have a concept of user code vs library code when typechecking

[ddfisher]

Certain warning options shouldn't trigger on typeshed (and other library code). See #1554 and #1884, for example. This is currently handled with the is_typeshed_stub bool in TypeChecker. Unfortunately, this doesn't work for other libraries and doesn't allow us to purposefully run these checks on typeshed. Instead, we could have a concept of user code vs library code, where code mypy was specifically run on has these warnings enabled, and other code does not.

Down the line, we might also want to use this flag to avoid typechecking library code when it's referenced from user code -- we could limit ourselves to reading the types.

[gvanrossum]

Maybe there should be flags (in the setup file?) to specify patterns for what's considered to be in each category?

[ilevkivskyi]

I think this is now superseded by PEP 561.