ErrorCodes defined in mypy plugins seemingly cannot be ignored by --disable-error-code

[aleksanb]

It seems like errors registered in plugins are unignoreable by mypy, as in the following case from the django-stubs plugin for mypy for analyzing django code: typeddjango/django-stubs#993.

We have some custom error codes declared like this:

and presumably mypy checks the list of errors here:

mypy/mypy/main.py

Line 992 in 2004ae0

valid_error_codes = set(error_codes.keys())

before initializing plugins, so our custom errors haven't ended up in error_codes before we check them.

Any suggestions for workarounds or fixes?