[ty] Disallow Final in function parameter annotations

Author: sharkdp

crates/ty_python_semantic/resources/mdtest/type_qualifiers/final.md

@@ -275,7 +275,7 @@ class C:
         self.LEGAL_I: Final[int]
         self.LEGAL_I = 1
 
-# TODO: This should be an error
+# error: [invalid-type-form] "`Final` is not allowed in function parameter annotations"
 def f(ILLEGAL: Final[int]) -> None:
     pass
 ```

crates/ty_python_semantic/src/types/infer.rs

@@ -2431,10 +2431,16 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             default: _,
         } = parameter_with_default;
 
-        self.infer_optional_annotation_expression(
+        let annotated = self.infer_optional_annotation_expression(
             parameter.annotation.as_deref(),
             DeferredExpressionState::None,
         );
+
+        if annotated.is_some_and(|annotated| annotated.qualifiers.contains(TypeQualifiers::FINAL)) {
+            if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, parameter) {
+                builder.into_diagnostic("`Final` is not allowed in function parameter annotations");
+            }
+        }
     }
 
     fn infer_parameter(&mut self, parameter: &ast::Parameter) {