
     debugger::DebugContext,
-    engine::{Closure, EngineState, Redirection, Stack},
+    engine::{Closure, EngineState, Redirection, Stack, StateWorkingSet},
     eval_base::Eval,
     ByteStreamSource, Config, FromValue, IntoPipelineData, OutDest, PipelineData, ShellError, Span,
     Spanned, Type, Value, VarId, ENV_VARIABLE_ID,
@@ -174,7 +175,7 @@ pub fn eval_call<D: DebugContext>(
         // We pass caller_stack here with the knowledge that internal commands
         // are going to be specifically looking for global state in the stack
         // rather than any local state.
-        decl.run(engine_state, caller_stack, call, input)
+        decl.run(engine_state, caller_stack, &call.into(), input)
     }
 }
 
@@ -226,7 +227,7 @@ fn eval_external(
         }
     }
 
-    command.run(engine_state, stack, &call, input)
+    command.run(engine_state, stack, &(&call).into(), input)
 }
 
 pub fn eval_expression<D: DebugContext>(
@@ -510,6 +511,11 @@ pub fn eval_block<D: DebugContext>(
     block: &Block,
     mut input: PipelineData,
 ) -> Result<PipelineData, ShellError> {
+    // Remove once IR is the default.
+    if stack.use_ir {
+        return eval_ir_block::<D>(engine_state, stack, block, input);
+    }
+
     D::enter_block(engine_state, block);
 
     let num_pipelines = block.len();
@@ -524,7 +530,7 @@ pub fn eval_block<D: DebugContext>(
 
         for (i, element) in elements.iter().enumerate() {
             let next = elements.get(i + 1).unwrap_or(last);
-            let (next_out, next_err) = next.pipe_redirection(engine_state);
+            let (next_out, next_err) = next.pipe_redirection(&StateWorkingSet::new(engine_state));
             let (stdout, stderr) = eval_element_redirection::<D>(
                 engine_state,
                 stack,
@@ -913,7 +919,7 @@ impl Eval for EvalRuntime {
 ///
 /// An automatic environment variable cannot be assigned to by user code.
 /// Current there are three of them: $env.PWD, $env.FILE_PWD, $env.CURRENT_FILE
-fn is_automatic_env_var(var: &str) -> bool {
+pub(crate) fn is_automatic_env_var(var: &str) -> bool {
     let names = ["PWD", "FILE_PWD", "CURRENT_FILE"];
     names.iter().any(|&name| {
         if cfg!(windows) {

crates/nu-engine/src/eval_helpers.rs

@@ -1,6 +1,6 @@
 use crate::{
     eval_block, eval_block_with_early_return, eval_expression, eval_expression_with_input,
-    eval_subexpression,
+    eval_ir_block, eval_subexpression,
 };
 use nu_protocol::{
     ast::{Block, Expression},
@@ -13,6 +13,10 @@ use nu_protocol::{
 pub type EvalBlockFn =
     fn(&EngineState, &mut Stack, &Block, PipelineData) -> Result<PipelineData, ShellError>;
 
+/// Type of eval_ir_block() function
+pub type EvalIrBlockFn =
+    fn(&EngineState, &mut Stack, &Block, PipelineData) -> Result<PipelineData, ShellError>;
+
 /// Type of eval_block_with_early_return() function
 pub type EvalBlockWithEarlyReturnFn =
     fn(&EngineState, &mut Stack, &Block, PipelineData) -> Result<PipelineData, ShellError>;
@@ -42,6 +46,16 @@ pub fn get_eval_block(engine_state: &EngineState) -> EvalBlockFn {
     }
 }
 
+/// Helper function to fetch `eval_ir_block()` with the correct type parameter based on whether
+/// engine_state is configured with or without a debugger.
+pub fn get_eval_ir_block(engine_state: &EngineState) -> EvalIrBlockFn {
+    if engine_state.is_debugging() {
+        eval_ir_block::<WithDebug>
+    } else {
+        eval_ir_block::<WithoutDebug>
+    }
+}
+
 /// Helper function to fetch `eval_block_with_early_return()` with the correct type parameter based
 /// on whether engine_state is configured with or without a debugger.
 pub fn get_eval_block_with_early_return(engine_state: &EngineState) -> EvalBlockWithEarlyReturnFn {

crates/nu-engine/src/lib.rs

@@ -2,16 +2,19 @@ mod call_ext;
 mod closure_eval;
 pub mod column;
 pub mod command_prelude;
+mod compile;
 pub mod documentation;
 pub mod env;
 mod eval;
 mod eval_helpers;
+mod eval_ir;
 mod glob_from;
 pub mod scope;
 
 pub use call_ext::CallExt;
 pub use closure_eval::*;
 pub use column::get_columns;
+pub use compile::compile;
 pub use documentation::get_full_help;
 pub use env::*;
 pub use eval::{
@@ -19,4 +22,5 @@ pub use eval::{
     eval_expression_with_input, eval_subexpression, eval_variable, redirect_env,
 };
 pub use eval_helpers::*;
+pub use eval_ir::eval_ir_block;
 pub use glob_from::glob_from;

crates/nu-parser/src/known_external.rs

@@ -1,7 +1,8 @@
 use nu_engine::command_prelude::*;
 use nu_protocol::{
-    ast::{Argument, Expr, Expression},
-    engine::{CommandType, UNKNOWN_SPAN_ID},
+    ast::{self, Expr, Expression},
+    engine::{self, CallImpl, CommandType, UNKNOWN_SPAN_ID},
+    ir::{self, DataSlice},
 };
 
 #[derive(Clone)]
@@ -43,8 +44,6 @@ impl Command for KnownExternal {
 
         let command = engine_state.get_decl(decl_id);
 
-        let mut extern_call = Call::new(head_span);
-
         let extern_name = if let Some(name_bytes) = engine_state.find_decl_name(call.decl_id, &[]) {
             String::from_utf8_lossy(name_bytes)
         } else {
@@ -56,59 +55,166 @@ impl Command for KnownExternal {
         };
 
         let extern_name: Vec<_> = extern_name.split(' ').collect();
-        let call_head_id = engine_state
-            .find_span_id(call.head)
-            .unwrap_or(UNKNOWN_SPAN_ID);
 
-        let arg_extern_name = Expression::new_existing(
-            Expr::String(extern_name[0].to_string()),
+        match &call.inner {
+            CallImpl::AstRef(call) => {
+                let extern_call = ast_call_to_extern_call(engine_state, call, &extern_name)?;
+                command.run(engine_state, stack, &(&extern_call).into(), input)
+            }
+            CallImpl::AstBox(call) => {
+                let extern_call = ast_call_to_extern_call(engine_state, call, &extern_name)?;
+                command.run(engine_state, stack, &(&extern_call).into(), input)
+            }
+            CallImpl::IrRef(call) => {
+                let extern_call = ir_call_to_extern_call(stack, call, &extern_name)?;
+                command.run(engine_state, stack, &(&extern_call).into(), input)
+            }
+            CallImpl::IrBox(call) => {
+                let extern_call = ir_call_to_extern_call(stack, call, &extern_name)?;
+                command.run(engine_state, stack, &(&extern_call).into(), input)
+            }
+        }
+    }
+}
+
+/// Transform the args from an `ast::Call` onto a `run-external` call
+fn ast_call_to_extern_call(
+    engine_state: &EngineState,
+    call: &ast::Call,
+    extern_name: &[&str],
+) -> Result<ast::Call, ShellError> {
+    let head_span = call.head;
+
+    let mut extern_call = ast::Call::new(head_span);
+
+    let call_head_id = engine_state
+        .find_span_id(call.head)
+        .unwrap_or(UNKNOWN_SPAN_ID);
+
+    let arg_extern_name = Expression::new_existing(
+        Expr::String(extern_name[0].to_string()),
+        call.head,
+        call_head_id,
+        Type::String,
+    );
+
+    extern_call.add_positional(arg_extern_name);
+
+    for subcommand in extern_name.iter().skip(1) {
+        extern_call.add_positional(Expression::new_existing(
+            Expr::String(subcommand.to_string()),
             call.head,
             call_head_id,
             Type::String,
-        );
+        ));
+    }
 
-        extern_call.add_positional(arg_extern_name);
-
-        for subcommand in extern_name.into_iter().skip(1) {
-            extern_call.add_positional(Expression::new_existing(
-                Expr::String(subcommand.to_string()),
-                call.head,
-                call_head_id,
-                Type::String,
-            ));
-        }
-
-        for arg in &call.arguments {
-            match arg {
-                Argument::Positional(positional) => extern_call.add_positional(positional.clone()),
-                Argument::Named(named) => {
-                    let named_span_id = engine_state
-                        .find_span_id(named.0.span)
-                        .unwrap_or(UNKNOWN_SPAN_ID);
-                    if let Some(short) = &named.1 {
-                        extern_call.add_positional(Expression::new_existing(
-                            Expr::String(format!("-{}", short.item)),
-                            named.0.span,
-                            named_span_id,
-                            Type::String,
-                        ));
-                    } else {
-                        extern_call.add_positional(Expression::new_existing(
-                            Expr::String(format!("--{}", named.0.item)),
-                            named.0.span,
-                            named_span_id,
-                            Type::String,
-                        ));
-                    }
-                    if let Some(arg) = &named.2 {
-                        extern_call.add_positional(arg.clone());
-                    }
+    for arg in &call.arguments {
+        match arg {
+            ast::Argument::Positional(positional) => extern_call.add_positional(positional.clone()),
+            ast::Argument::Named(named) => {
+                let named_span_id = engine_state
+                    .find_span_id(named.0.span)
+                    .unwrap_or(UNKNOWN_SPAN_ID);
+                if let Some(short) = &named.1 {
+                    extern_call.add_positional(Expression::new_existing(
+                        Expr::String(format!("-{}", short.item)),
+                        named.0.span,
+                        named_span_id,
+                        Type::String,
+                    ));
+                } else {
+                    extern_call.add_positional(Expression::new_existing(
+                        Expr::String(format!("--{}", named.0.item)),
+                        named.0.span,
+                        named_span_id,
+                        Type::String,
+                    ));
                 }
-                Argument::Unknown(unknown) => extern_call.add_unknown(unknown.clone()),
-                Argument::Spread(args) => extern_call.add_spread(args.clone()),
+                if let Some(arg) = &named.2 {
+                    extern_call.add_positional(arg.clone());
+                }
+            }
+            ast::Argument::Unknown(unknown) => extern_call.add_unknown(unknown.clone()),
+            ast::Argument::Spread(args) => extern_call.add_spread(args.clone()),
+        }
+    }
+
+    Ok(extern_call)
+}
+
+/// Transform the args from an `ir::Call` onto a `run-external` call
+fn ir_call_to_extern_call(
+    stack: &mut Stack,
+    call: &ir::Call,
+    extern_name: &[&str],
+) -> Result<ir::Call, ShellError> {
+    let mut extern_call = ir::Call::build(call.decl_id, call.head);
+
+    // Add the command and subcommands
+    for name in extern_name {
+        extern_call.add_positional(stack, call.head, Value::string(*name, call.head));
+    }
+
+    // Add the arguments, reformatting named arguments into string positionals
+    for index in 0..call.args_len {
+        match &call.arguments(stack)[index] {
+            engine::Argument::Flag {
+                data,
+                name,
+                short,
+                span,
+            } => {
+                let name_arg = engine::Argument::Positional {
+                    span: *span,
+                    val: Value::string(known_external_option_name(data, *name, *short), *span),
+                    ast: None,
+                };
+                extern_call.add_argument(stack, name_arg);
+            }
+            engine::Argument::Named {
+                data,
+                name,
+                short,
+                span,
+                val,
+                ..
+            } => {
+                let name_arg = engine::Argument::Positional {
+                    span: *span,
+                    val: Value::string(known_external_option_name(data, *name, *short), *span),
+                    ast: None,
+                };
+                let val_arg = engine::Argument::Positional {
+                    span: *span,
+                    val: val.clone(),
+                    ast: None,
+                };
+                extern_call.add_argument(stack, name_arg);
+                extern_call.add_argument(stack, val_arg);
+            }
+            a @ (engine::Argument::Positional { .. }
+            | engine::Argument::Spread { .. }
+            | engine::Argument::ParserInfo { .. }) => {
+                let argument = a.clone();
+                extern_call.add_argument(stack, argument);
             }
         }
+    }
 
-        command.run(engine_state, stack, &extern_call, input)
+    Ok(extern_call.finish())
+}
+
+fn known_external_option_name(data: &[u8], name: DataSlice, short: DataSlice) -> String {
+    if !data[name].is_empty() {
+        format!(
+            "--{}",
+            std::str::from_utf8(&data[name]).expect("invalid utf-8 in flag name")
+        )
+    } else {
+        format!(
+            "-{}",
+            std::str::from_utf8(&data[short]).expect("invalid utf-8 in flag short name")
+        )
     }
 }

crates/nu-parser/src/lex.rs

@@ -238,6 +238,10 @@ pub fn lex_item(
                     Some(e),
                 );
             }
+        } else if c == b'|' && is_redirection(&input[token_start..*curr_offset]) {
+            // matches err>| etc.
+            *curr_offset += 1;
+            break;
         } else if is_item_terminator(&block_level, c, additional_whitespace, special_tokens) {
             break;
         }
@@ -301,6 +305,16 @@ pub fn lex_item(
             contents: TokenContents::OutGreaterGreaterThan,
             span,
         },
+        b"out>|" | b"o>|" => {
+            err = Some(ParseError::Expected(
+                "`|`.  Redirecting stdout to a pipe is the same as normal piping.",
+                span,
+            ));
+            Token {
+                contents: TokenContents::Item,
+                span,
+            }
+        }
         b"err>" | b"e>" => Token {
             contents: TokenContents::ErrGreaterThan,
             span,
@@ -309,6 +323,10 @@ pub fn lex_item(
             contents: TokenContents::ErrGreaterGreaterThan,
             span,
         },
+        b"err>|" | b"e>|" => Token {
+            contents: TokenContents::ErrGreaterPipe,
+            span,
+        },
         b"out+err>" | b"err+out>" | b"o+e>" | b"e+o>" => Token {
             contents: TokenContents::OutErrGreaterThan,
             span,
@@ -317,6 +335,10 @@ pub fn lex_item(
             contents: TokenContents::OutErrGreaterGreaterThan,
             span,
         },
+        b"out+err>|" | b"err+out>|" | b"o+e>|" | b"e+o>|" => Token {
+            contents: TokenContents::OutErrGreaterPipe,
+            span,
+        },
         b"&&" => {
             err = Some(ParseError::ShellAndAnd(span));
             Token {
@@ -580,17 +602,6 @@ fn lex_internal(
             // If the next character is non-newline whitespace, skip it.
             curr_offset += 1;
         } else {
-            let (token, err) = try_lex_special_piped_item(input, &mut curr_offset, span_offset);
-            if error.is_none() {
-                error = err;
-            }
-            if let Some(token) = token {
-                output.push(token);
-                is_complete = false;
-                continue;
-            }
-
-            // Otherwise, try to consume an unclassified token.
             let (token, err) = lex_item(
                 input,
                 &mut curr_offset,
@@ -609,68 +620,10 @@ fn lex_internal(
     (output, error)
 }
 
