fix: preserve slots declared on slotted attrs classes

changelog.d/1642.change.md

@@ -0,0 +1 @@
+Fixed `@attrs.define` dropping slots declared on the slotted class itself when the attribute set is rewritten.

src/attr/_make.py

@@ -947,6 +947,21 @@ def _create_slots_class(self):
         # Setting __slots__ to inherited attributes wastes memory.
         slot_names = [name for name in names if name not in base_names]
 
+        # Preserve slots declared on the class itself: they would otherwise
+        # be lost when we rewrite `__slots__` below.
+        own_slots = tuple(self._cls.__dict__.get("__slots__", ()))
+        own_new_slots = [
+            name
+            for name in own_slots
+            if name not in slot_names and name not in existing_slots
+        ]
+        slot_names = own_new_slots + slot_names
+        # Remove stale descriptors for the class's own slots from the new
+        # class dict -- the type machinery recreates them from `__slots__`
+        # and keeping both raises ValueError on class creation.
+        for name in own_new_slots:
+            cd.pop(name, None)
+
         # There are slots for attributes from current class
         # that are defined in parent classes.
         # As their descriptors may be overridden by a child class,

tests/test_slots.py

@@ -282,6 +282,24 @@ class C2(C1):
     assert {"x": 1, "y": 2, "z": "test"} == attr.asdict(c2)
 
 
+def test_own_slots_preserved():
+    """
+    Slots declared on the slotted attrs class itself are preserved.
+    """
+
+    @attrs.define
+    class A:
+        __slots__ = ("private",)
+
+        public: str = "foo"
+
+    a = A()
+    a.private = 1
+    assert 1 == a.private
+    assert "foo" == a.public
+    assert "private" in A.__slots__
+
+
 def test_inheritance_from_slots_with_attribute_override():
     """
     Inheriting from a slotted class doesn't re-create existing slots