Disallow spreading lists automatically when calling externals (#11857)

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# Description
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Spreading lists automatically when calling externals was deprecated in
0.89 (#11289), and this PR is to remove it in 0.91.

# User-Facing Changes
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helps us keep track of breaking changes. -->

The new error message looks like this:

```
>  ^echo [1 2]
Error: nu:🐚:cannot_pass_list_to_external

  × Lists are not automatically spread when calling external commands
   ╭─[entry #13:1:8]
 1 │  ^echo [1 2]
   ·        ──┬──
   ·          ╰── Spread operator (...) is necessary to spread lists
   ╰────
  help: Either convert the list to a string or use the spread operator, like so: ...[1 2]
```

The old error message didn't say exactly where to put the `...` and
seemed to confuse a lot of people, so hopefully this helps.

# Tests + Formatting
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There was one test to check that implicit spread was deprecated before,
updated that to check that it's disallowed now.

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

@@ -135,29 +135,23 @@ pub fn create_external_command(
     let mut spanned_args = vec![];
     let mut arg_keep_raw = vec![];
     for (arg, spread) in call.rest_iter(1) {
-        // TODO: Disallow automatic spreading entirely later. This match block will
-        // have to be refactored, and lists will have to be disallowed in the parser too
         match eval_expression(engine_state, stack, arg)? {
             Value::List { vals, .. } => {
-                if !spread {
-                    nu_protocol::report_error_new(
-                        engine_state,
-                        &ShellError::GenericError {
-                            error: "Automatically spreading lists is deprecated".into(),
-                            msg: "Spreading lists automatically when calling external commands is deprecated and will be removed in 0.91.".into(),
-                            span: Some(arg.span),
-                            help: Some("Use the spread operator (put a '...' before the argument)".into()),
-                            inner: vec![],
-                        },
-                    );
-                }
-                // turn all the strings in the array into params.
-                // Example: one_arg may be something like ["ls" "-a"]
-                // convert it to "ls" "-a"
-                for v in vals {
-                    spanned_args.push(value_as_spanned(v)?);
-                    // for arguments in list, it's always treated as a whole arguments
-                    arg_keep_raw.push(true);
+                if spread {
+                    // turn all the strings in the array into params.
+                    // Example: one_arg may be something like ["ls" "-a"]
+                    // convert it to "ls" "-a"
+                    for v in vals {
+                        spanned_args.push(value_as_spanned(v)?);
+                        // for arguments in list, it's always treated as a whole arguments
+                        arg_keep_raw.push(true);
+                    }
+                } else {
+                    return Err(ShellError::CannotPassListToExternal {
+                        arg: String::from_utf8_lossy(engine_state.get_span_contents(arg.span))
+                            .into(),
+                        span: arg.span,
+                    });
                 }
             }
             val => {

crates/nu-protocol/src/shell_error.rs

@@ -1308,6 +1308,22 @@ This is an internal Nushell error, please file an issue https://github.com/nushe
         span: Span,
     },
 
+    /// Lists are not automatically spread when calling external commands
+    ///
+    /// ## Resolution
+    ///
+    /// Use the spread operator (put a '...' before the argument)
+    #[error("Lists are not automatically spread when calling external commands")]
+    #[diagnostic(
+        code(nu::shell::cannot_pass_list_to_external),
+        help("Either convert the list to a string or use the spread operator, like so: ...{arg}")
+    )]
+    CannotPassListToExternal {
+        arg: String,
+        #[label = "Spread operator (...) is necessary to spread lists"]
+        span: Span,
+    },
+
     /// Out of bounds.
     ///
     /// ## Resolution

src/tests/test_spread.rs

@@ -182,13 +182,11 @@ fn explain_spread_args() -> TestResult {
 }
 
 #[test]
-fn deprecate_implicit_spread_for_externals() {
-    // TODO: When automatic spreading is removed, test that list literals fail at parse time
-    let result = nu!(r#"nu --testbin cococo [1 2]"#);
-    assert!(result
-        .err
-        .contains("Automatically spreading lists is deprecated"));
-    assert_eq!(result.out, "1 2");
+fn disallow_implicit_spread_for_externals() -> TestResult {
+    fail_test(
+        r#"nu --testbin cococo [1 2]"#,
+        "Lists are not automatically spread",
+    )
 }
 
 #[test]