fix(lsp): preserve annotated type-variable names when adding annotations

```
fn wibble(a, b, c) {
   ↑
  let x: a = todo
  fn(a: b, b: c) -> d {
    todo
  }
}
```

After action:
```diff
- fn wibble(a: e, b: f, c: g) -> fn(b, c) -> h
+ fn wibble(a: e, b: f, c: g) -> fn(b, c) -> d
                                             ^ original name is preserved
```

When the "add annotations" code-action is used, the compiler now retains the
original type-variable names from the user's annotation instead of replacing them
with fresh or default names.

This ensures that added annotations reflect the user's naming exactly and prevents
confusing or misleading variable renaming.

Author: abs0luty

compiler-core/src/language_server/code_action.rs

@@ -1372,13 +1372,187 @@ fn collect_type_variables(printer: &mut Printer<'_>, function: &ast::TypedFuncti
 }
 
 impl<'ast, 'a, 'b> ast::visit::Visit<'ast> for TypeVariableCollector<'a, 'b> {
+    fn visit_typed_function(&mut self, fun: &'ast ast::TypedFunction) {
+        for argument in fun.arguments.iter() {
+            if let Some(annotation) = &argument.annotation {
+                register_type_variables_from_annotation(
+                    self.printer,
+                    annotation,
+                    argument.type_.as_ref(),
+                );
+            }
+        }
+
+        if let Some(annotation) = &fun.return_annotation {
+            register_type_variables_from_annotation(
+                self.printer,
+                annotation,
+                fun.return_type.as_ref(),
+            );
+        }
+
+        ast::visit::visit_typed_function(self, fun);
+    }
+
+    fn visit_typed_expr_fn(
+        &mut self,
+        location: &'ast SrcSpan,
+        type_: &'ast Arc<Type>,
+        kind: &'ast FunctionLiteralKind,
+        arguments: &'ast [TypedArg],
+        body: &'ast Vec1<TypedStatement>,
+        return_annotation: &'ast Option<ast::TypeAst>,
+    ) {
+        if let Type::Fn {
+            arguments: argument_types,
+            return_: return_type,
+            ..
+        } = type_.as_ref()
+        {
+            for (argument, argument_type) in arguments.iter().zip(argument_types) {
+                if let Some(annotation) = &argument.annotation {
+                    register_type_variables_from_annotation(
+                        self.printer,
+                        annotation,
+                        argument_type.as_ref(),
+                    );
+                }
+            }
+
+            if let Some(annotation) = return_annotation {
+                register_type_variables_from_annotation(
+                    self.printer,
+                    annotation,
+                    return_type.as_ref(),
+                );
+            }
+        }
+
+        ast::visit::visit_typed_expr_fn(
+            self,
+            location,
+            type_,
+            kind,
+            arguments,
+            body,
+            return_annotation,
+        );
+    }
+
     fn visit_type_ast_var(&mut self, _location: &'ast SrcSpan, name: &'ast EcoString) {
         // Register this type variable so that we don't duplicate names when
         // adding annotations.
         self.printer.register_type_variable(name.clone());
     }
 }
 
+fn register_type_variables_from_annotation(
+    printer: &mut Printer<'_>,
+    annotation: &ast::TypeAst,
+    type_: &Type,
+) {
+    // fn wibble(a, b, c) {
+    //   fn(a: b, b: c) -> d { ... }
+    //                     ^
+    // Without this tracking the printer could rename `d` to a fresh `h`.
+    match (annotation, type_) {
+        (ast::TypeAst::Var(ast::TypeAstVar { name, .. }), Type::Var { type_ }) => {
+            match &*type_.borrow() {
+                TypeVar::Generic { id } | TypeVar::Unbound { id } => {
+                    let id = *id;
+                    printer.register_type_variable(name.clone());
+                    printer.register_type_variable_with_id(id, name.clone());
+                }
+                TypeVar::Link { type_ } => {
+                    register_type_variables_from_annotation(printer, annotation, type_.as_ref());
+                }
+            }
+        }
+
+        (
+            ast::TypeAst::Fn(ast::TypeAstFn {
+                arguments: annotation_arguments,
+                return_: annotation_return,
+                ..
+            }),
+            Type::Fn {
+                arguments: type_arguments,
+                return_: type_return,
+                ..
+            },
+        ) => {
+            for (argument_annotation, argument_type) in
+                annotation_arguments.iter().zip(type_arguments)
+            {
+                // Maintain the names from each `fn(arg: name, ...)` position.
+                register_type_variables_from_annotation(
+                    printer,
+                    argument_annotation,
+                    argument_type.as_ref(),
+                );
+            }
+
+            // And likewise propagate the annotated return variable.
+            register_type_variables_from_annotation(
+                printer,
+                annotation_return.as_ref(),
+                type_return.as_ref(),
+            );
+        }
+
+        (
+            ast::TypeAst::Constructor(ast::TypeAstConstructor {
+                arguments: annotation_arguments,
+                ..
+            }),
+            Type::Named {
+                arguments: type_arguments,
+                ..
+            },
+        ) => {
+            for (argument_annotation, argument_type) in
+                annotation_arguments.iter().zip(type_arguments)
+            {
+                // Track aliases introduced inside named type arguments.
+                register_type_variables_from_annotation(
+                    printer,
+                    argument_annotation,
+                    argument_type.as_ref(),
+                );
+            }
+        }
+
+        (
+            ast::TypeAst::Tuple(ast::TypeAstTuple {
+                elements: annotation_elements,
+                ..
+            }),
+            Type::Tuple {
+                elements: type_elements,
+                ..
+            },
+        ) => {
+            for (element_annotation, element_type) in annotation_elements.iter().zip(type_elements)
+            {
+                // Tuples can hide extra annotations; ensure each slot retains its label.
+                register_type_variables_from_annotation(
+                    printer,
+                    element_annotation,
+                    element_type.as_ref(),
+                );
+            }
+        }
+
+        (_, Type::Var { type_ }) => {
+            if let TypeVar::Link { type_ } = &*type_.borrow() {
+                register_type_variables_from_annotation(printer, annotation, type_.as_ref());
+            }
+        }
+
+        _ => {}
+    }
+}
+
 pub struct QualifiedConstructor<'a> {
     import: &'a Import<EcoString>,
     used_name: EcoString,

compiler-core/src/language_server/tests/snapshots/gleam_core__language_server__tests__action__type_variables_in_let_bindings_are_considered_when_adding_annotations.snap

@@ -15,7 +15,7 @@ fn wibble(a, b, c) {
 
 ----- AFTER ACTION
 
-fn wibble(a: e, b: f, c: g) -> fn(b, c) -> h {
+fn wibble(a: e, b: f, c: g) -> fn(b, c) -> d {
   let x: a = todo
   fn(a: b, b: c) -> d {
     todo

compiler-core/src/type_/printer.rs

@@ -454,6 +454,12 @@ impl<'a> Printer<'a> {
         _ = self.printed_type_variable_names.insert(name);
     }
 
+    /// Record that the given type-variable id is already using the supplied name.
+    pub fn register_type_variable_with_id(&mut self, id: u64, name: EcoString) {
+        _ = self.printed_type_variable_names.insert(name.clone());
+        _ = self.printed_type_variables.insert(id, name);
+    }
+
     pub fn print_type(&mut self, type_: &Type) -> EcoString {
         let mut buffer = EcoString::new();
         self.print(type_, &mut buffer, PrintMode::Normal);