Spotlight shows traits with generic impls that do not apply to the specific type

[Manishearth]

In both these cases, the mutability or the generic parameter is not dealt with when showing spotlight impls:

#74370 (comment)

We should do something similar to what we do for auto traits and blanket impls.

[fmease]

Seems to be fixed (the Box<str> no longer has any "notable trait" spotlight & the "notable trait" spotlight for &[u8] now only contains the Read impl).

Not sure which PR has fixed it (needs success bisection). Tentatively marking E-needs-test but we likely already have sufficient test coverage.