warn(missing_docs) doesn't trigger on extern declarations

[Lokathor]

playground example

It seems like this lint should fire if you have an extern declaration without docs on it.

[camelid]

@rustbot modify labels: A-lint T-compiler C-bug

[lcnr]

I think I know what causes this and will write a more general fix here this week.

@rustbot claim

[lcnr]

the fix wasn't more general but it's a fix none the less #77032