[ty] Support frozen dataclasses

17675

Author: thejchap

crates/ty_python_semantic/resources/mdtest/dataclasses.md

@@ -369,7 +369,26 @@ To do
 
 ### `frozen`
 
-To do
+If true (the default is False), assigning to fields will generate a diagnostic. This emulates
+read-only frozen instances. If __setattr__() or __delattr__() is defined in the class, we should
+emit a diagnostic.
+
+```py
+from dataclasses import dataclass
+
+@dataclass(frozen=True)
+class Frozen:
+    x: int
+
+    # TODO: Emit a diagnostic here
+    def __setattr__(self, name: str, value: object) -> None: ...
+
+    # TODO: Emit a diagnostic here
+    def __delattr__(self, name: str) -> None: ...
+
+frozen = Frozen(1)
+frozen.x = 2  # error: [invalid-assignment]
+```
 
 ### `match_args`
 

crates/ty_python_semantic/src/types/infer.rs

@@ -2924,6 +2924,23 @@ impl<'db> TypeInferenceBuilder<'db> {
             | Type::TypeVar(..)
             | Type::AlwaysTruthy
             | Type::AlwaysFalsy => {
+                if let Type::NominalInstance(instance) = object_ty {
+                    let dataclass_params = match instance.class() {
+                        ClassType::NonGeneric(cls) => cls.dataclass_params(self.db()),
+                        ClassType::Generic(_) => None,
+                    };
+                    let frozen = dataclass_params
+                        .is_some_and(|params| params.contains(DataclassParams::FROZEN));
+                    if frozen && emit_diagnostics {
+                        if let Some(builder) = self.context.report_lint(&INVALID_ASSIGNMENT, target)
+                        {
+                            builder.into_diagnostic(format_args!(
+                                "Property `{attribute}` defined in `{ty}` is read-only",
+                                ty = object_ty.display(self.db()),
+                            ));
+                        }
+                    }
+                }
                 match object_ty.class_member(db, attribute.into()) {
                     meta_attr @ SymbolAndQualifiers { .. } if meta_attr.is_class_var() => {
                         if emit_diagnostics {