fix: preserve explicitly declared slots

changelog.d/1637.change.md

@@ -0,0 +1 @@
+Preserve slots declared directly on a class decorated with `@attrs.define` or `@attr.s`.

src/attr/_make.py

@@ -860,10 +860,22 @@ def _create_slots_class(self):
         """
         Build and return a new class with a `__slots__` attribute.
         """
+        own_slots = self._cls.__dict__.get("__slots__", ())
+        if isinstance(own_slots, str):
+            own_slots = (own_slots,)
+        else:
+            own_slots = tuple(own_slots)
+
         cd = {
             k: v
             for k, v in self._cls_dict.items()
-            if k not in (*tuple(self._attr_names), "__dict__", "__weakref__")
+            if k
+            not in (
+                *tuple(self._attr_names),
+                *own_slots,
+                "__dict__",
+                "__weakref__",
+            )
         }
 
         # 3.14.0rc2+
@@ -903,7 +915,9 @@ def _create_slots_class(self):
 
         base_names = set(self._base_names)
 
-        names = self._attr_names
+        names = self._attr_names + tuple(
+            name for name in own_slots if name not in self._attr_names
+        )
         if (
             self._weakref_slot
             and "__weakref__" not in getattr(self._cls, "__slots__", ())

tests/test_slots.py

@@ -104,6 +104,57 @@ def test_slots_being_used():
     assert attr.asdict(slot_instance) == attr.asdict(non_slot_instance)
 
 
+def test_explicit_slots_are_preserved():
+    """Slots declared on the class are retained when attrs creates the class."""
+
+    @attrs.define
+    class ExplicitSlots:
+        __slots__ = ("private",)
+
+        public: str = "foo"
+
+    instance = ExplicitSlots()
+    instance.private = 1
+
+    assert instance.private == 1
+    assert instance.public == "foo"
+    assert {"__weakref__", "private", "public"} == set(ExplicitSlots.__slots__)
+
+
+def test_explicit_string_slot_is_preserved():
+    """A single string slot declaration survives attrs class rebuilding."""
+
+    @attrs.define
+    class ExplicitStringSlot:
+        __slots__ = "private"  # noqa: PLC0205 - exercise the supported string form
+
+        public: str = "foo"
+
+    instance = ExplicitStringSlot()
+    instance.private = 1
+
+    assert instance.private == 1
+    assert instance.public == "foo"
+    assert {"__weakref__", "private", "public"} == set(
+        ExplicitStringSlot.__slots__
+    )
+
+
+def test_explicit_slot_for_attribute_is_preserved():
+    """A declared slot matching an attrs field is not duplicated."""
+
+    @attrs.define
+    class ExplicitAttributeSlot:
+        __slots__ = ("public",)
+
+        public: str
+
+    instance = ExplicitAttributeSlot("foo")
+
+    assert instance.public == "foo"
+    assert {"__weakref__", "public"} == set(ExplicitAttributeSlot.__slots__)
+
+
 def test_slots_base_of_slotted():
     """
     The (hopefully gc'ed) temporary base class of a slotted class contains a