add check for error redirection for pipe error mode

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

@@ -172,6 +172,7 @@ impl BlockBuilder {
             Instruction::PushParserInfo { name: _, info: _ } => Ok(()),
             Instruction::RedirectOut { mode: _ } => Ok(()),
             Instruction::RedirectErr { mode: _ } => Ok(()),
+            Instruction::CheckErrRedirected { src } => allocate(&[*src], &[*src]),
             Instruction::OpenFile {
                 file_num: _,
                 path,

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

@@ -133,6 +133,12 @@ pub(crate) fn finish_redirection(
         }) => {
             builder.push(Instruction::CloseFile { file_num }.into_spanned(span))?;
         }
+        Some(Spanned {
+            item: RedirectMode::Pipe | RedirectMode::Capture,
+            span,
+        }) => {
+            builder.push(Instruction::CheckErrRedirected { src: out_reg }.into_spanned(span))?;
+        }
         _ => (),
     }
 

crates/nu-engine/src/eval_ir.rs

@@ -6,9 +6,9 @@ use nu_protocol::{
     debugger::DebugContext,
     engine::{Argument, Closure, EngineState, ErrorHandler, Matcher, Redirection, Stack},
     ir::{Call, DataSlice, Instruction, IrAstRef, IrBlock, Literal, RedirectMode},
-    record, DeclId, ErrSpan, Flag, IntoPipelineData, IntoSpanned, ListStream, OutDest,
-    PipelineData, PositionalArg, Range, Record, RegId, ShellError, Signature, Span, Spanned, Value,
-    VarId, ENV_VARIABLE_ID,
+    record, ByteStreamSource, DeclId, ErrSpan, Flag, IntoPipelineData, IntoSpanned, ListStream,
+    OutDest, PipelineData, PositionalArg, Range, Record, RegId, ShellError, Signature, Span,
+    Spanned, Value, VarId, ENV_VARIABLE_ID,
 };
 use nu_utils::IgnoreCaseExt;
 
@@ -394,6 +394,24 @@ fn eval_instruction<D: DebugContext>(
             ctx.redirect_err = eval_redirection(ctx, mode, *span, RedirectionStream::Err)?;
             Ok(Continue)
         }
+        Instruction::CheckErrRedirected { src } => {
+            let data = ctx.take_reg(*src);
+            match &data {
+                PipelineData::ByteStream(stream, _)
+                    if matches!(stream.source(), ByteStreamSource::Child(_)) =>
+                {
+                    ctx.put_reg(*src, data);
+                    Ok(Continue)
+                }
+                _ => Err(ShellError::GenericError {
+                    error: "Can't redirect stderr of internal command output".into(),
+                    msg: "piping stderr only works on external commands".into(),
+                    span: Some(*span),
+                    help: None,
+                    inner: vec![],
+                }),
+            }
+        }
         Instruction::OpenFile {
             file_num,
             path,

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

@@ -127,6 +127,9 @@ impl<'a> fmt::Display for FmtInstruction<'a> {
             Instruction::RedirectErr { mode } => {
                 write!(f, "{:WIDTH$} {mode}", "redirect-err")
             }
+            Instruction::CheckErrRedirected { src } => {
+                write!(f, "{:WIDTH$} {src}", "check-err-redirected")
+            }
             Instruction::OpenFile {
                 file_num,
                 path,

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

@@ -144,6 +144,8 @@ pub enum Instruction {
     ///
     /// 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,