Adds slots=True support for @attr.s, refs #11487 (#11489)

Closes #11487
Refs #11482
Refs #10801

Author: sobolevn

mypy/plugins/attrs.py

@@ -272,6 +272,7 @@ def attr_class_maker_callback(ctx: 'mypy.plugin.ClassDefContext',
     init = _get_decorator_bool_argument(ctx, 'init', True)
     frozen = _get_frozen(ctx, frozen_default)
     order = _determine_eq_order(ctx)
+    slots = _get_decorator_bool_argument(ctx, 'slots', False)
 
     auto_attribs = _get_decorator_optional_bool_argument(ctx, 'auto_attribs', auto_attribs_default)
     kw_only = _get_decorator_bool_argument(ctx, 'kw_only', False)
@@ -302,6 +303,8 @@ def attr_class_maker_callback(ctx: 'mypy.plugin.ClassDefContext',
             return
 
     _add_attrs_magic_attribute(ctx, raw_attr_types=[info[attr.name].type for attr in attributes])
+    if slots:
+        _add_slots(ctx, attributes)
 
     # Save the attributes so that subclasses can reuse them.
     ctx.cls.info.metadata['attrs'] = {
@@ -727,6 +730,12 @@ def _add_attrs_magic_attribute(ctx: 'mypy.plugin.ClassDefContext',
     )
 
 
+def _add_slots(ctx: 'mypy.plugin.ClassDefContext',
+               attributes: List[Attribute]) -> None:
+    # Unlike `@dataclasses.dataclass`, `__slots__` is rewritten here.
+    ctx.cls.info.slots = {attr.name for attr in attributes}
+
+
 class MethodAdder:
     """Helper to add methods to a TypeInfo.
 

test-data/unit/check-attr.test

@@ -1402,3 +1402,33 @@ class A:
 reveal_type(A.__attrs_attrs__)  # N: Revealed type is "Tuple[attr.Attribute[builtins.int], attr.Attribute[builtins.str]]"
 
 [builtins fixtures/attr.pyi]
+
+[case testAttrsClassWithSlots]
+import attr
+
+@attr.s(slots=True)
+class A:
+    b: int = attr.ib()
+
+    def __attrs_post_init__(self) -> None:
+        self.b = 1
+        self.c = 2  # E: Trying to assign name "c" that is not in "__slots__" of type "__main__.A"
+
+@attr.dataclass(slots=True)
+class B:
+    __slots__ = ()  # would be replaced
+    b: int
+
+    def __attrs_post_init__(self) -> None:
+        self.b = 1
+        self.c = 2  # E: Trying to assign name "c" that is not in "__slots__" of type "__main__.B"
+
+@attr.dataclass(slots=False)
+class C:
+    __slots__ = ()  # would not be replaced
+    b: int
+
+    def __attrs_post_init__(self) -> None:
+        self.b = 1  # E: Trying to assign name "b" that is not in "__slots__" of type "__main__.C"
+        self.c = 2  # E: Trying to assign name "c" that is not in "__slots__" of type "__main__.C"
+[builtins fixtures/attr.pyi]