I tried to use a compiled class with weakrefs and had mixed results. Here are my findings:

Not working:

class MyFailingClass0:
    pass

obj = MyFailingClass0()
ref = weakrefs.ref(obj)

@final
class MyFailingClass1:
    pass

obj = MyFailingClass1()
ref = weakrefs.ref(obj)

I then thought about my experience with cython and tried these, also not working:

class MyFailingClass2:
    def __init__(self):
        self.__weakref__: Final[Optional[ReferenceType]] = None

obj = MyFailingClass2()
ref = weakrefs.ref(obj)

@final
class MyFailingClass3:
    def __init__(self):
        self.__weakref__: Final[Optional[ReferenceType]] = None

obj = MyFailingClass3()
ref = weakrefs.ref(obj)

Next I tried giving my class a dict attribute manually, also not working:

class MyFailingClass4:
    def __init__(self):
        self.__dict__: Final[Dict[str, Any]] = {}

obj = MyFailingClass4()
ref = weakrefs.ref(obj)

@final
class MyFailingClass5:
    def __init__(self):
        self.__dict__: Final[Dict[str, Any]] = {}

obj = MyFailingClass5()
ref = weakrefs.ref(obj)

I think the above results show us a few things are not working as they probably should be:

1. a user should be able to add a dict attribute to a compiled class, either by defining it in the class init or with mypyc_attr(dict=True) similar to how msgspec handles it, or both. Based on what I saw here https://github.com/python/mypy/blob/501a07b45af8e44eda665e53526fc590dc5a014e/mypyc/codegen/emitclass.py#L285 I believe if this is implemented then weakrefs would also work for those classes. It might also be good to autodetect uses of functools.cached_property and give those classes a dict attr as well.
2. a user should be able to add a weakref attribute to a compiled class, by one or more of the following: defining it in the class init, defining it in slots, with mypyc_attr(supports_weakref=True)

However! I was able to find a pattern that succeeds with weakrefs currently:

noncompiledfile.py

class WeakrefBase:
    """We need our class to inherit from a python class in order for weakrefs to work."""

compiledfile.py

from noncompiledfile import WeakrefBase

class Weakrefable(WeakrefBase):
    ...

obj = Weakreafable()
ref = weakrefs.ref(obj)

Success!