-/// trying to lex for the following item:
-/// e>|, e+o>|, o+e>|
-///
-/// It returns Some(token) if we find the item, or else return None.
-fn try_lex_special_piped_item(
-    input: &[u8],
-    curr_offset: &mut usize,
-    span_offset: usize,
-) -> (Option<Token>, Option<ParseError>) {
-    let c = input[*curr_offset];
-    let e_pipe_len = 3;
-    let eo_pipe_len = 5;
-    let o_pipe_len = 3;
-    let offset = *curr_offset;
-    if c == b'e' {
-        // expect `e>|`
-        if (offset + e_pipe_len <= input.len()) && (&input[offset..offset + e_pipe_len] == b"e>|") {
-            *curr_offset += e_pipe_len;
-            return (
-                Some(Token::new(
-                    TokenContents::ErrGreaterPipe,
-                    Span::new(span_offset + offset, span_offset + offset + e_pipe_len),
-                )),
-                None,
-            );
-        }
-        if (offset + eo_pipe_len <= input.len())
-            && (&input[offset..offset + eo_pipe_len] == b"e+o>|")
-        {
-            *curr_offset += eo_pipe_len;
-            return (
-                Some(Token::new(
-                    TokenContents::OutErrGreaterPipe,
-                    Span::new(span_offset + offset, span_offset + offset + eo_pipe_len),
-                )),
-                None,
-            );
-        }
-    } else if c == b'o' {
-        // indicates an error if user happened to type `o>|`
-        if offset + o_pipe_len <= input.len() && (&input[offset..offset + o_pipe_len] == b"o>|") {
-            return (
-                None,
-                Some(ParseError::Expected(
-                    "`|`.  Redirecting stdout to a pipe is the same as normal piping.",
-                    Span::new(span_offset + offset, span_offset + offset + o_pipe_len),
-                )),
-            );
-        }
-        // it can be the following case: `o+e>|`
-        if (offset + eo_pipe_len <= input.len())
-            && (&input[offset..offset + eo_pipe_len] == b"o+e>|")
-        {
-            *curr_offset += eo_pipe_len;
-            return (
-                Some(Token::new(
-                    TokenContents::OutErrGreaterPipe,
-                    Span::new(span_offset + offset, span_offset + offset + eo_pipe_len),
-                )),
-                None,
-            );
-        }
-    }
-    (None, None)
+/// True if this the start of a redirection. Does not match `>>` or `>|` forms.
+fn is_redirection(token: &[u8]) -> bool {
+    matches!(
+        token,
+        b"o>" | b"out>" | b"e>" | b"err>" | b"o+e>" | b"e+o>" | b"out+err>" | b"err+out>"
+    )
 }

crates/nu-parser/src/parse_patterns.rs

@@ -39,7 +39,7 @@ pub fn parse_pattern(working_set: &mut StateWorkingSet, span: Span) -> MatchPatt
         let value = parse_value(working_set, span, &SyntaxShape::Any);
 
         MatchPattern {
-            pattern: Pattern::Value(value),
+            pattern: Pattern::Value(Box::new(value)),
             guard: None,
             span,
         }

crates/nu-parser/src/parser.rs

@@ -3302,6 +3302,8 @@ pub fn parse_row_condition(working_set: &mut StateWorkingSet, spans: &[Span]) ->
                 default_value: None,
             });
 
+            compile_block(working_set, &mut block);
+
             working_set.add_block(Arc::new(block))
         }
     };
@@ -4445,7 +4447,7 @@ pub fn parse_match_block_expression(working_set: &mut StateWorkingSet, span: Spa
                 &SyntaxShape::MathExpression,
             );
 
-            pattern.guard = Some(guard);
+            pattern.guard = Some(Box::new(guard));
             position += if found { start + 1 } else { start };
             connector = working_set.get_span_contents(output[position].span);
         }
@@ -5298,6 +5300,8 @@ pub fn parse_expression(working_set: &mut StateWorkingSet, spans: &[Span]) -> Ex
             let ty = output.ty.clone();
             block.pipelines = vec![Pipeline::from_vec(vec![output])];
 
+            compile_block(working_set, &mut block);
+
             let block_id = working_set.add_block(Arc::new(block));
 
             let mut env_vars = vec![];
@@ -5853,9 +5857,25 @@ pub fn parse_block(
         working_set.parse_errors.extend_from_slice(&errors);
     }
 
+    // Do not try to compile blocks that are subexpressions, or when we've already had a parse
+    // failure as that definitely will fail to compile
+    if !is_subexpression && working_set.parse_errors.is_empty() {
+        compile_block(working_set, &mut block);
+    }
+
     block
 }
 
+/// Compile an IR block for the `Block`, adding a compile error on failure
+fn compile_block(working_set: &mut StateWorkingSet<'_>, block: &mut Block) {
+    match nu_engine::compile(working_set, block) {
+        Ok(ir_block) => {
+            block.ir_block = Some(ir_block);
+        }
+        Err(err) => working_set.compile_errors.push(err),
+    }
+}
+
 pub fn discover_captures_in_closure(
     working_set: &StateWorkingSet,
     block: &Block,
@@ -6298,12 +6318,14 @@ fn wrap_expr_with_collect(working_set: &mut StateWorkingSet, expr: &Expression)
             default_value: None,
         });
 
-        let block = Block {
+        let mut block = Block {
             pipelines: vec![Pipeline::from_vec(vec![expr.clone()])],
             signature: Box::new(signature),
             ..Default::default()
         };
 
+        compile_block(working_set, &mut block);
+
         let block_id = working_set.add_block(Arc::new(block));
 
         output.push(Argument::Positional(Expression::new(

crates/nu-parser/tests/test_parser.rs

@@ -1,7 +1,7 @@
 use nu_parser::*;
 use nu_protocol::{
-    ast::{Argument, Call, Expr, Expression, ExternalArgument, PathMember, Range},
-    engine::{Command, EngineState, Stack, StateWorkingSet},
+    ast::{Argument, Expr, Expression, ExternalArgument, PathMember, Range},
+    engine::{Call, Command, EngineState, Stack, StateWorkingSet},
     ParseError, PipelineData, ShellError, Signature, Span, SyntaxShape, Type,
 };
 use rstest::rstest;
@@ -1759,10 +1759,7 @@ mod range {
 #[cfg(test)]
 mod input_types {
     use super::*;
-    use nu_protocol::{
-        ast::{Argument, Call},
-        Category, PipelineData, ShellError, Type,
-    };
+    use nu_protocol::{ast::Argument, engine::Call, Category, PipelineData, ShellError, Type};
 
     #[derive(Clone)]
     pub struct LsTest;

crates/nu-plugin-engine/src/context.rs

@@ -1,8 +1,7 @@
 use crate::util::MutableCow;
 use nu_engine::{get_eval_block_with_early_return, get_full_help, ClosureEvalOnce};
 use nu_protocol::{
-    ast::Call,
-    engine::{Closure, EngineState, Redirection, Stack},
+    engine::{Call, Closure, EngineState, Redirection, Stack},
     Config, IntoSpanned, OutDest, PipelineData, PluginIdentity, ShellError, Signals, Span, Spanned,
     Value,
 };
@@ -54,7 +53,7 @@ pub struct PluginExecutionCommandContext<'a> {
     identity: Arc<PluginIdentity>,
     engine_state: Cow<'a, EngineState>,
     stack: MutableCow<'a, Stack>,
-    call: Cow<'a, Call>,
+    call: Call<'a>,
 }
 
 impl<'a> PluginExecutionCommandContext<'a> {
@@ -62,13 +61,13 @@ impl<'a> PluginExecutionCommandContext<'a> {
         identity: Arc<PluginIdentity>,
         engine_state: &'a EngineState,
         stack: &'a mut Stack,
-        call: &'a Call,
+        call: &'a Call<'a>,
     ) -> PluginExecutionCommandContext<'a> {
         PluginExecutionCommandContext {
             identity,
             engine_state: Cow::Borrowed(engine_state),
             stack: MutableCow::Borrowed(stack),
-            call: Cow::Borrowed(call),
+            call: call.clone(),
         }
     }
 }
@@ -217,7 +216,7 @@ impl<'a> PluginExecutionContext for PluginExecutionCommandContext<'a> {
             identity: self.identity.clone(),
             engine_state: Cow::Owned(self.engine_state.clone().into_owned()),
             stack: self.stack.owned(),
-            call: Cow::Owned(self.call.clone().into_owned()),
+            call: self.call.to_owned(),
         })
     }
 }

crates/nu-plugin-protocol/src/evaluated_call.rs

@@ -1,7 +1,7 @@
 use nu_protocol::{
-    ast::{Call, Expression},
-    engine::{EngineState, Stack},
-    FromValue, ShellError, Span, Spanned, Value,
+    ast::{self, Expression},
+    engine::{Call, CallImpl, EngineState, Stack},
+    ir, FromValue, ShellError, Span, Spanned, Value,
 };
 use serde::{Deserialize, Serialize};
 
@@ -33,6 +33,24 @@ impl EvaluatedCall {
         engine_state: &EngineState,
         stack: &mut Stack,
         eval_expression_fn: fn(&EngineState, &mut Stack, &Expression) -> Result<Value, ShellError>,
+    ) -> Result<Self, ShellError> {
+        match &call.inner {
+            CallImpl::AstRef(call) => {
+                Self::try_from_ast_call(call, engine_state, stack, eval_expression_fn)
+            }
+            CallImpl::AstBox(call) => {
+                Self::try_from_ast_call(call, engine_state, stack, eval_expression_fn)
+            }
+            CallImpl::IrRef(call) => Self::try_from_ir_call(call, stack),
+            CallImpl::IrBox(call) => Self::try_from_ir_call(call, stack),
+        }
+    }
+
+    fn try_from_ast_call(
+        call: &ast::Call,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        eval_expression_fn: fn(&EngineState, &mut Stack, &Expression) -> Result<Value, ShellError>,
     ) -> Result<Self, ShellError> {
         let positional =
             call.rest_iter_flattened(0, |expr| eval_expression_fn(engine_state, stack, expr))?;
@@ -54,6 +72,22 @@ impl EvaluatedCall {
         })
     }
 
+    fn try_from_ir_call(call: &ir::Call, stack: &Stack) -> Result<Self, ShellError> {
+        let positional = call.rest_iter_flattened(stack, 0)?;
+
+        let mut named = Vec::with_capacity(call.named_len(stack));
+        named.extend(
+            call.named_iter(stack)
+                .map(|(name, value)| (name.map(|s| s.to_owned()), value.cloned())),
+        );
+
+        Ok(Self {
+            head: call.head,
+            positional,
+            named,
+        })
+    }
+
     /// Check if a flag (named parameter that does not take a value) is set
     /// Returns Ok(true) if flag is set or passed true value
     /// Returns Ok(false) if flag is not set or passed false value

crates/nu-protocol/Cargo.toml

@@ -33,6 +33,7 @@ serde = { workspace = true, default-features = false }
 thiserror = "1.0"
 typetag = "0.2"
 os_pipe = { workspace = true, features = ["io_safety"] }
+log = { workspace = true }
 
 [target.'cfg(unix)'.dependencies]
 nix = { workspace = true, default-features = false, features = ["signal"] }
@@ -54,4 +55,4 @@ tempfile = { workspace = true }
 os_pipe = { workspace = true }
 
 [package.metadata.docs.rs]
-all-features = true
+all-features = true

crates/nu-protocol/src/alias.rs

@@ -1,6 +1,6 @@
 use crate::{
-    ast::{Call, Expression},
-    engine::{Command, CommandType, EngineState, Stack},
+    ast::Expression,
+    engine::{Call, Command, CommandType, EngineState, Stack},
     PipelineData, ShellError, Signature,
 };
 

crates/nu-protocol/src/ast/block.rs

@@ -1,5 +1,5 @@
 use super::Pipeline;
-use crate::{engine::EngineState, OutDest, Signature, Span, Type, VarId};
+use crate::{engine::StateWorkingSet, ir::IrBlock, OutDest, Signature, Span, Type, VarId};
 use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -8,6 +8,8 @@ pub struct Block {
     pub pipelines: Vec<Pipeline>,
     pub captures: Vec<VarId>,
     pub redirect_env: bool,
+    /// The block compiled to IR instructions. Not available for subexpressions.
+    pub ir_block: Option<IrBlock>,
     pub span: Option<Span>, // None option encodes no span to avoid using test_span()
 }
 
@@ -22,10 +24,10 @@ impl Block {
 
     pub fn pipe_redirection(
         &self,
-        engine_state: &EngineState,
+        working_set: &StateWorkingSet,
     ) -> (Option<OutDest>, Option<OutDest>) {
         if let Some(first) = self.pipelines.first() {
-            first.pipe_redirection(engine_state)
+            first.pipe_redirection(working_set)
         } else {
             (None, None)
         }
@@ -45,6 +47,7 @@ impl Block {
             pipelines: vec![],
             captures: vec![],
             redirect_env: false,
+            ir_block: None,
             span: None,
         }
     }
@@ -55,6 +58,7 @@ impl Block {
             pipelines: Vec::with_capacity(capacity),
             captures: vec![],
             redirect_env: false,
+            ir_block: None,
             span: None,
         }
     }
@@ -86,6 +90,7 @@ where
             pipelines: pipelines.collect(),
             captures: vec![],
             redirect_env: false,
+            ir_block: None,
             span: None,
         }
     }

crates/nu-protocol/src/ast/expr.rs

@@ -5,7 +5,9 @@ use super::{
     Call, CellPath, Expression, ExternalArgument, FullCellPath, Keyword, MatchPattern, Operator,
     Range, Table, ValueWithUnit,
 };
-use crate::{ast::ImportPattern, engine::EngineState, BlockId, OutDest, Signature, Span, VarId};
+use crate::{
+    ast::ImportPattern, engine::StateWorkingSet, BlockId, OutDest, Signature, Span, VarId,
+};
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub enum Expr {
@@ -60,17 +62,17 @@ const _: () = assert!(std::mem::size_of::<Expr>() <= 40);
 impl Expr {
     pub fn pipe_redirection(
         &self,
-        engine_state: &EngineState,
+        working_set: &StateWorkingSet,
     ) -> (Option<OutDest>, Option<OutDest>) {
         // Usages of `$in` will be wrapped by a `collect` call by the parser,
         // so we do not have to worry about that when considering
         // which of the expressions below may consume pipeline output.
         match self {
-            Expr::Call(call) => engine_state.get_decl(call.decl_id).pipe_redirection(),
-            Expr::Subexpression(block_id) | Expr::Block(block_id) => engine_state
+            Expr::Call(call) => working_set.get_decl(call.decl_id).pipe_redirection(),
+            Expr::Subexpression(block_id) | Expr::Block(block_id) => working_set
                 .get_block(*block_id)
-                .pipe_redirection(engine_state),
-            Expr::FullCellPath(cell_path) => cell_path.head.expr.pipe_redirection(engine_state),
+                .pipe_redirection(working_set),
+            Expr::FullCellPath(cell_path) => cell_path.head.expr.pipe_redirection(working_set),
             Expr::Bool(_)
             | Expr::Int(_)
             | Expr::Float(_)

crates/nu-protocol/src/ast/match_pattern.rs

@@ -5,7 +5,7 @@ use serde::{Deserialize, Serialize};
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct MatchPattern {
     pub pattern: Pattern,
-    pub guard: Option<Expression>,
+    pub guard: Option<Box<Expression>>,
     pub span: Span,
 }
 
