Add runtime `__slots__` attribute to `attrs` (#15651)

This is similar to https://github.com/python/mypy/pull/15649 but for
`attrs` :)
Refs https://github.com/python/mypy/issues/15647

mypy/plugins/attrs.py

diff --git a/mypy/plugins/attrs.py b/mypy/plugins/attrs.py
index 7f1196c..0f748cc 100644
--- a/mypy/plugins/attrs.py
+++ b/mypy/plugins/attrs.py
@@ -896,6 +896,13 @@
     # Unlike `@dataclasses.dataclass`, `__slots__` is rewritten here.
     ctx.cls.info.slots = {attr.name for attr in attributes}
 
+    # Also, inject `__slots__` attribute to class namespace:
+    slots_type = TupleType(
+        [ctx.api.named_type("builtins.str") for _ in attributes],
+        fallback=ctx.api.named_type("builtins.tuple"),
+    )
+    add_attribute_to_class(api=ctx.api, cls=ctx.cls, name="__slots__", typ=slots_type)
+
 
 def _add_match_args(ctx: mypy.plugin.ClassDefContext, attributes: list[Attribute]) -> None:
     if (

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

diff --git a/test-data/unit/check-plugin-attrs.test b/test-data/unit/check-plugin-attrs.test
index 3d1e2d7..88a541c 100644
--- a/test-data/unit/check-plugin-attrs.test
+++ b/test-data/unit/check-plugin-attrs.test
@@ -1676,6 +1676,33 @@
         self.c = 2  # E: Trying to assign name "c" that is not in "__slots__" of type "__main__.C"
 [builtins fixtures/plugin_attrs.pyi]
 
+[case testRuntimeSlotsAttr]
+from attr import dataclass
+
+@dataclass(slots=True)
+class Some:
+    x: int
+    y: str
+    z: bool
+
+reveal_type(Some.__slots__)  # N: Revealed type is "Tuple[builtins.str, builtins.str, builtins.str]"
+
+@dataclass(slots=True)
+class Other:
+    x: int
+    y: str
+
+reveal_type(Other.__slots__)  # N: Revealed type is "Tuple[builtins.str, builtins.str]"
+
+
+@dataclass
+class NoSlots:
+    x: int
+    y: str
+
+NoSlots.__slots__  # E: "Type[NoSlots]" has no attribute "__slots__"
+[builtins fixtures/plugin_attrs.pyi]
+
 [case testAttrsWithMatchArgs]
 # flags: --python-version 3.10
 import attr