Fixes the panic when using externs + string interpolation (#4799)

crates/nu-parser/src/known_external.rs

@@ -36,6 +36,11 @@ impl Command for KnownExternal {
  ) -> Result<PipelineData, ShellError> {
  // FIXME: This is a bit of a hack, and it'd be nice for the parser/AST to be able to handle the original
  // order of the parameters. Until then, we need to recover the original order.
+
+ // FIXME: This is going to be a bit expensive, but we need to do it to ensure any new block/subexpression
+ // we find when parsing the external call is handled properly.
+ let mut engine_state = engine_state.clone();
+
  let call_span = call.span();
  let contents = engine_state.get_span_contents(&call_span);
 
@@ -45,8 +50,10 @@ impl Command for KnownExternal {
  let (lexed, _) = crate::lex(contents, call_span.start, &[], &[], true);
 
  let spans: Vec<_> = lexed.into_iter().map(|x| x.span).collect();
- let mut working_set = StateWorkingSet::new(engine_state);
+ let mut working_set = StateWorkingSet::new(&engine_state);
  let (external_call, _) = crate::parse_external_call(&mut working_set, &spans);
+ let delta = working_set.render();
+ engine_state.merge_delta(delta, None, ".")?;
 
  match external_call.expr {
  Expr::ExternalCall(head, args) => {
@@ -84,7 +91,7 @@ impl Command for KnownExternal {
  ))
  }
 
- command.run(engine_state, stack, &call, input)
+ command.run(&engine_state, stack, &call, input)
  }
  x => {
  println!("{:?}", x);