Add hint for unresolved associated trait items if the trait has a single item
This PR introduces a special-cased hint for unresolved trait items paths. It is shown if:
- the path was not resolved to any existing trait item
- and no existing trait item's name was reasonably close with regard to edit distance
- and the trait only has a single item in the corresponding namespace
I didn't know where I should put tests, therefore so far I just managed to bless two existing tests. I would be glad for hints where should tests for a hint like this be created, how should they be named (with reference to the original issue?) and what tests should I create (is it enough to test it just for types? or create separate tests also for functions and constants?).
It could also be turned into a machine applicable suggestion I suppose.
This is my first `rustc` PR, so please go easy on me :)
Fixes: https://github.com/rust-lang/rust/issues/87638
Yuki Okushi
8ee962f88e