Reference cycle for slot classes

Even the birds on the branches nowadays know we do a class switcheroo when defining slot classes.

The initial class (the one we throw away) is part of a reference cycle with something else, so it doesn't get GC'd right away. A tiny example:

from attrs import define


@define
class A:
    a: int


@define
class B(A):
    b: int


# collect()
print(A.__subclasses__())  # [<class '__main__.B'>, <class '__main__.B'>]

If gc.collect() is called right afterwards, the old class gets cleaned up, so it's almost certainly a reference cycle.

So, a good issue for someone getting started with attrs: find out what the reference cycle is and break it so this doesn't happen.

Alternatively, we could call gc.collect() ourselves. We could also check for the old class in __subclasses__() and emit a warning if we find it there; it probably means someone is using a bare super somewhere we didn't rewrite it.

Start by reproducing the attrs class-switching example from the issue and inspect the references that keep the initial slot class alive. Use gc.collect() and A.subclasses() to confirm the cycle and compare behavior before and after class creation. Done means the discarded class is no longer retained without requiring an explicit collection.