explicit-outlives-requirements should remove entire `where` clause if it removes the only bound

[matthiaskrgr]

Given the following code:

// run-pass
#![allow(dead_code)]
#![allow(unused_variables)]
#![feature(unsize, coerce_unsized)]

#[repr(packed)]
struct UnalignedPtr<'a, T: ?Sized>
    where T: 'a,
{
    data: &'a T,
}

fn main() {

    impl<'a, T, U> std::ops::CoerceUnsized<UnalignedPtr<'a, U>> for UnalignedPtr<'a, T>
        where
        T: std::marker::Unsize<U> + ?Sized,
        U: ?Sized,
    { }

    let arr = [1, 2, 3];
    let arr_unaligned: UnalignedPtr<[i32; 3]> = UnalignedPtr { data: &arr };
    let arr_unaligned: UnalignedPtr<[i32]> = arr_unaligned;
}

The current output is:

warning: outlives requirements can be inferred
 --> src/main.rs:8:11
  |
8 |     where T: 'a,
  |           ^^^^^ help: remove this bound
  |
  = note: requested on the command line with `-W explicit-outlives-requirements`

Rustfix will fail to apply this:

The following errors were reported:
error: expected `{` after struct name, found `,`
 --> src/main.rs:8:11
  |
8 |     where ,
  |           ^ expected `{` after struct name

error: aborting due to previous error

So we need to extend span to include the where: as well?

[cjgillot]

The relevant code is in rustc_lint/src/builtin.rs, with impl<'tcx> LateLintPass<'tcx> for ExplicitOutlivesRequirements.
The span should be computed using generics.span_for_predicate_removal(i) in the loop, instead of crafting a specific computation. Likewise, collect_outlives_bound_spans should use hir::Generics::span_for_bound_removal method. If using those 2 methods from hir::Generics is not enough to solve the issue, those methods should perhaps be adapted.

[nirmal-j-patel]

@rustbot claim

[nirmal-j-patel]

@rustbot release-assignment

Haven't been able to focus on this. Releasing assignment.

[reez12g]

@rustbot claim

[reez12g]

Progress report: I'm working on it.