Rule Proposal: Enforce generator on contextlib.contextmanager decorated functions

[max-muoto]

In the past, it was fairly common for people to simply put Iterator on context managers simply out of convenience, as typing out Generator, required adding in two (mostly useless) generic params. However, this can lead to runtime failures not caught by a type-checker, take this example from python/typeshed#2772.

import contextlib
from typing import Iterator

@contextlib.contextmanager
def f() -> Iterator[int]:
    return iter([1])


with f():
    pass


with f():
    raise TypeError('wat')

Due to how breaking this change is, the typeshed maintainers were are reluctant to change the underlying annotation to require a Generator. It was actually proposed offloading this responsibility to linting tules, or special rules within the type-checkers themselves.

Now, Iterator[int] can be simplified to Generator[int] with the addition of type var defaults, and their usage in the cpython implementation, and the typesheds, see: python/typeshed#11867

I would propose adding a rule to autofix Iterator[T] to Generator[T] on 3.13+ for any functions decorated with contextlib.contextmanager, or when deferred annotations are present in the file, as otherwise a runtime error will be encountered. In cases where neither are true, an error can simply be flagged and fixed manually (either through quoting Generator[T], or doing Generator[T, None, None]).

This will help ensure that type-checkers such as Pyright and MyPy can flag and catch issues that might be hidden as a result.