[ty] synthesize __setattr__ and __delattr__ for frozen dataclasses

Author: thejchap

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

@@ -425,7 +425,39 @@ from dataclasses import dataclass
 class MyFrozenClass: ...
 
 frozen = MyFrozenClass()
-frozen.x = 2  # error: [unresolved-attribute]
+frozen.x = 2  # error: [invalid-assignment] "Property `x` defined in `MyFrozenClass` is read-only"
+```
+
+diagnostic is also emitted if a frozen dataclass is inherited, and an attempt is made to mutate an
+attribute in the child class:
+
+```py
+from dataclasses import dataclass
+
+@dataclass(frozen=True)
+class MyFrozenClass:
+    x: int = 1
+
+class MyFrozenChildClass(MyFrozenClass): ...
+
+frozen = MyFrozenChildClass()
+frozen.x = 2  # error: [invalid-assignment]
+```
+
+The same diagnostic is emitted if a frozen dataclass is inherited, and an attempt is made to delete
+an attribute:
+
+```py
+from dataclasses import dataclass
+
+@dataclass(frozen=True)
+class MyFrozenClass:
+    x: int = 1
+
+class MyFrozenChildClass(MyFrozenClass): ...
+
+frozen = MyFrozenChildClass()
+del frozen.x  # TODO this should emit an [invalid-assignment]
 ```
 
 ### `match_args`

crates/ty_python_semantic/src/types/class.rs

@@ -1589,6 +1589,25 @@ impl<'db> ClassLiteral<'db> {
                     .place
                     .ignore_possibly_unbound()
             }
+            (CodeGeneratorKind::DataclassLike, "__setattr__") => {
+                if has_dataclass_param(DataclassParams::FROZEN) {
+                    let signature = Signature::new(
+                        Parameters::new([
+                            Parameter::positional_or_keyword(Name::new_static("self"))
+                                .with_annotated_type(Type::instance(
+                                    db,
+                                    self.apply_optional_specialization(db, specialization),
+                                )),
+                            Parameter::positional_or_keyword(Name::new_static("name")),
+                            Parameter::positional_or_keyword(Name::new_static("value")),
+                        ]),
+                        Some(Type::Never),
+                    );
+
+                    return Some(CallableType::function_like(db, signature));
+                }
+                None
+            }
             _ => None,
         }
     }

crates/ty_python_semantic/src/types/infer.rs

@@ -3468,18 +3468,39 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                     MemberLookupPolicy::MRO_NO_OBJECT_FALLBACK,
                 );
 
+                let get_setattr_dunder_attr_ty =
+                    || object_ty.class_member(db, "__setattr__".into());
+
                 let check_setattr_return_type = |result: Bindings<'db>| -> bool {
                     match result.return_type(db) {
                         Type::Never => {
                             if emit_diagnostics {
                                 if let Some(builder) =
                                     self.context.report_lint(&INVALID_ASSIGNMENT, target)
                                 {
-                                    builder.into_diagnostic(format_args!(
-                                                                   "Cannot assign to attribute `{attribute}` on type `{}` \
-                                                                    whose `__setattr__` method returns `Never`/`NoReturn`",
-                                                                   object_ty.display(db)
-                                                               ));
+                                    let is_setattr_synthesized = match get_setattr_dunder_attr_ty()
+                                    {
+                                        PlaceAndQualifiers {
+                                            place: Place::Type(attr_ty, _),
+                                            qualifiers: _,
+                                        } => attr_ty.is_callable_type(),
+                                        _ => false,
+                                    };
+
+                                    let msg = if is_setattr_synthesized {
+                                        format!(
+                                            "Property `{attribute}` defined in `{}` is read-only",
+                                            object_ty.display(db)
+                                        )
+                                    } else {
+                                        format!(
+                                            "Cannot assign to attribute `{attribute}` on type `{}` \
+                                                 whose `__setattr__` method returns `Never`/`NoReturn`",
+                                            object_ty.display(db)
+                                        )
+                                    };
+
+                                    builder.into_diagnostic(msg);
                                 }
                             }
                             false