@@ -19,7 +19,9 @@ impl MatchPattern {
 pub enum Pattern {
     Record(Vec<(String, MatchPattern)>),
     List(Vec<MatchPattern>),
-    Value(Expression),
+    // TODO: it would be nice if this didn't depend on AST
+    // maybe const evaluation can get us to a Value instead?
+    Value(Box<Expression>),
     Variable(VarId),
     Or(Vec<MatchPattern>),
     Rest(VarId), // the ..$foo pattern

crates/nu-protocol/src/ast/pipeline.rs

@@ -1,8 +1,4 @@
-use crate::{
-    ast::Expression,
-    engine::{EngineState, StateWorkingSet},
-    OutDest, Span,
-};
+use crate::{ast::Expression, engine::StateWorkingSet, OutDest, Span};
 use serde::{Deserialize, Serialize};
 use std::fmt::Display;
 
@@ -120,9 +116,9 @@ impl PipelineElement {
 
     pub fn pipe_redirection(
         &self,
-        engine_state: &EngineState,
+        working_set: &StateWorkingSet,
     ) -> (Option<OutDest>, Option<OutDest>) {
-        self.expr.expr.pipe_redirection(engine_state)
+        self.expr.expr.pipe_redirection(working_set)
     }
 }
 
@@ -166,10 +162,10 @@ impl Pipeline {
 
     pub fn pipe_redirection(
         &self,
-        engine_state: &EngineState,
+        working_set: &StateWorkingSet,
     ) -> (Option<OutDest>, Option<OutDest>) {
         if let Some(first) = self.elements.first() {
-            first.pipe_redirection(engine_state)
+            first.pipe_redirection(working_set)
         } else {
             (None, None)
         }

crates/nu-protocol/src/engine/argument.rs

@@ -0,0 +1,124 @@
+use std::sync::Arc;
+
+use crate::{ast::Expression, ir::DataSlice, Span, Value};
+
+/// Represents a fully evaluated argument to a call.
+#[derive(Debug, Clone)]
+pub enum Argument {
+    /// A positional argument
+    Positional {
+        span: Span,
+        val: Value,
+        ast: Option<Arc<Expression>>,
+    },
+    /// A spread argument, e.g. `...$args`
+    Spread {
+        span: Span,
+        vals: Value,
+        ast: Option<Arc<Expression>>,
+    },
+    /// A named argument with no value, e.g. `--flag`
+    Flag {
+        data: Arc<[u8]>,
+        name: DataSlice,
+        short: DataSlice,
+        span: Span,
+    },
+    /// A named argument with a value, e.g. `--flag value` or `--flag=`
+    Named {
+        data: Arc<[u8]>,
+        name: DataSlice,
+        short: DataSlice,
+        span: Span,
+        val: Value,
+        ast: Option<Arc<Expression>>,
+    },
+    /// Information generated by the parser for use by certain keyword commands
+    ParserInfo {
+        data: Arc<[u8]>,
+        name: DataSlice,
+        // TODO: rather than `Expression`, this would probably be best served by a specific enum
+        // type for this purpose.
+        info: Box<Expression>,
+    },
+}
+
+impl Argument {
+    /// The span encompassing the argument's usage within the call, distinct from the span of the
+    /// actual value of the argument.
+    pub fn span(&self) -> Option<Span> {
+        match self {
+            Argument::Positional { span, .. } => Some(*span),
+            Argument::Spread { span, .. } => Some(*span),
+            Argument::Flag { span, .. } => Some(*span),
+            Argument::Named { span, .. } => Some(*span),
+            // Because `ParserInfo` is generated, its span shouldn't be used
+            Argument::ParserInfo { .. } => None,
+        }
+    }
+
+    /// The original AST [`Expression`] for the argument's value. This is not usually available;
+    /// declarations have to opt-in if they require this.
+    pub fn ast_expression(&self) -> Option<&Arc<Expression>> {
+        match self {
+            Argument::Positional { ast, .. } => ast.as_ref(),
+            Argument::Spread { ast, .. } => ast.as_ref(),
+            Argument::Flag { .. } => None,
+            Argument::Named { ast, .. } => ast.as_ref(),
+            Argument::ParserInfo { .. } => None,
+        }
+    }
+}
+
+/// Stores the argument context for calls in IR evaluation.
+#[derive(Debug, Clone)]
+pub struct ArgumentStack {
+    arguments: Vec<Argument>,
+}
+
+impl ArgumentStack {
+    /// Create a new, empty argument stack.
+    pub const fn new() -> Self {
+        ArgumentStack { arguments: vec![] }
+    }
+
+    /// Returns the index of the end of the argument stack. Call and save this before adding
+    /// arguments.
+    pub fn get_base(&self) -> usize {
+        self.arguments.len()
+    }
+
+    /// Calculates the number of arguments past the given [previously retrieved](.get_base) base
+    /// pointer.
+    pub fn get_len(&self, base: usize) -> usize {
+        self.arguments.len().checked_sub(base).unwrap_or_else(|| {
+            panic!(
+                "base ({}) is beyond the end of the arguments stack ({})",
+                base,
+                self.arguments.len()
+            );
+        })
+    }
+
+    /// Push an argument onto the end of the argument stack.
+    pub fn push(&mut self, argument: Argument) {
+        self.arguments.push(argument);
+    }
+
+    /// Clear all of the arguments after the given base index, to prepare for the next frame.
+    pub fn leave_frame(&mut self, base: usize) {
+        self.arguments.truncate(base);
+    }
+
+    /// Get arguments for the frame based on the given [`base`](`.get_base()`) and
+    /// [`len`](`.get_len()`) parameters.
+    pub fn get_args(&self, base: usize, len: usize) -> &[Argument] {
+        &self.arguments[base..(base + len)]
+    }
+
+    /// Move arguments for the frame based on the given [`base`](`.get_base()`) and
+    /// [`len`](`.get_len()`) parameters.
+    pub fn drain_args(&mut self, base: usize, len: usize) -> impl Iterator<Item = Argument> + '_ {
+        self.arguments.drain(base..(base + len))
+    }
+}

crates/nu-protocol/src/engine/call.rs

@@ -0,0 +1,223 @@
+use crate::{
+    ast::{self, Expression},
+    ir, DeclId, FromValue, ShellError, Span, Value,
+};
+
+use super::{EngineState, Stack, StateWorkingSet};
+
+/// This is a HACK to help [`Command`](super::Command) support both the old AST evaluator and the
+/// new IR evaluator at the same time. It should be removed once we are satisfied with the new
+/// evaluator.
+#[derive(Debug, Clone)]
+pub struct Call<'a> {
+    pub head: Span,
+    pub decl_id: DeclId,
+    pub inner: CallImpl<'a>,
+}
+
+#[derive(Debug, Clone)]
+pub enum CallImpl<'a> {
+    AstRef(&'a ast::Call),
+    AstBox(Box<ast::Call>),
+    IrRef(&'a ir::Call),
+    IrBox(Box<ir::Call>),
+}
+
+impl Call<'_> {
+    /// Returns a new AST call with the given span. This is often used by commands that need an
+    /// empty call to pass to a command. It's not easily possible to add anything to this.
+    pub fn new(span: Span) -> Self {
+        // this is using the boxed variant, which isn't so efficient... but this is only temporary
+        // anyway.
+        Call {
+            head: span,
+            decl_id: 0,
+            inner: CallImpl::AstBox(Box::new(ast::Call::new(span))),
+        }
+    }
+
+    /// Convert the `Call` from any lifetime into `'static`, by cloning the data within onto the
+    /// heap.
+    pub fn to_owned(&self) -> Call<'static> {
+        Call {
+            head: self.head,
+            decl_id: self.decl_id,
+            inner: self.inner.to_owned(),
+        }
+    }
+
+    /// Assert that the call is `ast::Call`, and fail with an error if it isn't.
+    ///
+    /// Provided as a stop-gap for commands that can't work with `ir::Call`, or just haven't been
+    /// implemented yet. Eventually these issues should be resolved and then this can be removed.
+    pub fn assert_ast_call(&self) -> Result<&ast::Call, ShellError> {
+        match &self.inner {
+            CallImpl::AstRef(call) => Ok(call),
+            CallImpl::AstBox(call) => Ok(call),
+            _ => Err(ShellError::NushellFailedSpanned {
+                msg: "Can't be used in IR context".into(),
+                label: "this command is not yet supported by IR evaluation".into(),
+                span: self.head,
+            }),
+        }
+    }
+
+    /// FIXME: implementation asserts `ast::Call` and proxies to that
+    pub fn has_flag_const(
+        &self,
+        working_set: &StateWorkingSet,
+        flag_name: &str,
+    ) -> Result<bool, ShellError> {
+        self.assert_ast_call()?
+            .has_flag_const(working_set, flag_name)
+    }
+
+    /// FIXME: implementation asserts `ast::Call` and proxies to that
+    pub fn get_flag_const<T: FromValue>(
+        &self,
+        working_set: &StateWorkingSet,
+        name: &str,
+    ) -> Result<Option<T>, ShellError> {
+        self.assert_ast_call()?.get_flag_const(working_set, name)
+    }
+
+    /// FIXME: implementation asserts `ast::Call` and proxies to that
+    pub fn req_const<T: FromValue>(
+        &self,
+        working_set: &StateWorkingSet,
+        pos: usize,
+    ) -> Result<T, ShellError> {
+        self.assert_ast_call()?.req_const(working_set, pos)
+    }
+
+    /// FIXME: implementation asserts `ast::Call` and proxies to that
+    pub fn rest_const<T: FromValue>(
+        &self,
+        working_set: &StateWorkingSet,
+        starting_pos: usize,
+    ) -> Result<Vec<T>, ShellError> {
+        self.assert_ast_call()?
+            .rest_const(working_set, starting_pos)
+    }
+
+    /// Returns a span covering the call's arguments.
+    pub fn arguments_span(&self) -> Span {
+        match &self.inner {
+            CallImpl::AstRef(call) => call.arguments_span(),
+            CallImpl::AstBox(call) => call.arguments_span(),
+            CallImpl::IrRef(call) => call.arguments_span(),
+            CallImpl::IrBox(call) => call.arguments_span(),
+        }
+    }
+
+    /// Returns a span covering the whole call.
+    pub fn span(&self) -> Span {
+        match &self.inner {
+            CallImpl::AstRef(call) => call.span(),
+            CallImpl::AstBox(call) => call.span(),
+            CallImpl::IrRef(call) => call.span(),
+            CallImpl::IrBox(call) => call.span(),
+        }
+    }
+
+    /// Get a parser info argument by name.
+    pub fn get_parser_info<'a>(&'a self, stack: &'a Stack, name: &str) -> Option<&'a Expression> {
+        match &self.inner {
+            CallImpl::AstRef(call) => call.get_parser_info(name),
+            CallImpl::AstBox(call) => call.get_parser_info(name),
+            CallImpl::IrRef(call) => call.get_parser_info(stack, name),
+            CallImpl::IrBox(call) => call.get_parser_info(stack, name),
+        }
+    }
+
+    /// Evaluator-agnostic implementation of `rest_iter_flattened()`. Evaluates or gets all of the
+    /// positional and spread arguments, flattens spreads, and then returns one list of values.
+    pub fn rest_iter_flattened(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        eval_expression: fn(
+            &EngineState,
+            &mut Stack,
+            &ast::Expression,
+        ) -> Result<Value, ShellError>,
+        starting_pos: usize,
+    ) -> Result<Vec<Value>, ShellError> {
+        fn by_ast(
+            call: &ast::Call,
+            engine_state: &EngineState,
+            stack: &mut Stack,
+            eval_expression: fn(
+                &EngineState,
+                &mut Stack,
+                &ast::Expression,
+            ) -> Result<Value, ShellError>,
+            starting_pos: usize,
+        ) -> Result<Vec<Value>, ShellError> {
+            call.rest_iter_flattened(starting_pos, |expr| {
+                eval_expression(engine_state, stack, expr)
+            })
+        }
+
+        fn by_ir(
+            call: &ir::Call,
+            stack: &Stack,
+            starting_pos: usize,
+        ) -> Result<Vec<Value>, ShellError> {
+            call.rest_iter_flattened(stack, starting_pos)
+        }
+
+        match &self.inner {
+            CallImpl::AstRef(call) => {
+                by_ast(call, engine_state, stack, eval_expression, starting_pos)
+            }
+            CallImpl::AstBox(call) => {
+                by_ast(call, engine_state, stack, eval_expression, starting_pos)
+            }
+            CallImpl::IrRef(call) => by_ir(call, stack, starting_pos),
+            CallImpl::IrBox(call) => by_ir(call, stack, starting_pos),
+        }
+    }
+
+    /// Get the original AST expression for a positional argument. Does not usually work for IR
+    /// unless the decl specified `requires_ast_for_arguments()`
+    pub fn positional_nth<'a>(&'a self, stack: &'a Stack, index: usize) -> Option<&'a Expression> {
+        match &self.inner {
+            CallImpl::AstRef(call) => call.positional_nth(index),
+            CallImpl::AstBox(call) => call.positional_nth(index),
+            CallImpl::IrRef(call) => call.positional_ast(stack, index).map(|arc| arc.as_ref()),
+            CallImpl::IrBox(call) => call.positional_ast(stack, index).map(|arc| arc.as_ref()),
+        }
+    }
+}
+
+impl CallImpl<'_> {
+    pub fn to_owned(&self) -> CallImpl<'static> {
+        match self {
+            CallImpl::AstRef(call) => CallImpl::AstBox(Box::new((*call).clone())),
+            CallImpl::AstBox(call) => CallImpl::AstBox(call.clone()),
+            CallImpl::IrRef(call) => CallImpl::IrBox(Box::new((*call).clone())),
+            CallImpl::IrBox(call) => CallImpl::IrBox(call.clone()),
+        }
+    }
+}
+
+impl<'a> From<&'a ast::Call> for Call<'a> {
+    fn from(call: &'a ast::Call) -> Self {
+        Call {
+            head: call.head,
+            decl_id: call.decl_id,
+            inner: CallImpl::AstRef(call),
+        }
+    }
+}
+
+impl<'a> From<&'a ir::Call> for Call<'a> {
+    fn from(call: &'a ir::Call) -> Self {
+        Call {
+            head: call.head,
+            decl_id: call.decl_id,
+            inner: CallImpl::IrRef(call),
+        }
+    }
+}

