Lint for unused `extern mod`

[klutzy]

extern mod extra;
fn main() {}

Would be good if rustc prints lint warning for unused extern mod.

[alexcrichton]

This would be a bit tricky because currently every program links to the rustuv crate, and I don't think that any of the ever explicitly use it at all. It registers something to be put in the crate map, which is considered as "using' the rustuv crate, but this doesn't happen until translation time.

That being said, we have lots of lints for other unused things, so why not extern mod as well!

[elinorbgr]

Hello,
Seeing the tag E-easy, I gave a try to this. While creating the lint itself was indeed pretty easy, there is currently no tracking of whether an extern crate as been used or not in librustc/middle/resolve.rs (where the tacking of used imports is done). So at my point, every extern crate is marked as unused by the lint checker.
But the tracking code of imports itself seems to be a side effect of the whole resolve code, and thus it is quite difficult to understand the whole thing enough to add tracking of extern crates as well. I'm not sure whether I'll manage to figure it out or not.

So I think this is done. Thank you, @vberger!