Fix cryptic parser error (#5731)

# Problem

Before: "unexpected token |>", highlights the |>

After: "This argument needs a label, but it doesn't have one", highlights the argument with no label

Closes #5724 

# Discussion

I am trying a new approach to the parser: instead of parsing the specific correct thing we need, parse a more general form, then later, narrow it down to specifics and return a nice error if it's wrong. For example, instead of parsing labeled arguments, parse labeled OR unlabeled arguments. Then later check that the only unlabeled arg is the first one, and return a nice error if there's any other. This worked nicely for this PR, hopefully the approach will work for other "cryptic error" issues.

Author: adamchalmers

e2e/playwright/snapshot-tests.spec.ts-snapshots/Grid-visibility-Grid-turned-off-to-on-via-command-bar-1-Google-Chrome-linux.png

@@ -2809,29 +2809,45 @@ fn fn_call_kw(i: &mut TokenSlice) -> PResult<Node<CallExpressionKw>> {
     let _ = open_paren.parse_next(i)?;
     ignore_whitespace(i);
 
+    #[allow(clippy::large_enum_variant)]
+    pub enum ArgPlace {
+        NonCode(Node<NonCodeNode>),
+        LabeledArg(LabeledArg),
+        UnlabeledArg(Expr),
+    }
     let initial_unlabeled_arg = opt((expression, comma, opt(whitespace)).map(|(arg, _, _)| arg)).parse_next(i)?;
     let args: Vec<_> = repeat(
         0..,
         alt((
-            terminated(non_code_node.map(NonCodeOr::NonCode), whitespace),
-            terminated(labeled_argument, labeled_arg_separator).map(NonCodeOr::Code),
+            terminated(non_code_node.map(ArgPlace::NonCode), whitespace),
+            terminated(labeled_argument, labeled_arg_separator).map(ArgPlace::LabeledArg),
+            expression.map(ArgPlace::UnlabeledArg),
         )),
     )
     .parse_next(i)?;
-    let (args, non_code_nodes): (Vec<_>, BTreeMap<usize, _>) = args.into_iter().enumerate().fold(
+    let (args, non_code_nodes): (Vec<_>, BTreeMap<usize, _>) = args.into_iter().enumerate().try_fold(
         (Vec::new(), BTreeMap::new()),
         |(mut args, mut non_code_nodes), (i, e)| {
             match e {
-                NonCodeOr::NonCode(x) => {
+                ArgPlace::NonCode(x) => {
                     non_code_nodes.insert(i, vec![x]);
                 }
-                NonCodeOr::Code(x) => {
+                ArgPlace::LabeledArg(x) => {
                     args.push(x);
                 }
+                ArgPlace::UnlabeledArg(arg) => {
+                    return Err(ErrMode::Cut(
+                        CompilationError::fatal(
+                            SourceRange::from(arg),
+                            "This argument needs a label, but it doesn't have one",
+                        )
+                        .into(),
+                    ));
+                }
             }
-            (args, non_code_nodes)
+            Ok((args, non_code_nodes))
         },
-    );
+    )?;
     if let Some(std_fn) = crate::std::get_stdlib_fn(&fn_name.name) {
         let just_args: Vec<_> = args.iter().collect();
         typecheck_all_kw(std_fn, &just_args)?;
@@ -4641,6 +4657,27 @@ baz = 2
         assert_eq!(actual.operator, UnaryOperator::Not);
         crate::parsing::top_level_parse(some_program_string).unwrap(); // Updated import path
     }
+
+    #[test]
+    fn test_sensible_error_when_missing_equals_in_kwarg() {
+        for (i, program) in ["f(x=1,y)", "f(x=1,y,z)", "f(x=1,y,z=1)", "f(x=1, y, z=1)"]
+            .into_iter()
+            .enumerate()
+        {
+            let tokens = crate::parsing::token::lex(program, ModuleId::default()).unwrap();
+            let err = fn_call_kw.parse(tokens.as_slice()).unwrap_err();
+            let cause = err.inner().cause.as_ref().unwrap();
+            assert_eq!(
+                cause.message, "This argument needs a label, but it doesn't have one",
+                "failed test {i}: {program}"
+            );
+            assert_eq!(
+                cause.source_range.start(),
+                program.find("y").unwrap(),
+                "failed test {i}: {program}"
+            );
+        }
+    }
 }
 
 #[cfg(test)]