crates/nu-protocol/src/engine/command.rs

@@ -1,5 +1,5 @@
 use super::{EngineState, Stack, StateWorkingSet};
-use crate::{ast::Call, Alias, BlockId, Example, OutDest, PipelineData, ShellError, Signature};
+use crate::{engine::Call, Alias, BlockId, Example, OutDest, PipelineData, ShellError, Signature};
 use std::fmt::Display;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -124,6 +124,12 @@ pub trait Command: Send + Sync + CommandClone {
     fn pipe_redirection(&self) -> (Option<OutDest>, Option<OutDest>) {
         (None, None)
     }
+
+    /// Return true if the AST nodes for the arguments are required for IR evaluation. This is
+    /// currently inefficient so is not generally done.
+    fn requires_ast_for_arguments(&self) -> bool {
+        false
+    }
 }
 
 pub trait CommandClone {

crates/nu-protocol/src/engine/engine_state.rs

@@ -302,7 +302,7 @@ impl EngineState {
         cwd: impl AsRef<Path>,
     ) -> Result<(), ShellError> {
         for mut scope in stack.env_vars.drain(..) {
-            for (overlay_name, mut env) in scope.drain() {
+            for (overlay_name, mut env) in Arc::make_mut(&mut scope).drain() {
                 if let Some(env_vars) = Arc::make_mut(&mut self.env_vars).get_mut(&overlay_name) {
                     // Updating existing overlay
                     env_vars.extend(env.drain());

crates/nu-protocol/src/engine/error_handler.rs

@@ -0,0 +1,55 @@
+use crate::RegId;
+
+/// Describes an error handler stored during IR evaluation.
+#[derive(Debug, Clone, Copy)]
+pub struct ErrorHandler {
+    /// Instruction index within the block that will handle the error
+    pub handler_index: usize,
+    /// Register to put the error information into, when an error occurs
+    pub error_register: Option<RegId>,
+}
+
+/// Keeps track of error handlers pushed during evaluation of an IR block.
+#[derive(Debug, Clone)]
+pub struct ErrorHandlerStack {
+    handlers: Vec<ErrorHandler>,
+}
+
+impl ErrorHandlerStack {
+    pub const fn new() -> ErrorHandlerStack {
+        ErrorHandlerStack { handlers: vec![] }
+    }
+
+    /// Get the current base of the stack, which establishes a frame.
+    pub fn get_base(&self) -> usize {
+        self.handlers.len()
+    }
+
+    /// Push a new error handler onto the stack.
+    pub fn push(&mut self, handler: ErrorHandler) {
+        self.handlers.push(handler);
+    }
+
+    /// Try to pop an error handler from the stack. Won't go below `base`, to avoid retrieving a
+    /// handler belonging to a parent frame.
+    pub fn pop(&mut self, base: usize) -> Option<ErrorHandler> {
+        if self.handlers.len() > base {
+            self.handlers.pop()
+        } else {
+            None
+        }
+    }
+
+    /// Reset the stack to the state it was in at the beginning of the frame, in preparation to
+    /// return control to the parent frame.
+    pub fn leave_frame(&mut self, base: usize) {
+        if self.handlers.len() >= base {
+            self.handlers.truncate(base);
+        } else {
+            panic!(
+                "ErrorHandlerStack bug: tried to leave frame at {base}, but current base is {}",
+                self.get_base()
+            )
+        }
+    }
+}

crates/nu-protocol/src/engine/mod.rs

@@ -1,8 +1,11 @@
+mod argument;
 mod cached_file;
+mod call;
 mod call_info;
 mod capture_block;
 mod command;
 mod engine_state;
+mod error_handler;
 mod overlay;
 mod pattern_match;
 mod stack;
@@ -14,10 +17,13 @@ mod variable;
 
 pub use cached_file::CachedFile;
 
+pub use argument::*;
+pub use call::*;
 pub use call_info::*;
 pub use capture_block::*;
 pub use command::*;
 pub use engine_state::*;
+pub use error_handler::*;
 pub use overlay::*;
 pub use pattern_match::*;
 pub use stack::*;

crates/nu-protocol/src/engine/stack.rs

@@ -1,7 +1,7 @@
 use crate::{
     engine::{
-        EngineState, Redirection, StackCallArgGuard, StackCaptureGuard, StackIoGuard, StackOutDest,
-        DEFAULT_OVERLAY_NAME,
+        ArgumentStack, EngineState, ErrorHandlerStack, Redirection, StackCallArgGuard,
+        StackCaptureGuard, StackIoGuard, StackOutDest, DEFAULT_OVERLAY_NAME,
     },
     Config, OutDest, ShellError, Span, Value, VarId, ENV_VARIABLE_ID, NU_VARIABLE_ID,
 };
@@ -36,11 +36,17 @@ pub struct Stack {
     /// Variables
     pub vars: Vec<(VarId, Value)>,
     /// Environment variables arranged as a stack to be able to recover values from parent scopes
-    pub env_vars: Vec<EnvVars>,
+    pub env_vars: Vec<Arc<EnvVars>>,
     /// Tells which environment variables from engine state are hidden, per overlay.
-    pub env_hidden: HashMap<String, HashSet<String>>,
+    pub env_hidden: Arc<HashMap<String, HashSet<String>>>,
     /// List of active overlays
     pub active_overlays: Vec<String>,
+    /// Argument stack for IR evaluation
+    pub arguments: ArgumentStack,
+    /// Error handler stack for IR evaluation
+    pub error_handlers: ErrorHandlerStack,
+    /// Set true to always use IR mode
+    pub use_ir: bool,
     pub recursion_count: u64,
     pub parent_stack: Option<Arc<Stack>>,
     /// Variables that have been deleted (this is used to hide values from parent stack lookups)
@@ -68,8 +74,11 @@ impl Stack {
         Self {
             vars: Vec::new(),
             env_vars: Vec::new(),
-            env_hidden: HashMap::new(),
+            env_hidden: Arc::new(HashMap::new()),
             active_overlays: vec![DEFAULT_OVERLAY_NAME.to_string()],
+            arguments: ArgumentStack::new(),
+            error_handlers: ErrorHandlerStack::new(),
+            use_ir: false,
             recursion_count: 0,
             parent_stack: None,
             parent_deletions: vec![],
@@ -88,6 +97,9 @@ impl Stack {
             env_vars: parent.env_vars.clone(),
             env_hidden: parent.env_hidden.clone(),
             active_overlays: parent.active_overlays.clone(),
+            arguments: ArgumentStack::new(),
+            error_handlers: ErrorHandlerStack::new(),
+            use_ir: parent.use_ir,
             recursion_count: parent.recursion_count,
             vars: vec![],
             parent_deletions: vec![],
@@ -124,8 +136,8 @@ impl Stack {
 
     pub fn with_env(
         &mut self,
-        env_vars: &[EnvVars],
-        env_hidden: &HashMap<String, HashSet<String>>,
+        env_vars: &[Arc<EnvVars>],
+        env_hidden: &Arc<HashMap<String, HashSet<String>>>,
     ) {
         // Do not clone the environment if it hasn't changed
         if self.env_vars.iter().any(|scope| !scope.is_empty()) {
@@ -242,23 +254,24 @@ impl Stack {
 
     pub fn add_env_var(&mut self, var: String, value: Value) {
         if let Some(last_overlay) = self.active_overlays.last() {
-            if let Some(env_hidden) = self.env_hidden.get_mut(last_overlay) {
+            if let Some(env_hidden) = Arc::make_mut(&mut self.env_hidden).get_mut(last_overlay) {
                 // if the env var was hidden, let's activate it again
                 env_hidden.remove(&var);
             }
 
             if let Some(scope) = self.env_vars.last_mut() {
+                let scope = Arc::make_mut(scope);
                 if let Some(env_vars) = scope.get_mut(last_overlay) {
                     env_vars.insert(var, value);
                 } else {
                     scope.insert(last_overlay.into(), [(var, value)].into_iter().collect());
                 }
             } else {
-                self.env_vars.push(
+                self.env_vars.push(Arc::new(
                     [(last_overlay.into(), [(var, value)].into_iter().collect())]
                         .into_iter()
                         .collect(),
-                );
+                ));
             }
         } else {
             // TODO: Remove panic
@@ -280,15 +293,17 @@ impl Stack {
     }
 
     pub fn captures_to_stack_preserve_out_dest(&self, captures: Vec<(VarId, Value)>) -> Stack {
-        // FIXME: this is probably slow
         let mut env_vars = self.env_vars.clone();
-        env_vars.push(HashMap::new());
+        env_vars.push(Arc::new(HashMap::new()));
 
         Stack {
             vars: captures,
             env_vars,
             env_hidden: self.env_hidden.clone(),
             active_overlays: self.active_overlays.clone(),
+            arguments: ArgumentStack::new(),
+            error_handlers: ErrorHandlerStack::new(),
+            use_ir: self.use_ir,
             recursion_count: self.recursion_count,
             parent_stack: None,
             parent_deletions: vec![],
@@ -298,7 +313,7 @@ impl Stack {
     }
 
     pub fn gather_captures(&self, engine_state: &EngineState, captures: &[VarId]) -> Stack {
-        let mut vars = vec![];
+        let mut vars = Vec::with_capacity(captures.len());
 
         let fake_span = Span::new(0, 0);
 
@@ -313,13 +328,16 @@ impl Stack {
         }
 
         let mut env_vars = self.env_vars.clone();
-        env_vars.push(HashMap::new());
+        env_vars.push(Arc::new(HashMap::new()));
 
         Stack {
             vars,
             env_vars,
             env_hidden: self.env_hidden.clone(),
             active_overlays: self.active_overlays.clone(),
+            arguments: ArgumentStack::new(),
+            error_handlers: ErrorHandlerStack::new(),
+            use_ir: self.use_ir,
             recursion_count: self.recursion_count,
             parent_stack: None,
             parent_deletions: vec![],
@@ -481,6 +499,7 @@ impl Stack {
 
     pub fn remove_env_var(&mut self, engine_state: &EngineState, name: &str) -> bool {
         for scope in self.env_vars.iter_mut().rev() {
+            let scope = Arc::make_mut(scope);
             for active_overlay in self.active_overlays.iter().rev() {
                 if let Some(env_vars) = scope.get_mut(active_overlay) {
                     if env_vars.remove(name).is_some() {
@@ -493,10 +512,11 @@ impl Stack {
         for active_overlay in self.active_overlays.iter().rev() {
             if let Some(env_vars) = engine_state.env_vars.get(active_overlay) {
                 if env_vars.get(name).is_some() {
-                    if let Some(env_hidden) = self.env_hidden.get_mut(active_overlay) {
-                        env_hidden.insert(name.into());
+                    let env_hidden = Arc::make_mut(&mut self.env_hidden);
+                    if let Some(env_hidden_in_overlay) = env_hidden.get_mut(active_overlay) {
+                        env_hidden_in_overlay.insert(name.into());
                     } else {
-                        self.env_hidden
+                        env_hidden
                             .insert(active_overlay.into(), [name.into()].into_iter().collect());
                     }
 

crates/nu-protocol/src/engine/state_working_set.rs

@@ -4,8 +4,8 @@ use crate::{
         usage::build_usage, CachedFile, Command, CommandType, EngineState, OverlayFrame,
         StateDelta, Variable, VirtualPath, Visibility,
     },
-    BlockId, Category, Config, DeclId, FileId, GetSpan, Module, ModuleId, ParseError, ParseWarning,
-    Span, SpanId, Type, Value, VarId, VirtualPathId,
+    BlockId, Category, CompileError, Config, DeclId, FileId, GetSpan, Module, ModuleId, ParseError,
+    ParseWarning, Span, SpanId, Type, Value, VarId, VirtualPathId,
 };
 use core::panic;
 use std::{
@@ -31,6 +31,7 @@ pub struct StateWorkingSet<'a> {
     pub search_predecls: bool,
     pub parse_errors: Vec<ParseError>,
     pub parse_warnings: Vec<ParseWarning>,
+    pub compile_errors: Vec<CompileError>,
 }
 
 impl<'a> StateWorkingSet<'a> {
@@ -50,6 +51,7 @@ impl<'a> StateWorkingSet<'a> {
             search_predecls: true,
             parse_errors: vec![],
             parse_warnings: vec![],
+            compile_errors: vec![],
         }
     }
 
