Support inheriting from slots classes with gc=False

Previously we prevented setting `gc=False` when inheriting from any
non-Struct type. In reality this is safe to set when inheriting from any
non-c-extension class lacking a `__dict__`.

msgspec/_core.c

@@ -5456,7 +5456,7 @@ typedef struct {
     bool already_has_dict;
     int cache_hash;
     Py_ssize_t hash_offset;
-    bool has_non_struct_bases;
+    bool has_non_slots_bases;
 } StructMetaInfo;
 
 static int
@@ -5505,7 +5505,9 @@ structmeta_collect_base(StructMetaInfo *info, MsgspecState *mod, PyObject *base)
     }
 
     if (Py_TYPE(base) != &StructMetaType) {
-        info->has_non_struct_bases = true;
+        if (((PyTypeObject *)base)->tp_dictoffset) {
+            info->has_non_slots_bases = true;
+        }
         /* XXX: in Python 3.8 Generic defines __new__, but we can ignore it.
          * This can be removed when Python 3.8 support is dropped */
         if (base == mod->typing_generic) return 0;
@@ -6221,7 +6223,7 @@ StructMeta_new_inner(
         .already_has_dict = false,
         .cache_hash = arg_cache_hash,
         .hash_offset = 0,
-        .has_non_struct_bases = false,
+        .has_non_slots_bases = false,
     };
 
     info.defaults_lk = PyDict_New();
@@ -6273,10 +6275,10 @@ StructMeta_new_inner(
     }
 
     if (info.gc == OPT_FALSE) {
-        if (info.has_non_struct_bases) {
+        if (info.has_non_slots_bases) {
             PyErr_SetString(
                 PyExc_ValueError,
-                "Cannot set gc=False when inheriting from non-struct types"
+                "Cannot set gc=False when inheriting from non-struct types with a __dict__"
             );
             goto cleanup;
         }

tests/test_struct.py

@@ -1049,18 +1049,28 @@ class Test(Struct, gc=has_gc):
         assert not gc.is_tracked(copy.copy(Test(1, ())))
         assert gc.is_tracked(copy.copy(Test(1, []))) == has_gc
 
-    def test_struct_gc_false_forbids_mixins(self):
-        class Mixin:
+    def test_struct_gc_false_cannot_inherit_from_non_slots_classes(self):
+        class Base:
             pass
 
         with pytest.raises(
             ValueError,
-            match="Cannot set gc=False when inheriting from non-struct types",
+            match="Cannot set gc=False when inheriting from non-struct types with a __dict__",
         ):
 
-            class Test(Struct, Mixin, gc=False):
+            class Test(Struct, Base, gc=False):
                 pass
 
+    def test_struct_gc_false_can_inherit_from_slots_class_mixin(self):
+        class Base:
+            __slots__ = ()
+
+        class Test(Struct, Base, gc=False):
+            x: int
+
+        t = Test(1)
+        assert not gc.is_tracked(t)
+
     @pytest.mark.parametrize("case", ["base-dict", "base-nogc", "nobase"])
     def test_struct_gc_false_forbids_dict_true(self, case):
         if case == "base-dict":