fix inspection for structs that use the StructMeta metaclass (#941) (#945)

Co-authored-by: Ofek Lev <ofekmeister@gmail.com>

Author: provinzkraut

docs/structs.rst

@@ -931,8 +931,11 @@ has a signature similar to `dataclasses.make_dataclass`. See
     Point(x=1.0, y=2.0)
 
 
+Advanced
+--------
+
 Metaclasses
------------
+~~~~~~~~~~~
 
 You can define project-wide :class:`msgspec.Struct` policies at class-creation
 time by extending the :class:`msgspec.StructMeta` metaclass.
@@ -1002,19 +1005,22 @@ abstract base Structs (like ``Event``) cannot be instantiated, and
 :func:`isinstance` and :func:`issubclass` checks behave the same as for normal
 ABCs.
 
-.. warning::
+.. important::
 
-    Mixing :class:`msgspec.StructMeta` with arbitrary metaclasses
-    is not supported. Only combinations involving :class:`abc.ABCMeta`
-    (or its subclasses) are guaranteed to work. Prefer using
-    :meth:`object.__init_subclass__` on a :class:`msgspec.Struct` base class
-    instead of additional custom metaclasses.
+    - Classes with a :class:`msgspec.StructMeta`-derived metaclass do not
+      *technically* need to inherit from :class:`msgspec.Struct`, but it is
+      recommended to do so for static typing support in IDEs and other tools.
+    - Mixing :class:`msgspec.StructMeta` with arbitrary metaclasses
+      is not supported. Only combinations involving :class:`abc.ABCMeta`
+      (or its subclasses) are guaranteed to work. Prefer using
+      :meth:`object.__init_subclass__` on a :class:`msgspec.Struct` base class
+      instead of additional custom metaclasses.
 
 
 .. _struct-gc:
 
-Disabling Garbage Collection (Advanced)
----------------------------------------
+Disabling Garbage Collection
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 .. warning::
 

src/msgspec/inspect.py

@@ -675,7 +675,7 @@ def _origin_args_metadata(t):
 
 
 def _is_struct(t):
-    return type(t) is type(msgspec.Struct)
+    return isinstance(t, msgspec.StructMeta)
 
 
 def _is_enum(t):

src/msgspec/structs.py

@@ -6,6 +6,7 @@
 from ._core import (  # noqa
     Factory as _Factory,
     StructConfig,
+    StructMeta,
     asdict,
     astuple,
     force_setattr,
@@ -72,12 +73,19 @@ def fields(type_or_instance: Struct | type[Struct]) -> tuple[FieldInfo]:
     -------
     tuple[FieldInfo]
     """
-    if isinstance(type_or_instance, Struct):
-        annotated_cls = cls = type(type_or_instance)
+    obj = type_or_instance
+
+    # Struct class
+    if isinstance(obj, StructMeta):
+        annotated_cls = cls = obj
+    # Struct instance
+    elif isinstance(type(obj), StructMeta):
+        annotated_cls = cls = type(obj)
+    # Generic alias
     else:
-        annotated_cls = type_or_instance
-        cls = getattr(type_or_instance, "__origin__", type_or_instance)
-        if not (isinstance(cls, type) and issubclass(cls, Struct)):
+        annotated_cls = obj
+        cls = getattr(obj, "__origin__", obj)
+        if not isinstance(cls, StructMeta):
             raise TypeError("Must be called with a struct type or instance")
 
     hints = _get_class_annotations(annotated_cls)

src/msgspec/structs.pyi

@@ -34,4 +34,4 @@ class FieldInfo(Struct):
     @property
     def required(self) -> bool: ...
 
-def fields(type_or_instance: Struct | type[Struct]) -> tuple[FieldInfo]: ...
+def fields(type_or_instance: S | type[S]) -> tuple[FieldInfo]: ...

tests/unit/test_inspect.py

@@ -786,3 +786,32 @@ class Example(msgspec.Struct):
     res = mi.multi_type_info([Example, List[Example]])
     assert res == (ex_type, mi.ListType(ex_type))
     assert res[0] is res[1].item_type
+
+
+def test_type_info_custom_base_class():
+    class CustomMeta(msgspec.StructMeta):
+        pass
+
+    class Base(metaclass=CustomMeta):
+        pass
+
+    class Model(Base):
+        foo: str
+
+    assert mi.type_info(Model) == mi.StructType(
+        cls=Model,
+        fields=(
+            mi.Field(
+                name="foo",
+                encode_name="foo",
+                type=mi.StrType(min_length=None, max_length=None, pattern=None),
+                required=True,
+                default=msgspec.NODEFAULT,
+                default_factory=msgspec.NODEFAULT,
+            ),
+        ),
+        tag_field=None,
+        tag=None,
+        array_like=False,
+        forbid_unknown_fields=False,
+    )

tests/unit/test_struct.py

@@ -2412,6 +2412,30 @@ class Bad(Generic[T]):
             with pytest.raises(TypeError, match="struct type or instance"):
                 msgspec.structs.fields(val)
 
+    def test_fields_struct_meta(self):
+        class CustomMeta(msgspec.StructMeta):
+            pass
+
+        class Base(metaclass=CustomMeta):
+            pass
+
+        class Model(Base):
+            pass
+
+        assert msgspec.structs.fields(Model) == ()
+
+    def test_fields_struct_meta_instance(self):
+        class CustomMeta(msgspec.StructMeta):
+            pass
+
+        class Base(metaclass=CustomMeta):
+            pass
+
+        class Model(Base):
+            pass
+
+        assert msgspec.structs.fields(Model()) == ()
+
     def test_fields_no_fields(self):
         assert msgspec.structs.fields(msgspec.Struct) == ()