@@ -260,6 +262,12 @@ impl<'a> StateWorkingSet<'a> {
     }
 
     pub fn add_block(&mut self, block: Arc<Block>) -> BlockId {
+        log::trace!(
+            "block id={} added, has IR = {:?}",
+            self.num_blocks(),
+            block.ir_block.is_some()
+        );
+
         self.delta.blocks.push(block);
 
         self.num_blocks() - 1

crates/nu-protocol/src/errors/cli_error.rs

@@ -107,4 +107,8 @@ impl<'src> miette::Diagnostic for CliError<'src> {
     fn related<'a>(&'a self) -> Option<Box<dyn Iterator<Item = &'a dyn miette::Diagnostic> + 'a>> {
         self.0.related()
     }
+
+    fn diagnostic_source(&self) -> Option<&dyn miette::Diagnostic> {
+        self.0.diagnostic_source()
+    }
 }

crates/nu-protocol/src/errors/compile_error.rs

@@ -0,0 +1,238 @@
+use crate::{RegId, Span};
+use miette::Diagnostic;
+use serde::{Deserialize, Serialize};
+use thiserror::Error;
+
+/// An internal compiler error, generally means a Nushell bug rather than an issue with user error
+/// since parsing and typechecking has already passed.
+#[derive(Debug, Clone, Error, Diagnostic, PartialEq, Serialize, Deserialize)]
+pub enum CompileError {
+    #[error("Register overflow.")]
+    #[diagnostic(code(nu::compile::register_overflow))]
+    RegisterOverflow {
+        #[label("the code being compiled is probably too large")]
+        block_span: Option<Span>,
+    },
+
+    #[error("Register {reg_id} was uninitialized when used, possibly reused.")]
+    #[diagnostic(
+        code(nu::compile::register_uninitialized),
+        help("this is a compiler bug. Please report it at https://github.com/nushell/nushell/issues/new\nfrom: {caller}"),
+    )]
+    RegisterUninitialized { reg_id: RegId, caller: String },
+
+    #[error("Register {reg_id} was uninitialized when used, possibly reused.")]
+    #[diagnostic(
+        code(nu::compile::register_uninitialized),
+        help("this is a compiler bug. Please report it at https://github.com/nushell/nushell/issues/new\nfrom: {caller}"),
+    )]
+    RegisterUninitializedWhilePushingInstruction {
+        reg_id: RegId,
+        caller: String,
+        instruction: String,
+        #[label("while adding this instruction: {instruction}")]
+        span: Span,
+    },
+
+    #[error("Block contains too much string data: maximum 4 GiB exceeded.")]
+    #[diagnostic(
+        code(nu::compile::data_overflow),
+        help("try loading the string data from a file instead")
+    )]
+    DataOverflow {
+        #[label("while compiling this block")]
+        block_span: Option<Span>,
+    },
+
+    #[error("Block contains too many files.")]
+    #[diagnostic(
+        code(nu::compile::register_overflow),
+        help("try using fewer file redirections")
+    )]
+    FileOverflow {
+        #[label("while compiling this block")]
+        block_span: Option<Span>,
+    },
+
+    #[error("Invalid redirect mode: File should not be specified by commands.")]
+    #[diagnostic(
+        code(nu::compile::invalid_redirect_mode),
+        help("this is a command bug. Please report it at https://github.com/nushell/nushell/issues/new")
+    )]
+    InvalidRedirectMode {
+        #[label("while compiling this expression")]
+        span: Span,
+    },
+
+    #[error("Encountered garbage, likely due to parse error.")]
+    #[diagnostic(code(nu::compile::garbage))]
+    Garbage {
+        #[label("garbage found here")]
+        span: Span,
+    },
+
+    #[error("Unsupported operator expression.")]
+    #[diagnostic(code(nu::compile::unsupported_operator_expression))]
+    UnsupportedOperatorExpression {
+        #[label("this expression is in operator position but is not an operator")]
+        span: Span,
+    },
+
+    #[error("Attempted access of $env by integer path.")]
+    #[diagnostic(code(nu::compile::access_env_by_int))]
+    AccessEnvByInt {
+        #[label("$env keys should be strings")]
+        span: Span,
+    },
+
+    #[error("Encountered invalid `{keyword}` keyword call.")]
+    #[diagnostic(code(nu::compile::invalid_keyword_call))]
+    InvalidKeywordCall {
+        keyword: String,
+        #[label("this call is not properly formed")]
+        span: Span,
+    },
+
+    #[error("Attempted to set branch target of non-branch instruction.")]
+    #[diagnostic(
+        code(nu::compile::set_branch_target_of_non_branch_instruction),
+        help("this is a compiler bug. Please report it at https://github.com/nushell/nushell/issues/new"),
+    )]
+    SetBranchTargetOfNonBranchInstruction {
+        instruction: String,
+        #[label("tried to modify: {instruction}")]
+        span: Span,
+    },
+
+    /// You're trying to run an unsupported external command.
+    ///
+    /// ## Resolution
+    ///
+    /// Make sure there's an appropriate `run-external` declaration for this external command.
+    #[error("External calls are not supported.")]
+    #[diagnostic(
+        code(nu::compile::run_external_not_found),
+        help("`run-external` was not found in scope")
+    )]
+    RunExternalNotFound {
+        #[label("can't be run in this context")]
+        span: Span,
+    },
+
+    /// Invalid assignment left-hand side
+    ///
+    /// ## Resolution
+    ///
+    /// Assignment requires that you assign to a variable or variable cell path.
+    #[error("Assignment operations require a variable.")]
+    #[diagnostic(
+        code(nu::compile::assignment_requires_variable),
+        help("try assigning to a variable or a cell path of a variable")
+    )]
+    AssignmentRequiresVar {
+        #[label("needs to be a variable")]
+        span: Span,
+    },
+
+    /// Invalid assignment left-hand side
+    ///
+    /// ## Resolution
+    ///
+    /// Assignment requires that you assign to a mutable variable or cell path.
+    #[error("Assignment to an immutable variable.")]
+    #[diagnostic(
+        code(nu::compile::assignment_requires_mutable_variable),
+        help("declare the variable with `mut`, or shadow it again with `let`")
+    )]
+    AssignmentRequiresMutableVar {
+        #[label("needs to be a mutable variable")]
+        span: Span,
+    },
+
+    /// This environment variable cannot be set manually.
+    ///
+    /// ## Resolution
+    ///
+    /// This environment variable is set automatically by Nushell and cannot not be set manually.
+    #[error("{envvar_name} cannot be set manually.")]
+    #[diagnostic(
+        code(nu::compile::automatic_env_var_set_manually),
+        help(
+            r#"The environment variable '{envvar_name}' is set automatically by Nushell and cannot be set manually."#
+        )
+    )]
+    AutomaticEnvVarSetManually {
+        envvar_name: String,
+        #[label("cannot set '{envvar_name}' manually")]
+        span: Span,
+    },
+
+    /// It is not possible to replace the entire environment at once
+    ///
+    /// ## Resolution
+    ///
+    /// Setting the entire environment is not allowed. Change environment variables individually
+    /// instead.
+    #[error("Cannot replace environment.")]
+    #[diagnostic(
+        code(nu::compile::cannot_replace_env),
+        help("Assigning a value to '$env' is not allowed.")
+    )]
+    CannotReplaceEnv {
+        #[label("setting '$env' not allowed")]
+        span: Span,
+    },
+
+    #[error("Unexpected expression.")]
+    #[diagnostic(code(nu::compile::unexpected_expression))]
+    UnexpectedExpression {
+        expr_name: String,
+        #[label("{expr_name} is not allowed in this context")]
+        span: Span,
+    },
+
+    #[error("Missing required declaration: `{decl_name}`")]
+    #[diagnostic(code(nu::compile::missing_required_declaration))]
+    MissingRequiredDeclaration {
+        decl_name: String,
+        #[label("`{decl_name}` must be in scope to compile this expression")]
+        span: Span,
+    },
+
+    #[error("Invalid literal")]
+    #[diagnostic(code(nu::compile::invalid_literal))]
+    InvalidLiteral {
+        msg: String,
+        #[label("{msg}")]
+        span: Span,
+    },
+
+    #[error("{msg}")]
+    #[diagnostic(code(nu::compile::not_in_a_loop))]
+    NotInALoop {
+        msg: String,
+        #[label("can't be used outside of a loop")]
+        span: Option<Span>,
+    },
+
+    #[error("Incoherent loop state: the loop that ended was not the one we were expecting.")]
+    #[diagnostic(
+        code(nu::compile::incoherent_loop_state),
+        help("this is a compiler bug. Please report it at https://github.com/nushell/nushell/issues/new"),
+    )]
+    IncoherentLoopState {
+        #[label("while compiling this block")]
+        block_span: Option<Span>,
+    },
+
+    #[error("Undefined label `{label_id}`.")]
+    #[diagnostic(
+        code(nu::compile::undefined_label),
+        help("this is a compiler bug. Please report it at https://github.com/nushell/nushell/issues/new"),
+    )]
+    UndefinedLabel {
+        label_id: usize,
+        #[label("label was used while compiling this code")]
+        span: Option<Span>,
+    },
+}

crates/nu-protocol/src/errors/mod.rs

@@ -1,10 +1,12 @@
 pub mod cli_error;
+mod compile_error;
 mod labeled_error;
 mod parse_error;
 mod parse_warning;
 mod shell_error;
 
 pub use cli_error::{format_error, report_error, report_error_new};
+pub use compile_error::CompileError;
 pub use labeled_error::{ErrorLabel, LabeledError};
 pub use parse_error::{DidYouMean, ParseError};
 pub use parse_warning::ParseWarning;

crates/nu-protocol/src/errors/shell_error.rs

@@ -1376,6 +1376,23 @@ On Windows, this would be %USERPROFILE%\AppData\Roaming"#
         help("Set XDG_CONFIG_HOME to an absolute path, or set it to an empty string to ignore it")
     )]
     InvalidXdgConfig { xdg: String, default: String },
+
+    /// An unexpected error occurred during IR evaluation.
+    ///
+    /// ## Resolution
+    ///
+    /// This is most likely a correctness issue with the IR compiler or evaluator. Please file a
+    /// bug with the minimum code needed to reproduce the issue, if possible.
+    #[error("IR evaluation error: {msg}")]
+    #[diagnostic(
+        code(nu::shell::ir_eval_error),
+        help("this is a bug, please report it at https://github.com/nushell/nushell/issues/new along with the code you were running if able")
+    )]
+    IrEvalError {
+        msg: String,
+        #[label = "while running this code"]
+        span: Option<Span>,
+    },
 }
 
 // TODO: Implement as From trait

crates/nu-protocol/src/eval_const.rs

@@ -307,7 +307,7 @@ fn eval_const_call(
         return Err(ShellError::NotAConstHelp { span: call.head });
     }
 
-    decl.run_const(working_set, call, input)
+    decl.run_const(working_set, &call.into(), input)
 }
 
 pub fn eval_const_subexpression(

crates/nu-protocol/src/id.rs

@@ -7,5 +7,19 @@ pub type ModuleId = usize;
 pub type OverlayId = usize;
 pub type FileId = usize;
 pub type VirtualPathId = usize;
+
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
 pub struct SpanId(pub usize); // more robust ID style used in the new parser
+
+/// An ID for an [IR](crate::ir) register. `%n` is a common shorthand for `RegId(n)`.
+///
+/// Note: `%0` is allocated with the block input at the beginning of a compiled block.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
+#[repr(transparent)]
+pub struct RegId(pub u32);
+
+impl std::fmt::Display for RegId {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "%{}", self.0)
+    }
+}

