[ty] Disallow assignment to Final class attributes (#19457)

## Summary

Emit errors for the following assignments:
```py
class C:
    CLASS_LEVEL_CONSTANT: Final[int] = 1

C.CLASS_LEVEL_CONSTANT = 2
C().CLASS_LEVEL_CONSTANT = 2
```

## Test Plan

Updated and new MD tests

Author: sharkdp

crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.md

@@ -498,7 +498,7 @@ class C:
 reveal_type(C.__init__)  # revealed: (self: C, instance_variable_no_default: int, instance_variable: int = Literal[1]) -> None
 
 c = C(1)
-# TODO: this should be an error
+# error: [invalid-assignment] "Cannot assign to final attribute `instance_variable` on type `C`"
 c.instance_variable = 2
 ```
 

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

@@ -170,26 +170,37 @@ Assignments to attributes qualified with `Final` are also not allowed:
 ```py
 from typing import Final
 
-class C:
-    FINAL_A: Final[int] = 1
-    FINAL_B: Final = 1
+class Meta(type):
+    META_FINAL_A: Final[int] = 1
+    META_FINAL_B: Final = 1
+
+class C(metaclass=Meta):
+    CLASS_FINAL_A: Final[int] = 1
+    CLASS_FINAL_B: Final = 1
 
     def __init__(self):
-        self.FINAL_C: Final[int] = 1
-        self.FINAL_D: Final = 1
+        self.INSTANCE_FINAL_A: Final[int] = 1
+        self.INSTANCE_FINAL_B: Final = 1
+
+# error: [invalid-assignment] "Cannot assign to final attribute `META_FINAL_A` on type `<class 'C'>`"
+C.META_FINAL_A = 2
+# error: [invalid-assignment] "Cannot assign to final attribute `META_FINAL_B` on type `<class 'C'>`"
+C.META_FINAL_B = 2
 
-# TODO: these should be errors (that mention `Final`)
-C.FINAL_A = 2
-# error: [invalid-assignment] "Object of type `Literal[2]` is not assignable to attribute `FINAL_B` of type `Literal[1]`"
-C.FINAL_B = 2
+# error: [invalid-assignment] "Cannot assign to final attribute `CLASS_FINAL_A` on type `<class 'C'>`"
+C.CLASS_FINAL_A = 2
+# error: [invalid-assignment] "Cannot assign to final attribute `CLASS_FINAL_B` on type `<class 'C'>`"
+C.CLASS_FINAL_B = 2
 
-# TODO: these should be errors (that mention `Final`)
 c = C()
-c.FINAL_A = 2
-# error: [invalid-assignment] "Object of type `Literal[2]` is not assignable to attribute `FINAL_B` of type `Literal[1]`"
-c.FINAL_B = 2
-c.FINAL_C = 2
-c.FINAL_D = 2
+# error: [invalid-assignment] "Cannot assign to final attribute `CLASS_FINAL_A` on type `C`"
+c.CLASS_FINAL_A = 2
+# error: [invalid-assignment] "Cannot assign to final attribute `CLASS_FINAL_B` on type `C`"
+c.CLASS_FINAL_B = 2
+# TODO: this should be an error
+c.INSTANCE_FINAL_A = 2
+# TODO: this should be an error
+c.INSTANCE_FINAL_B = 2
 ```
 
 ## Mutability

crates/ty_python_semantic/src/types/infer.rs

@@ -3363,6 +3363,24 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             assignable
         };
 
+        // Return true (and emit a diagnostic) if this is an invalid assignment to a `Final` attribute.
+        let invalid_assignment_to_final = |qualifiers: TypeQualifiers| -> bool {
+            if qualifiers.contains(TypeQualifiers::FINAL) {
+                if emit_diagnostics {
+                    if let Some(builder) = self.context.report_lint(&INVALID_ASSIGNMENT, target) {
+                        builder.into_diagnostic(format_args!(
+                            "Cannot assign to final attribute `{attribute}` \
+                                         on type `{}`",
+                            object_ty.display(db)
+                        ));
+                    }
+                }
+                true
+            } else {
+                false
+            }
+        };
+
         match object_ty {
             Type::Union(union) => {
                 if union.elements(self.db()).iter().all(|elem| {
@@ -3558,8 +3576,12 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                             }
                             PlaceAndQualifiers {
                                 place: Place::Type(meta_attr_ty, meta_attr_boundness),
-                                qualifiers: _,
+                                qualifiers,
                             } => {
+                                if invalid_assignment_to_final(qualifiers) {
+                                    return false;
+                                }
+
                                 let assignable_to_meta_attr =
                                     if let Place::Type(meta_dunder_set, _) =
                                         meta_attr_ty.class_member(db, "__set__".into()).place
@@ -3669,8 +3691,12 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 match object_ty.class_member(db, attribute.into()) {
                     PlaceAndQualifiers {
                         place: Place::Type(meta_attr_ty, meta_attr_boundness),
-                        qualifiers: _,
+                        qualifiers,
                     } => {
+                        if invalid_assignment_to_final(qualifiers) {
+                            return false;
+                        }
+
                         let assignable_to_meta_attr = if let Place::Type(meta_dunder_set, _) =
                             meta_attr_ty.class_member(db, "__set__".into()).place
                         {
@@ -3733,11 +3759,17 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                         place: Place::Unbound,
                         ..
                     } => {
-                        if let Place::Type(class_attr_ty, class_attr_boundness) = object_ty
+                        if let PlaceAndQualifiers {
+                            place: Place::Type(class_attr_ty, class_attr_boundness),
+                            qualifiers,
+                        } = object_ty
                             .find_name_in_mro(db, attribute)
                             .expect("called on Type::ClassLiteral or Type::SubclassOf")
-                            .place
                         {
+                            if invalid_assignment_to_final(qualifiers) {
+                                return false;
+                            }
+
                             if class_attr_boundness == Boundness::PossiblyUnbound {
                                 report_possibly_unbound_attribute(
                                     &self.context,