Add an optional error code for unimported reveal_type / reveal_locals

[sobolevn]

Some of our users report that reveal_type causes problems in runtime: typeddjango/django-stubs#590

I agree that this is indeed problematic. For example:

x = 1
reveal_type(x)

Mypy checks this code, finds 0 issues, returns 0 as the return code. This script will pass the CI, but will break in runtime. This is not what mypy is expected to do: it is expected to find bugs.

So, I propose adding a new error code:

• We can name it unimported-reveal
• It will be turned off by default
• It will raise errors on all plain reveal_type / reveal_locals usages
• If you want to keep reveal_type even in production (for some reason), use from typing[_extensions] import reveal_type or type: ignore[unimported-reveal]

Do others have any objections?