crates/nu-protocol/src/ir/call.rs

@@ -0,0 +1,351 @@
+use std::sync::Arc;
+
+use crate::{
+    ast::Expression,
+    engine::{self, Argument, Stack},
+    DeclId, ShellError, Span, Spanned, Value,
+};
+
+use super::DataSlice;
+
+/// Contains the information for a call being made to a declared command.
+#[derive(Debug, Clone)]
+pub struct Call {
+    /// The declaration ID of the command to be invoked.
+    pub decl_id: DeclId,
+    /// The span encompassing the command name, before the arguments.
+    pub head: Span,
+    /// The span encompassing the command name and all arguments.
+    pub span: Span,
+    /// The base index of the arguments for this call within the
+    /// [argument stack](crate::engine::ArgumentStack).
+    pub args_base: usize,
+    /// The number of [`Argument`]s for the call. Note that this just counts the number of
+    /// `Argument` entries on the stack, and has nothing to do with the actual number of positional
+    /// or spread arguments.
+    pub args_len: usize,
+}
+
+impl Call {
+    /// Build a new call with arguments.
+    pub fn build(decl_id: DeclId, head: Span) -> CallBuilder {
+        CallBuilder {
+            inner: Call {
+                decl_id,
+                head,
+                span: head,
+                args_base: 0,
+                args_len: 0,
+            },
+        }
+    }
+
+    /// Get the arguments for this call from the arguments stack.
+    pub fn arguments<'a>(&self, stack: &'a Stack) -> &'a [Argument] {
+        stack.arguments.get_args(self.args_base, self.args_len)
+    }
+
+    /// The span encompassing the arguments
+    ///
+    /// If there are no arguments the span covers where the first argument would exist
+    ///
+    /// If there are one or more arguments the span encompasses the start of the first argument to
+    /// end of the last argument
+    pub fn arguments_span(&self) -> Span {
+        let past = self.head.past();
+        Span::new(past.start, self.span.end)
+    }
+
+    /// The number of named arguments, with or without values.
+    pub fn named_len(&self, stack: &Stack) -> usize {
+        self.arguments(stack)
+            .iter()
+            .filter(|arg| matches!(arg, Argument::Named { .. } | Argument::Flag { .. }))
+            .count()
+    }
+
+    /// Iterate through named arguments, with or without values.
+    pub fn named_iter<'a>(
+        &'a self,
+        stack: &'a Stack,
+    ) -> impl Iterator<Item = (Spanned<&'a str>, Option<&'a Value>)> + 'a {
+        self.arguments(stack).iter().filter_map(
+            |arg: &Argument| -> Option<(Spanned<&str>, Option<&Value>)> {
+                match arg {
+                    Argument::Flag {
+                        data, name, span, ..
+                    } => Some((
+                        Spanned {
+                            item: std::str::from_utf8(&data[*name]).expect("invalid arg name"),
+                            span: *span,
+                        },
+                        None,
+                    )),
+                    Argument::Named {
+                        data,
+                        name,
+                        span,
+                        val,
+                        ..
+                    } => Some((
+                        Spanned {
+                            item: std::str::from_utf8(&data[*name]).expect("invalid arg name"),
+                            span: *span,
+                        },
+                        Some(val),
+                    )),
+                    _ => None,
+                }
+            },
+        )
+    }
+
+    /// Get a named argument's value by name. Returns [`None`] for named arguments with no value as
+    /// well.
+    pub fn get_named_arg<'a>(&self, stack: &'a Stack, flag_name: &str) -> Option<&'a Value> {
+        // Optimized to avoid str::from_utf8()
+        self.arguments(stack)
+            .iter()
+            .find_map(|arg: &Argument| -> Option<Option<&Value>> {
+                match arg {
+                    Argument::Flag { data, name, .. } if &data[*name] == flag_name.as_bytes() => {
+                        Some(None)
+                    }
+                    Argument::Named {
+                        data, name, val, ..
+                    } if &data[*name] == flag_name.as_bytes() => Some(Some(val)),
+                    _ => None,
+                }
+            })
+            .flatten()
+    }
+
+    /// The number of positional arguments, excluding spread arguments.
+    pub fn positional_len(&self, stack: &Stack) -> usize {
+        self.arguments(stack)
+            .iter()
+            .filter(|arg| matches!(arg, Argument::Positional { .. }))
+            .count()
+    }
+
+    /// Iterate through positional arguments. Does not include spread arguments.
+    pub fn positional_iter<'a>(&self, stack: &'a Stack) -> impl Iterator<Item = &'a Value> {
+        self.arguments(stack).iter().filter_map(|arg| match arg {
+            Argument::Positional { val, .. } => Some(val),
+            _ => None,
+        })
+    }
+
+    /// Get a positional argument by index. Does not include spread arguments.
+    pub fn positional_nth<'a>(&self, stack: &'a Stack, index: usize) -> Option<&'a Value> {
+        self.positional_iter(stack).nth(index)
+    }
+
+    /// Get the AST node for a positional argument by index. Not usually available unless the decl
+    /// required it.
+    pub fn positional_ast<'a>(
+        &self,
+        stack: &'a Stack,
+        index: usize,
+    ) -> Option<&'a Arc<Expression>> {
+        self.arguments(stack)
+            .iter()
+            .filter_map(|arg| match arg {
+                Argument::Positional { ast, .. } => Some(ast),
+                _ => None,
+            })
+            .nth(index)
+            .and_then(|option| option.as_ref())
+    }
+
+    /// Returns every argument to the rest parameter, as well as whether each argument
+    /// is spread or a normal positional argument (true for spread, false for normal)
+    pub fn rest_iter<'a>(
+        &self,
+        stack: &'a Stack,
+        start: usize,
+    ) -> impl Iterator<Item = (&'a Value, bool)> + 'a {
+        self.arguments(stack)
+            .iter()
+            .filter_map(|arg| match arg {
+                Argument::Positional { val, .. } => Some((val, false)),
+                Argument::Spread { vals, .. } => Some((vals, true)),
+                _ => None,
+            })
+            .skip(start)
+    }
+
+    /// Returns all of the positional arguments including and after `start`, with spread arguments
+    /// flattened into a single `Vec`.
+    pub fn rest_iter_flattened(
+        &self,
+        stack: &Stack,
+        start: usize,
+    ) -> Result<Vec<Value>, ShellError> {
+        let mut acc = vec![];
+        for (rest_val, spread) in self.rest_iter(stack, start) {
+            if spread {
+                match rest_val {
+                    Value::List { vals, .. } => acc.extend(vals.iter().cloned()),
+                    Value::Error { error, .. } => return Err(ShellError::clone(error)),
+                    _ => {
+                        return Err(ShellError::CannotSpreadAsList {
+                            span: rest_val.span(),
+                        })
+                    }
+                }
+            } else {
+                acc.push(rest_val.clone());
+            }
+        }
+        Ok(acc)
+    }
+
+    /// Get a parser info argument by name.
+    pub fn get_parser_info<'a>(&self, stack: &'a Stack, name: &str) -> Option<&'a Expression> {
+        self.arguments(stack)
+            .iter()
+            .find_map(|argument| match argument {
+                Argument::ParserInfo {
+                    data,
+                    name: name_slice,
+                    info: expr,
+                } if &data[*name_slice] == name.as_bytes() => Some(expr.as_ref()),
+                _ => None,
+            })
+    }
+
+    /// Returns a span encompassing the entire call.
+    pub fn span(&self) -> Span {
+        self.span
+    }
+
+    /// Resets the [`Stack`] to its state before the call was made.
+    pub fn leave(&self, stack: &mut Stack) {
+        stack.arguments.leave_frame(self.args_base);
+    }
+}
+
+/// Utility struct for building a [`Call`] with arguments on the [`Stack`].
+pub struct CallBuilder {
+    inner: Call,
+}
+
+impl CallBuilder {
+    /// Add an argument to the [`Stack`] and reference it from the [`Call`].
+    pub fn add_argument(&mut self, stack: &mut Stack, argument: Argument) -> &mut Self {
+        if self.inner.args_len == 0 {
+            self.inner.args_base = stack.arguments.get_base();
+        }
+        self.inner.args_len += 1;
+        if let Some(span) = argument.span() {
+            self.inner.span = self.inner.span.append(span);
+        }
+        stack.arguments.push(argument);
+        self
+    }
+
+    /// Add a positional argument to the [`Stack`] and reference it from the [`Call`].
+    pub fn add_positional(&mut self, stack: &mut Stack, span: Span, val: Value) -> &mut Self {
+        self.add_argument(
+            stack,
+            Argument::Positional {
+                span,
+                val,
+                ast: None,
+            },
+        )
+    }
+
+    /// Add a spread argument to the [`Stack`] and reference it from the [`Call`].
+    pub fn add_spread(&mut self, stack: &mut Stack, span: Span, vals: Value) -> &mut Self {
+        self.add_argument(
+            stack,
+            Argument::Spread {
+                span,
+                vals,
+                ast: None,
+            },
+        )
+    }
+
+    /// Add a flag (no-value named) argument to the [`Stack`] and reference it from the [`Call`].
+    pub fn add_flag(
+        &mut self,
+        stack: &mut Stack,
+        name: impl AsRef<str>,
+        short: impl AsRef<str>,
+        span: Span,
+    ) -> &mut Self {
+        let (data, name, short) = data_from_name_and_short(name.as_ref(), short.as_ref());
+        self.add_argument(
+            stack,
+            Argument::Flag {
+                data,
+                name,
+                short,
+                span,
+            },
+        )
+    }
+
+    /// Add a named argument to the [`Stack`] and reference it from the [`Call`].
+    pub fn add_named(
+        &mut self,
+        stack: &mut Stack,
+        name: impl AsRef<str>,
+        short: impl AsRef<str>,
+        span: Span,
+        val: Value,
+    ) -> &mut Self {
+        let (data, name, short) = data_from_name_and_short(name.as_ref(), short.as_ref());
+        self.add_argument(
+            stack,
+            Argument::Named {
+                data,
+                name,
+                short,
+                span,
+                val,
+                ast: None,
+            },
+        )
+    }
+
+    /// Produce the finished [`Call`] from the builder.
+    ///
+    /// The call should be entered / run before any other calls are constructed, because the
+    /// argument stack will be reset when they exit.
+    pub fn finish(&self) -> Call {
+        self.inner.clone()
+    }
+
+    /// Run a closure with the [`Call`] as an [`engine::Call`] reference, and then clean up the
+    /// arguments that were added to the [`Stack`] after.
+    ///
+    /// For convenience. Calls [`Call::leave`] after the closure ends.
+    pub fn with<T>(
+        self,
+        stack: &mut Stack,
+        f: impl FnOnce(&mut Stack, &engine::Call<'_>) -> T,
+    ) -> T {
+        let call = engine::Call::from(&self.inner);
+        let result = f(stack, &call);
+        self.inner.leave(stack);
+        result
+    }
+}
+
+fn data_from_name_and_short(name: &str, short: &str) -> (Arc<[u8]>, DataSlice, DataSlice) {
+    let data: Vec<u8> = name.bytes().chain(short.bytes()).collect();
+    let data: Arc<[u8]> = data.into();
+    let name = DataSlice {
+        start: 0,
+        len: name.len().try_into().expect("flag name too big"),
+    };
+    let short = DataSlice {
+        start: name.start.checked_add(name.len).expect("flag name too big"),
+        len: short.len().try_into().expect("flag short name too big"),
+    };
+    (data, name, short)
+}

crates/nu-protocol/src/ir/display.rs

@@ -0,0 +1,452 @@
+use std::fmt;
+
+use crate::{ast::Pattern, engine::EngineState, DeclId, VarId};
+
+use super::{DataSlice, Instruction, IrBlock, Literal, RedirectMode};
+
+pub struct FmtIrBlock<'a> {
+    pub(super) engine_state: &'a EngineState,
+    pub(super) ir_block: &'a IrBlock,
+}
+
+impl<'a> fmt::Display for FmtIrBlock<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let plural = |count| if count == 1 { "" } else { "s" };
+        writeln!(
+            f,
+            "# {} register{}, {} instruction{}, {} byte{} of data",
+            self.ir_block.register_count,
+            plural(self.ir_block.register_count as usize),
+            self.ir_block.instructions.len(),
+            plural(self.ir_block.instructions.len()),
+            self.ir_block.data.len(),
+            plural(self.ir_block.data.len()),
+        )?;
+        if self.ir_block.file_count > 0 {
+            writeln!(
+                f,
+                "# {} file{} used for redirection",
+                self.ir_block.file_count,
+                plural(self.ir_block.file_count as usize)
+            )?;
+        }
+        for (index, instruction) in self.ir_block.instructions.iter().enumerate() {
+            let formatted = format!(
+                "{:-4}: {}",
+                index,
+                FmtInstruction {
+                    engine_state: self.engine_state,
+                    instruction,
+                    data: &self.ir_block.data,
+                }
+            );
+            let comment = &self.ir_block.comments[index];
+            if comment.is_empty() {
+                writeln!(f, "{formatted}")?;
+            } else {
+                writeln!(f, "{formatted:40} # {comment}")?;
+            }
+        }
+        Ok(())
+    }
+}
+
+pub struct FmtInstruction<'a> {
+    pub(super) engine_state: &'a EngineState,
+    pub(super) instruction: &'a Instruction,
+    pub(super) data: &'a [u8],
+}
+
+impl<'a> fmt::Display for FmtInstruction<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        const WIDTH: usize = 22;
+
+        match self.instruction {
+            Instruction::Unreachable => {
+                write!(f, "{:WIDTH$}", "unreachable")
+            }
+            Instruction::LoadLiteral { dst, lit } => {
+                let lit = FmtLiteral {
+                    literal: lit,
+                    data: self.data,
+                };
+                write!(f, "{:WIDTH$} {dst}, {lit}", "load-literal")
+            }
+            Instruction::LoadValue { dst, val } => {
