attrs: define defaults to slots=True (#15642)

Fixes #15639.

The new-style API `attrs.define` [enables slots by
default](https://www.attrs.org/en/stable/api.html#attrs.define).

mypy/plugins/attrs.py

@@ -294,6 +294,7 @@ def attr_class_maker_callback(
     ctx: mypy.plugin.ClassDefContext,
     auto_attribs_default: bool | None = False,
     frozen_default: bool = False,
+    slots_default: bool = False,
 ) -> bool:
     """Add necessary dunder methods to classes decorated with attr.s.
 
@@ -314,7 +315,7 @@ def attr_class_maker_callback(
     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)
+    slots = _get_decorator_bool_argument(ctx, "slots", slots_default)
 
     auto_attribs = _get_decorator_optional_bool_argument(ctx, "auto_attribs", auto_attribs_default)
     kw_only = _get_decorator_bool_argument(ctx, "kw_only", False)

mypy/plugins/default.py

@@ -159,7 +159,9 @@ def get_class_decorator_hook_2(
                 attrs.attr_class_maker_callback, auto_attribs_default=None, frozen_default=True
             )
         elif fullname in attrs.attr_define_makers:
-            return partial(attrs.attr_class_maker_callback, auto_attribs_default=None)
+            return partial(
+                attrs.attr_class_maker_callback, auto_attribs_default=None, slots_default=True
+            )
 
         return None
 

test-data/unit/check-plugin-attrs.test

@@ -1631,6 +1631,24 @@ reveal_type(A.__attrs_init__)  # N: Revealed type is "def (self: __main__.A, b:
 [case testAttrsClassWithSlots]
 import attr
 
+@attr.define
+class Define:
+    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__.Define"
+
+
+@attr.define(slots=False)
+class DefineSlotsFalse:
+    b: int = attr.ib()
+
+    def __attrs_post_init__(self) -> None:
+        self.b = 1
+        self.c = 2
+
+
 @attr.s(slots=True)
 class A:
     b: int = attr.ib()