IR: fix incorrect capturing of subexpressions

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

@@ -444,7 +444,15 @@ pub(crate) fn compile_expression(
                     working_set,
                     builder,
                     &full_cell_path.head,
-                    RedirectModes::capture_out(expr.span),
+                    // Only capture the output if there is a tail. This was a bit of a headscratcher
+                    // as the parser emits a FullCellPath with no tail for subexpressions in
+                    // general, which shouldn't be captured any differently than they otherwise
+                    // would be.
+                    if !full_cell_path.tail.is_empty() {
+                        RedirectModes::capture_out(expr.span)
+                    } else {
+                        redirect_modes
+                    },
                     in_reg,
                     out_reg,
                 )?;

tests/shell/pipeline/commands/external.rs

@@ -209,6 +209,12 @@ fn run_glob_if_pass_variable_to_external() {
     })
 }
 
+#[test]
+fn subexpression_does_not_implicitly_capture() {
+    let actual = nu!("(nu --testbin cococo); null");
+    assert_eq!(actual.out, "cococo");
+}
+
 mod it_evaluation {
     use super::nu;
     use nu_test_support::fs::Stub::{EmptyFile, FileWithContent, FileWithContentToBeTrimmed};