+                let val = val.to_debug_string();
+                write!(f, "{:WIDTH$} {dst}, {val}", "load-value")
+            }
+            Instruction::Move { dst, src } => {
+                write!(f, "{:WIDTH$} {dst}, {src}", "move")
+            }
+            Instruction::Clone { dst, src } => {
+                write!(f, "{:WIDTH$} {dst}, {src}", "clone")
+            }
+            Instruction::Collect { src_dst } => {
+                write!(f, "{:WIDTH$} {src_dst}", "collect")
+            }
+            Instruction::Span { src_dst } => {
+                write!(f, "{:WIDTH$} {src_dst}", "span")
+            }
+            Instruction::Drop { src } => {
+                write!(f, "{:WIDTH$} {src}", "drop")
+            }
+            Instruction::Drain { src } => {
+                write!(f, "{:WIDTH$} {src}", "drain")
+            }
+            Instruction::LoadVariable { dst, var_id } => {
+                let var = FmtVar::new(self.engine_state, *var_id);
+                write!(f, "{:WIDTH$} {dst}, {var}", "load-variable")
+            }
+            Instruction::StoreVariable { var_id, src } => {
+                let var = FmtVar::new(self.engine_state, *var_id);
+                write!(f, "{:WIDTH$} {var}, {src}", "store-variable")
+            }
+            Instruction::LoadEnv { dst, key } => {
+                let key = FmtData(self.data, *key);
+                write!(f, "{:WIDTH$} {dst}, {key}", "load-env")
+            }
+            Instruction::LoadEnvOpt { dst, key } => {
+                let key = FmtData(self.data, *key);
+                write!(f, "{:WIDTH$} {dst}, {key}", "load-env-opt")
+            }
+            Instruction::StoreEnv { key, src } => {
+                let key = FmtData(self.data, *key);
+                write!(f, "{:WIDTH$} {key}, {src}", "store-env")
+            }
+            Instruction::PushPositional { src } => {
+                write!(f, "{:WIDTH$} {src}", "push-positional")
+            }
+            Instruction::AppendRest { src } => {
+                write!(f, "{:WIDTH$} {src}", "append-rest")
+            }
+            Instruction::PushFlag { name } => {
+                let name = FmtData(self.data, *name);
+                write!(f, "{:WIDTH$} {name}", "push-flag")
+            }
+            Instruction::PushShortFlag { short } => {
+                let short = FmtData(self.data, *short);
+                write!(f, "{:WIDTH$} {short}", "push-short-flag")
+            }
+            Instruction::PushNamed { name, src } => {
+                let name = FmtData(self.data, *name);
+                write!(f, "{:WIDTH$} {name}, {src}", "push-named")
+            }
+            Instruction::PushShortNamed { short, src } => {
+                let short = FmtData(self.data, *short);
+                write!(f, "{:WIDTH$} {short}, {src}", "push-short-named")
+            }
+            Instruction::PushParserInfo { name, info } => {
+                let name = FmtData(self.data, *name);
+                write!(f, "{:WIDTH$} {name}, {info:?}", "push-parser-info")
+            }
+            Instruction::RedirectOut { mode } => {
+                write!(f, "{:WIDTH$} {mode}", "redirect-out")
+            }
+            Instruction::RedirectErr { mode } => {
+                write!(f, "{:WIDTH$} {mode}", "redirect-err")
+            }
+            Instruction::CheckErrRedirected { src } => {
+                write!(f, "{:WIDTH$} {src}", "check-err-redirected")
+            }
+            Instruction::OpenFile {
+                file_num,
+                path,
+                append,
+            } => {
+                write!(
+                    f,
+                    "{:WIDTH$} file({file_num}), {path}, append = {append:?}",
+                    "open-file"
+                )
+            }
+            Instruction::WriteFile { file_num, src } => {
+                write!(f, "{:WIDTH$} file({file_num}), {src}", "write-file")
+            }
+            Instruction::CloseFile { file_num } => {
+                write!(f, "{:WIDTH$} file({file_num})", "close-file")
+            }
+            Instruction::Call { decl_id, src_dst } => {
+                let decl = FmtDecl::new(self.engine_state, *decl_id);
+                write!(f, "{:WIDTH$} {decl}, {src_dst}", "call")
+            }
+            Instruction::StringAppend { src_dst, val } => {
+                write!(f, "{:WIDTH$} {src_dst}, {val}", "string-append")
+            }
+            Instruction::GlobFrom { src_dst, no_expand } => {
+                let no_expand = if *no_expand { "no-expand" } else { "expand" };
+                write!(f, "{:WIDTH$} {src_dst}, {no_expand}", "glob-from",)
+            }
+            Instruction::ListPush { src_dst, item } => {
+                write!(f, "{:WIDTH$} {src_dst}, {item}", "list-push")
+            }
+            Instruction::ListSpread { src_dst, items } => {
+                write!(f, "{:WIDTH$} {src_dst}, {items}", "list-spread")
+            }
+            Instruction::RecordInsert { src_dst, key, val } => {
+                write!(f, "{:WIDTH$} {src_dst}, {key}, {val}", "record-insert")
+            }
+            Instruction::RecordSpread { src_dst, items } => {
+                write!(f, "{:WIDTH$} {src_dst}, {items}", "record-spread")
+            }
+            Instruction::Not { src_dst } => {
+                write!(f, "{:WIDTH$} {src_dst}", "not")
+            }
+            Instruction::BinaryOp { lhs_dst, op, rhs } => {
+                write!(f, "{:WIDTH$} {lhs_dst}, {op:?}, {rhs}", "binary-op")
+            }
+            Instruction::FollowCellPath { src_dst, path } => {
+                write!(f, "{:WIDTH$} {src_dst}, {path}", "follow-cell-path")
+            }
+            Instruction::CloneCellPath { dst, src, path } => {
+                write!(f, "{:WIDTH$} {dst}, {src}, {path}", "clone-cell-path")
+            }
+            Instruction::UpsertCellPath {
+                src_dst,
+                path,
+                new_value,
+            } => {
+                write!(
+                    f,
+                    "{:WIDTH$} {src_dst}, {path}, {new_value}",
+                    "upsert-cell-path"
+                )
+            }
+            Instruction::Jump { index } => {
+                write!(f, "{:WIDTH$} {index}", "jump")
+            }
+            Instruction::BranchIf { cond, index } => {
+                write!(f, "{:WIDTH$} {cond}, {index}", "branch-if")
+            }
+            Instruction::BranchIfEmpty { src, index } => {
+                write!(f, "{:WIDTH$} {src}, {index}", "branch-if-empty")
+            }
+            Instruction::Match {
+                pattern,
+                src,
+                index,
+            } => {
+                let pattern = FmtPattern {
+                    engine_state: self.engine_state,
+                    pattern,
+                };
+                write!(f, "{:WIDTH$} ({pattern}), {src}, {index}", "match")
+            }
+            Instruction::CheckMatchGuard { src } => {
+                write!(f, "{:WIDTH$} {src}", "check-match-guard")
+            }
+            Instruction::Iterate {
+                dst,
+                stream,
+                end_index,
+            } => {
+                write!(f, "{:WIDTH$} {dst}, {stream}, end {end_index}", "iterate")
+            }
+            Instruction::OnError { index } => {
+                write!(f, "{:WIDTH$} {index}", "on-error")
+            }
+            Instruction::OnErrorInto { index, dst } => {
+                write!(f, "{:WIDTH$} {index}, {dst}", "on-error-into")
+            }
+            Instruction::PopErrorHandler => {
+                write!(f, "{:WIDTH$}", "pop-error-handler")
+            }
+            Instruction::CheckExternalFailed { dst, src } => {
+                write!(f, "{:WIDTH$} {dst}, {src}", "check-external-failed")
+            }
+            Instruction::ReturnEarly { src } => {
+                write!(f, "{:WIDTH$} {src}", "return-early")
+            }
+            Instruction::Return { src } => {
+                write!(f, "{:WIDTH$} {src}", "return")
+            }
+        }
+    }
+}
+
+struct FmtDecl<'a>(DeclId, &'a str);
+
+impl<'a> FmtDecl<'a> {
+    fn new(engine_state: &'a EngineState, decl_id: DeclId) -> Self {
+        FmtDecl(decl_id, engine_state.get_decl(decl_id).name())
+    }
+}
+
+impl fmt::Display for FmtDecl<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "decl {} {:?}", self.0, self.1)
+    }
+}
+
+struct FmtVar<'a>(DeclId, Option<&'a str>);
+
+impl<'a> FmtVar<'a> {
+    fn new(engine_state: &'a EngineState, var_id: VarId) -> Self {
+        // Search for the name of the variable
+        let name: Option<&str> = engine_state
+            .active_overlays(&[])
+            .flat_map(|overlay| overlay.vars.iter())
+            .find(|(_, v)| **v == var_id)
+            .map(|(k, _)| std::str::from_utf8(k).unwrap_or("<utf-8 error>"));
+        FmtVar(var_id, name)
+    }
+}
+
+impl fmt::Display for FmtVar<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let Some(name) = self.1 {
+            write!(f, "var {} {:?}", self.0, name)
+        } else {
+            write!(f, "var {}", self.0)
+        }
+    }
+}
+
+impl fmt::Display for RedirectMode {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            RedirectMode::Pipe => write!(f, "pipe"),
+            RedirectMode::Capture => write!(f, "capture"),
+            RedirectMode::Null => write!(f, "null"),
+            RedirectMode::Inherit => write!(f, "inherit"),
+            RedirectMode::File { file_num } => write!(f, "file({file_num})"),
+            RedirectMode::Caller => write!(f, "caller"),
+        }
+    }
+}
+
+struct FmtData<'a>(&'a [u8], DataSlice);
+
+impl<'a> fmt::Display for FmtData<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let Ok(s) = std::str::from_utf8(&self.0[self.1]) {
+            // Write as string
+            write!(f, "{s:?}")
+        } else {
+            // Write as byte array
+            write!(f, "0x{:x?}", self.0)
+        }
+    }
+}
+
+struct FmtLiteral<'a> {
+    literal: &'a Literal,
+    data: &'a [u8],
+}
+
+impl<'a> fmt::Display for FmtLiteral<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.literal {
+            Literal::Bool(b) => write!(f, "bool({b:?})"),
+            Literal::Int(i) => write!(f, "int({i:?})"),
+            Literal::Float(fl) => write!(f, "float({fl:?})"),
+            Literal::Filesize(q) => write!(f, "filesize({q}b)"),
+            Literal::Duration(q) => write!(f, "duration({q}ns)"),
+            Literal::Binary(b) => write!(f, "binary({})", FmtData(self.data, *b)),
+            Literal::Block(id) => write!(f, "block({id})"),
+            Literal::Closure(id) => write!(f, "closure({id})"),
+            Literal::RowCondition(id) => write!(f, "row_condition({id})"),
+            Literal::Range {
+                start,
+                step,
+                end,
+                inclusion,
+            } => write!(f, "range({start}, {step}, {end}, {inclusion:?})"),
+            Literal::List { capacity } => write!(f, "list(capacity = {capacity})"),
+            Literal::Record { capacity } => write!(f, "record(capacity = {capacity})"),
+            Literal::Filepath { val, no_expand } => write!(
+                f,
+                "filepath({}, no_expand = {no_expand:?})",
+                FmtData(self.data, *val)
+            ),
+            Literal::Directory { val, no_expand } => write!(
+                f,
+                "directory({}, no_expand = {no_expand:?})",
+                FmtData(self.data, *val)
+            ),
+            Literal::GlobPattern { val, no_expand } => write!(
+                f,
+                "glob-pattern({}, no_expand = {no_expand:?})",
+                FmtData(self.data, *val)
+            ),
+            Literal::String(s) => write!(f, "string({})", FmtData(self.data, *s)),
+            Literal::RawString(rs) => write!(f, "raw-string({})", FmtData(self.data, *rs)),
+            Literal::CellPath(p) => write!(f, "cell-path({p})"),
+            Literal::Date(dt) => write!(f, "date({dt})"),
+            Literal::Nothing => write!(f, "nothing"),
+        }
+    }
+}
+
+struct FmtPattern<'a> {
+    engine_state: &'a EngineState,
+    pattern: &'a Pattern,
+}
+
+impl<'a> fmt::Display for FmtPattern<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.pattern {
+            Pattern::Record(bindings) => {
+                f.write_str("{")?;
+                for (name, pattern) in bindings {
+                    write!(
+                        f,
+                        "{}: {}",
+                        name,
+                        FmtPattern {
+                            engine_state: self.engine_state,
+                            pattern: &pattern.pattern,
+                        }
+                    )?;
+                }
+                f.write_str("}")
+            }
+            Pattern::List(bindings) => {
+                f.write_str("[")?;
+                for pattern in bindings {
+                    write!(
+                        f,
+                        "{}",
+                        FmtPattern {
+                            engine_state: self.engine_state,
+                            pattern: &pattern.pattern
+                        }
+                    )?;
+                }
+                f.write_str("]")
+            }
+            Pattern::Value(expr) => {
+                let string =
+                    String::from_utf8_lossy(self.engine_state.get_span_contents(expr.span));
+                f.write_str(&string)
+            }
+            Pattern::Variable(var_id) => {
+                let variable = FmtVar::new(self.engine_state, *var_id);
+                write!(f, "{}", variable)
+            }
+            Pattern::Or(patterns) => {
+                for (index, pattern) in patterns.iter().enumerate() {
+                    if index > 0 {
+                        f.write_str(" | ")?;
+                    }
+                    write!(
+                        f,
+                        "{}",
+                        FmtPattern {
+                            engine_state: self.engine_state,
+                            pattern: &pattern.pattern
+                        }
+                    )?;
+                }
+                Ok(())
+            }
+            Pattern::Rest(var_id) => {
+                let variable = FmtVar::new(self.engine_state, *var_id);
+                write!(f, "..{}", variable)
+            }
+            Pattern::IgnoreRest => f.write_str(".."),
+            Pattern::IgnoreValue => f.write_str("_"),
+            Pattern::Garbage => f.write_str("<garbage>"),
+        }
+    }
+}

