Highlight identifier span instead of whole pattern span in `unused` lint
Fixes#81314
This pretty much just changes the span highlighted in the lint from `pat_sp` to `ident.span`. There's however an exception, which is in patterns with shorthands like `Point { y, ref mut x }`, where a suggestion to change just `x` would be invalid; in those cases I had to keep the pattern span. Another option would be suggesting something like `Point { y, x: ref mut _x }`.
I also added a new test since there weren't any test that checked the `unused` lint with optional patterns.
Guillaume Gomez
d259d1d98a