No warning for unused function using `impl Trait` on beta and nightly

[jonas-schievink]

The following code only warns that f is unused on stable, not on beta or nightly:

trait Trait {}
impl<T> Trait for T {}

fn f() -> impl Trait {}

(apologies if this is a duplicate, but a quick search didn't turn anything up)

[jonas-schievink]

This has appeared between 63d6649 and d41f21f (commits in that range).

Likely culprit is #53545 (cc @FelixMcFelix). It introduces a new reachability level to rustc (ReachableFromImplTrait), which marks any function with an impl Trait return type with that level. Maybe the level required by the lint to consider an item reachable needs to be higher than that?

[FelixMcFelix]

Yeah, the likely fix for this is to create a query for lints that removes all levels under AccessLevel::Reachable, or manually strip the entries each time. The former probably makes more sense. I wasn't sure we'd need to do this at the time, it shouldn't take too long.

[jonas-schievink]

I've implemented a more minimal fix at #54810. It should be safe to backport to beta, if this is wanted at all.

[pnkfelix]

(assigning self just to make sure everything gets followed up on here)

[pnkfelix]

tagged as P-high since it was a regression.