crates/nu-protocol/src/ir/mod.rs

@@ -0,0 +1,419 @@
+use std::{fmt, sync::Arc};
+
+use crate::{
+    ast::{CellPath, Expression, Operator, Pattern, RangeInclusion},
+    engine::EngineState,
+    BlockId, DeclId, RegId, Span, Value, VarId,
+};
+
+use chrono::{DateTime, FixedOffset};
+use serde::{Deserialize, Serialize};
+
+mod call;
+mod display;
+
+pub use call::*;
+pub use display::{FmtInstruction, FmtIrBlock};
+
+#[derive(Clone, Serialize, Deserialize)]
+pub struct IrBlock {
+    pub instructions: Vec<Instruction>,
+    pub spans: Vec<Span>,
+    #[serde(with = "serde_arc_u8_array")]
+    pub data: Arc<[u8]>,
+    pub ast: Vec<Option<IrAstRef>>,
+    /// Additional information that can be added to help with debugging
+    pub comments: Vec<Box<str>>,
+    pub register_count: u32,
+    pub file_count: u32,
+}
+
+impl fmt::Debug for IrBlock {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // the ast field is too verbose and doesn't add much
+        f.debug_struct("IrBlock")
+            .field("instructions", &self.instructions)
+            .field("spans", &self.spans)
+            .field("data", &self.data)
+            .field("comments", &self.comments)
+            .field("register_count", &self.register_count)
+            .field("file_count", &self.register_count)
+            .finish_non_exhaustive()
+    }
+}
+
+impl IrBlock {
+    /// Returns a value that can be formatted with [`Display`](std::fmt::Display) to show a detailed
+    /// listing of the instructions contained within this [`IrBlock`].
+    pub fn display<'a>(&'a self, engine_state: &'a EngineState) -> FmtIrBlock<'a> {
+        FmtIrBlock {
+            engine_state,
+            ir_block: self,
+        }
+    }
+}
+
+/// A slice into the `data` array of a block. This is a compact and cache-friendly way to store
+/// string data that a block uses.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
+pub struct DataSlice {
+    pub start: u32,
+    pub len: u32,
+}
+
+impl DataSlice {
+    /// A data slice that contains no data. This slice is always valid.
+    pub const fn empty() -> DataSlice {
+        DataSlice { start: 0, len: 0 }
+    }
+}
+
+impl std::ops::Index<DataSlice> for [u8] {
+    type Output = [u8];
+
+    fn index(&self, index: DataSlice) -> &Self::Output {
+        &self[index.start as usize..(index.start as usize + index.len as usize)]
+    }
+}
+
+/// A possible reference into the abstract syntax tree for an instruction. This is not present for
+/// most instructions and is just added when needed.
+#[derive(Debug, Clone)]
+pub struct IrAstRef(pub Arc<Expression>);
+
+impl Serialize for IrAstRef {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        self.0.as_ref().serialize(serializer)
+    }
+}
+
+impl<'de> Deserialize<'de> for IrAstRef {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        Expression::deserialize(deserializer).map(|expr| IrAstRef(Arc::new(expr)))
+    }
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub enum Instruction {
+    /// Unreachable code path (error)
+    Unreachable,
+    /// Load a literal value into the `dst` register
+    LoadLiteral { dst: RegId, lit: Literal },
+    /// Load a clone of a boxed value into the `dst` register (e.g. from const evaluation)
+    LoadValue { dst: RegId, val: Box<Value> },
+    /// Move a register. Value is taken from `src` (used by this instruction).
+    Move { dst: RegId, src: RegId },
+    /// Copy a register (must be a collected value). Value is still in `src` after this instruction.
+    Clone { dst: RegId, src: RegId },
+    /// Collect a stream in a register to a value
+    Collect { src_dst: RegId },
+    /// Change the span of the contents of a register to the span of this instruction.
+    Span { src_dst: RegId },
+    /// Drop the value/stream in a register, without draining
+    Drop { src: RegId },
+    /// Drain the value/stream in a register and discard (e.g. semicolon).
+    ///
+    /// If passed a stream from an external command, sets $env.LAST_EXIT_CODE to the resulting exit
+    /// code, and invokes any available error handler with Empty, or if not available, returns an
+    /// exit-code-only stream, leaving the block.
+    Drain { src: RegId },
+    /// Load the value of a variable into the `dst` register
+    LoadVariable { dst: RegId, var_id: VarId },
+    /// Store the value of a variable from the `src` register
+    StoreVariable { var_id: VarId, src: RegId },
+    /// Load the value of an environment variable into the `dst` register
+    LoadEnv { dst: RegId, key: DataSlice },
+    /// Load the value of an environment variable into the `dst` register, or `Nothing` if it
+    /// doesn't exist
+    LoadEnvOpt { dst: RegId, key: DataSlice },
+    /// Store the value of an environment variable from the `src` register
+    StoreEnv { key: DataSlice, src: RegId },
+    /// Add a positional arg to the next (internal) call.
+    PushPositional { src: RegId },
+    /// Add a list of args to the next (internal) call (spread/rest).
+    AppendRest { src: RegId },
+    /// Add a named arg with no value to the next (internal) call.
+    PushFlag { name: DataSlice },
+    /// Add a short named arg with no value to the next (internal) call.
+    PushShortFlag { short: DataSlice },
+    /// Add a named arg with a value to the next (internal) call.
+    PushNamed { name: DataSlice, src: RegId },
+    /// Add a short named arg with a value to the next (internal) call.
+    PushShortNamed { short: DataSlice, src: RegId },
+    /// Add parser info to the next (internal) call.
+    PushParserInfo {
+        name: DataSlice,
+        info: Box<Expression>,
+    },
+    /// Set the redirection for stdout for the next call (only).
+    ///
+    /// The register for a file redirection is not consumed.
+    RedirectOut { mode: RedirectMode },
+    /// Set the redirection for stderr for the next call (only).
+    ///
+    /// The register for a file redirection is not consumed.
+    RedirectErr { mode: RedirectMode },
+    /// Throw an error if stderr wasn't redirected in the given stream. `src` is preserved.
+    CheckErrRedirected { src: RegId },
+    /// Open a file for redirection, pushing it onto the file stack.
+    OpenFile {
+        file_num: u32,
+        path: RegId,
+        append: bool,
+    },
+    /// Write data from the register to a file. This is done to finish a file redirection, in case
+    /// an internal command or expression was evaluated rather than an external one.
+    WriteFile { file_num: u32, src: RegId },
+    /// Pop a file used for redirection from the file stack.
+    CloseFile { file_num: u32 },
+    /// Make a call. The input is taken from `src_dst`, and the output is placed in `src_dst`,
+    /// overwriting it. The argument stack is used implicitly and cleared when the call ends.
+    Call { decl_id: DeclId, src_dst: RegId },
+    /// Append a value onto the end of a string. Uses `to_expanded_string(", ", ...)` on the value.
+    /// Used for string interpolation literals. Not the same thing as the `++` operator.
+    StringAppend { src_dst: RegId, val: RegId },
+    /// Convert a string into a glob. Used for glob interpolation and setting glob variables. If the
+    /// value is already a glob, it won't be modified (`no_expand` will have no effect).
+    GlobFrom { src_dst: RegId, no_expand: bool },
+    /// Push a value onto the end of a list. Used to construct list literals.
+    ListPush { src_dst: RegId, item: RegId },
+    /// Spread a value onto the end of a list. Used to construct list literals.
+    ListSpread { src_dst: RegId, items: RegId },
+    /// Insert a key-value pair into a record. Used to construct record literals. Raises an error if
+    /// the key already existed in the record.
+    RecordInsert {
+        src_dst: RegId,
+        key: RegId,
+        val: RegId,
+    },
+    /// Spread a record onto a record. Used to construct record literals. Any existing value for the
+    /// key is overwritten.
+    RecordSpread { src_dst: RegId, items: RegId },
+    /// Negate a boolean.
+    Not { src_dst: RegId },
+    /// Do a binary operation on `lhs_dst` (left) and `rhs` (right) and write the result to
+    /// `lhs_dst`.
+    BinaryOp {
+        lhs_dst: RegId,
+        op: Operator,
+        rhs: RegId,
+    },
+    /// Follow a cell path on the value in `src_dst`, storing the result back to `src_dst`
+    FollowCellPath { src_dst: RegId, path: RegId },
+    /// Clone the value at a cell path in `src`, storing the result to `dst`. The original value
+    /// remains in `src`. Must be a collected value.
+    CloneCellPath { dst: RegId, src: RegId, path: RegId },
+    /// Update/insert a cell path to `new_value` on the value in `src_dst`, storing the modified
+    /// value back to `src_dst`
+    UpsertCellPath {
+        src_dst: RegId,
+        path: RegId,
+        new_value: RegId,
+    },
+    /// Jump to an offset in this block
+    Jump { index: usize },
+    /// Branch to an offset in this block if the value of the `cond` register is a true boolean,
+    /// otherwise continue execution
+    BranchIf { cond: RegId, index: usize },
+    /// Branch to an offset in this block if the value of the `src` register is Empty or Nothing,
+    /// otherwise continue execution. The original value in `src` is preserved.
+    BranchIfEmpty { src: RegId, index: usize },
+    /// Match a pattern on `src`. If the pattern matches, branch to `index` after having set any
+    /// variables captured by the pattern. If the pattern doesn't match, continue execution. The
+    /// original value is preserved in `src` through this instruction.
+    Match {
+        pattern: Box<Pattern>,
+        src: RegId,
+        index: usize,
+    },
+    /// Check that a match guard is a boolean, throwing
+    /// [`MatchGuardNotBool`](crate::ShellError::MatchGuardNotBool) if it isn't. Preserves `src`.
+    CheckMatchGuard { src: RegId },
+    /// Iterate on register `stream`, putting the next value in `dst` if present, or jumping to
+    /// `end_index` if the iterator is finished
+    Iterate {
+        dst: RegId,
+        stream: RegId,
+        end_index: usize,
+    },
+    /// Push an error handler, without capturing the error value
+    OnError { index: usize },
+    /// Push an error handler, capturing the error value into `dst`. If the error handler is not
+    /// called, the register should be freed manually.
+    OnErrorInto { index: usize, dst: RegId },
+    /// Pop an error handler. This is not necessary when control flow is directed to the error
+    /// handler due to an error.
+    PopErrorHandler,
+    /// Check if an external command failed. Boolean value into `dst`. `src` is preserved, but it
+    /// does require waiting for the command to exit.
+    CheckExternalFailed { dst: RegId, src: RegId },
+    /// Return early from the block, raising a `ShellError::Return` instead.
+    ///
+    /// Collecting the value is unavoidable.
+    ReturnEarly { src: RegId },
+    /// Return from the block with the value in the register
+    Return { src: RegId },
+}
+
+impl Instruction {
+    /// Returns a value that can be formatted with [`Display`](std::fmt::Display) to show a detailed
+    /// listing of the instruction.
+    pub fn display<'a>(
+        &'a self,
+        engine_state: &'a EngineState,
+        data: &'a [u8],
+    ) -> FmtInstruction<'a> {
+        FmtInstruction {
+            engine_state,
+            instruction: self,
+            data,
+        }
+    }
+
+    /// Returns the branch target index of the instruction if this is a branching instruction.
+    pub fn branch_target(&self) -> Option<usize> {
+        match self {
+            Instruction::Jump { index } => Some(*index),
+            Instruction::BranchIf { cond: _, index } => Some(*index),
+            Instruction::BranchIfEmpty { src: _, index } => Some(*index),
+            Instruction::Match {
+                pattern: _,
+                src: _,
+                index,
+            } => Some(*index),
+
+            Instruction::Iterate {
+                dst: _,
+                stream: _,
+                end_index,
+            } => Some(*end_index),
+            Instruction::OnError { index } => Some(*index),
+            Instruction::OnErrorInto { index, dst: _ } => Some(*index),
+            _ => None,
+        }
+    }
+
+    /// Sets the branch target of the instruction if this is a branching instruction.
+    ///
+    /// Returns `Err(target_index)` if it isn't a branching instruction.
+    pub fn set_branch_target(&mut self, target_index: usize) -> Result<(), usize> {
+        match self {
+            Instruction::Jump { index } => *index = target_index,
+            Instruction::BranchIf { cond: _, index } => *index = target_index,
+            Instruction::BranchIfEmpty { src: _, index } => *index = target_index,
+            Instruction::Match {
+                pattern: _,
+                src: _,
+                index,
+            } => *index = target_index,
+
+            Instruction::Iterate {
+                dst: _,
+                stream: _,
+                end_index,
+            } => *end_index = target_index,
+            Instruction::OnError { index } => *index = target_index,
+            Instruction::OnErrorInto { index, dst: _ } => *index = target_index,
+            _ => return Err(target_index),
+        }
+        Ok(())
+    }
+}
+
+// This is to document/enforce the size of `Instruction` in bytes.
+// We should try to avoid increasing the size of `Instruction`,
+// and PRs that do so will have to change the number below so that it's noted in review.
+const _: () = assert!(std::mem::size_of::<Instruction>() <= 24);
+
+/// A literal value that can be embedded in an instruction.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub enum Literal {
+    Bool(bool),
+    Int(i64),
+    Float(f64),
+    Filesize(i64),
+    Duration(i64),
+    Binary(DataSlice),
+    Block(BlockId),
+    Closure(BlockId),
+    RowCondition(BlockId),
+    Range {
+        start: RegId,
+        step: RegId,
+        end: RegId,
+        inclusion: RangeInclusion,
+    },
+    List {
+        capacity: usize,
+    },
+    Record {
+        capacity: usize,
+    },
+    Filepath {
+        val: DataSlice,
+        no_expand: bool,
+    },
+    Directory {
+        val: DataSlice,
+        no_expand: bool,
+    },
+    GlobPattern {
+        val: DataSlice,
+        no_expand: bool,
+    },
+    String(DataSlice),
+    RawString(DataSlice),
+    CellPath(Box<CellPath>),
+    Date(Box<DateTime<FixedOffset>>),
+    Nothing,
+}
+
+/// A redirection mode for the next call. See [`OutDest`](crate::OutDest).
+///
+/// This is generated by:
+///
+/// 1. Explicit redirection in a [`PipelineElement`](crate::ast::PipelineElement), or
+/// 2. The [`pipe_redirection()`](crate::engine::Command::pipe_redirection) of the command being
+///    piped into.
+///
+/// Not setting it uses the default, determined by [`Stack`](crate::engine::Stack).
+#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
+pub enum RedirectMode {
+    Pipe,
+    Capture,
+    Null,
+    Inherit,
+    /// Use the given numbered file.
+    File {
+        file_num: u32,
+    },
+    /// Use the redirection mode requested by the caller, for a pre-return call.
+    Caller,
+}
+
+/// Just a hack to allow `Arc<[u8]>` to be serialized and deserialized
+mod serde_arc_u8_array {
+    use serde::{Deserialize, Serialize};
+    use std::sync::Arc;
+
+    pub fn serialize<S>(data: &Arc<[u8]>, ser: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        data.as_ref().serialize(ser)
+    }
+
+    pub fn deserialize<'de, D>(de: D) -> Result<Arc<[u8]>, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let data: Vec<u8> = Deserialize::deserialize(de)?;
+        Ok(data.into())
+    }
+}

crates/nu-protocol/src/lib.rs

@@ -9,6 +9,7 @@ pub mod eval_base;
 pub mod eval_const;
 mod example;
 mod id;
+pub mod ir;
 mod lev_distance;
 mod module;
 pub mod parser_path;

crates/nu-protocol/src/pipeline/byte_stream.rs

@@ -352,6 +352,12 @@ impl ByteStream {
         self.span
     }
 
+    /// Changes the [`Span`] associated with the [`ByteStream`].
+    pub fn with_span(mut self, span: Span) -> Self {
+        self.span = span;
+        self
+    }
+
     /// Returns the [`ByteStreamType`] associated with the [`ByteStream`].
     pub fn type_(&self) -> ByteStreamType {
         self.type_

crates/nu-protocol/src/pipeline/list_stream.rs

@@ -31,11 +31,22 @@ impl ListStream {
         self.span
     }
 
+    /// Changes the [`Span`] associated with this [`ListStream`].
+    pub fn with_span(mut self, span: Span) -> Self {
+        self.span = span;
+        self
+    }
+
     /// Convert a [`ListStream`] into its inner [`Value`] `Iterator`.
     pub fn into_inner(self) -> ValueIterator {
         self.stream
     }
 
+    /// Take a single value from the inner `Iterator`, modifying the stream.
+    pub fn next_value(&mut self) -> Option<Value> {
+        self.stream.next()
+    }
+
     /// Converts each value in a [`ListStream`] into a string and then joins the strings together
     /// using the given separator.
     pub fn into_string(self, separator: &str, config: &Config) -> String {

crates/nu-protocol/src/pipeline/pipeline_data.rs

@@ -96,6 +96,24 @@ impl PipelineData {
         }
     }
 
+    /// Change the span of the [`PipelineData`].
+    ///
+    /// Returns `Value(Nothing)` with the given span if it was [`PipelineData::Empty`].
+    pub fn with_span(self, span: Span) -> Self {
+        match self {
+            PipelineData::Empty => PipelineData::Value(Value::nothing(span), None),
+            PipelineData::Value(value, metadata) => {
+                PipelineData::Value(value.with_span(span), metadata)
+            }
+            PipelineData::ListStream(stream, metadata) => {
+                PipelineData::ListStream(stream.with_span(span), metadata)
+            }
+            PipelineData::ByteStream(stream, metadata) => {
+                PipelineData::ByteStream(stream.with_span(span), metadata)
+            }
+        }
+    }
+
     /// Get a type that is representative of the `PipelineData`.
     ///
     /// The type returned here makes no effort to collect a stream, so it may be a different type
@@ -129,7 +147,8 @@ impl PipelineData {
     /// without consuming input and without writing anything.
     ///
     /// For the other [`OutDest`]s, the given `PipelineData` will be completely consumed
-    /// and `PipelineData::Empty` will be returned.
+    /// and `PipelineData::Empty` will be returned, unless the data is from an external stream,
+    /// in which case an external stream containing only that exit code will be returned.
     pub fn write_to_out_dests(
         self,
         engine_state: &EngineState,
@@ -137,7 +156,11 @@ impl PipelineData {
     ) -> Result<PipelineData, ShellError> {
         match (self, stack.stdout()) {
             (PipelineData::ByteStream(stream, ..), stdout) => {
-                stream.write_to_out_dests(stdout, stack.stderr())?;
+                if let Some(exit_status) = stream.write_to_out_dests(stdout, stack.stderr())? {
+                    return Ok(PipelineData::new_external_stream_with_only_exit_code(
+                        exit_status.code(),
+                    ));
+                }
             }
             (data, OutDest::Pipe | OutDest::Capture) => return Ok(data),
             (PipelineData::Empty, ..) => {}
@@ -570,7 +593,7 @@ impl PipelineData {
                         self.write_all_and_flush(engine_state, no_newline, to_stderr)
                     } else {
                         let call = Call::new(Span::new(0, 0));
-                        let table = command.run(engine_state, stack, &call, self)?;
+                        let table = command.run(engine_state, stack, &(&call).into(), self)?;
                         table.write_all_and_flush(engine_state, no_newline, to_stderr)
                     }
                 } else {

crates/nu-protocol/src/signature.rs

@@ -1,6 +1,5 @@
 use crate::{
-    ast::Call,
-    engine::{Command, CommandType, EngineState, Stack},
+    engine::{Call, Command, CommandType, EngineState, Stack},
     BlockId, PipelineData, ShellError, SyntaxShape, Type, Value, VarId,
 };
 use serde::{Deserialize, Serialize};
@@ -486,15 +485,14 @@ impl Signature {
         (name, s)
     }
 
-    pub fn get_positional(&self, position: usize) -> Option<PositionalArg> {
+    pub fn get_positional(&self, position: usize) -> Option<&PositionalArg> {
         if position < self.required_positional.len() {
-            self.required_positional.get(position).cloned()
+            self.required_positional.get(position)
         } else if position < (self.required_positional.len() + self.optional_positional.len()) {
             self.optional_positional
                 .get(position - self.required_positional.len())
-                .cloned()
         } else {
-            self.rest_positional.clone()
+            self.rest_positional.as_ref()
         }
     }
 

crates/nu-protocol/src/span.rs

@@ -53,6 +53,22 @@ impl<T> Spanned<T> {
     }
 }
 
+impl<T, E> Spanned<Result<T, E>> {
+    /// Move the `Result` to the outside, resulting in a spanned `Ok` or unspanned `Err`.
+    pub fn transpose(self) -> Result<Spanned<T>, E> {
+        match self {
+            Spanned {
+                item: Ok(item),
+                span,
+            } => Ok(Spanned { item, span }),
+            Spanned {
+                item: Err(err),
+                span: _,
+            } => Err(err),
+        }
+    }
+}
+
 /// Helper trait to create [`Spanned`] more ergonomically.
 pub trait IntoSpanned: Sized {
     /// Wrap items together with a span into [`Spanned`].

crates/nu-protocol/tests/test_signature.rs

@@ -39,7 +39,7 @@ fn test_signature_chained() {
 
     assert_eq!(
         signature.get_positional(0),
-        Some(PositionalArg {
+        Some(&PositionalArg {
             name: "required".to_string(),
             desc: "required description".to_string(),
             shape: SyntaxShape::String,
@@ -49,7 +49,7 @@ fn test_signature_chained() {
     );
     assert_eq!(
         signature.get_positional(1),
-        Some(PositionalArg {
+        Some(&PositionalArg {
             name: "optional".to_string(),
             desc: "optional description".to_string(),
             shape: SyntaxShape::String,
@@ -59,7 +59,7 @@ fn test_signature_chained() {
     );
     assert_eq!(
         signature.get_positional(2),
-        Some(PositionalArg {
+        Some(&PositionalArg {
             name: "rest".to_string(),
             desc: "rest description".to_string(),
             shape: SyntaxShape::String,