Concrete type only used in `impl Trait` is unused

[cramertj]

The following code gives "warning: enum is never used: Empty":

enum Empty { }
trait Bar<T> {}
impl Bar<Empty> for () {}

fn boo() -> impl Bar<Empty> {}

fn main() {
    boo();
}

This regressed pretty recently. cc @oli-obk

[jonas-schievink]

1.27.0 does not warn, 1.28.0 does.

[pnkfelix]

triage: P-medium; This seems like a papercut that should be easy to work-around (at least if one isn't using forbid...).

[pnkfelix]

switched to P-high after prompting from @nikomatsakis

[pnkfelix]

whoops forgot to tag with T-compiler in time for the meeting...

[pnkfelix]

assigning to @nikomatsakis

[oli-obk]

I'm grabbing this, I think I broke it and it should be easy to fix