msgspec.defstruct is not compatible with msgspec.StructMeta

[kunesj]

Description

Structs that were defined with custom metaclass (subclass of msgspec.StructMeta) can't be subclassed with msgspec.defstruct. (msgspec 0.20.0)

Following code:

import msgspec

class _FooStructMeta(msgspec.StructMeta):
    def __new__(mcls, name, bases, namespace, **struct_config):
        return super().__new__(mcls, name, bases, namespace, **struct_config)

class FooStruct(msgspec.Struct, metaclass=_FooStructMeta):
    pass

BarStruct = msgspec.defstruct(name="BarStruct", fields=[("id", int)], bases=(FooStruct,))

Raises this error:

Traceback (most recent call last):
  File "/home/jirka642/.config/JetBrains/PyCharm2025.3/scratches/scratch_1.py", line 13, in <module>
    BarStruct = msgspec.defstruct(name="BarStruct", fields=[("id", int)], bases=(FooStruct,))
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jirka642/.config/JetBrains/PyCharm2025.3/scratches/scratch_1.py", line 6, in __new__
    return super().__new__(mcls, name, bases, namespace, **struct_config)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Struct types cannot define __slots__

I have been able to hotfix it by running namespace.pop("__slots__", None) in the metaclass, but I'm not sure if that is safe.