Suppress E0621 perpetual borrow suggestion

[Dnreikronos]

Fixes #156682

E0621 currently suggests changing &mut Buffer<'a> to &'a mut Buffer<'a>. This creates a perpetual borrow, where the value borrows itself for its entire lifetime and becomes unusable after the call. The suggestion is almost never what the user wants.

What changed

When the suggested type would produce &'named [mut] T where 'named also appears inside T, this PR suppresses the misleading suggestion and instead emits a correct multipart suggestion that introduces a fresh lifetime parameter (e.g. 'b) and applies it only to the outer reference.

How the detection works

After computing new_ty (the type with the anonymous region replaced by the named one), we check if new_ty is ty::Ref(outer_region, inner_ty, _) where outer_region == named and tcx.any_free_region_meets(inner_ty, |r| r == named).

Before

help: add explicit lifetime `'a` to the type of `buffer`
  |
5 | pub fn foo<'a>(buffer: &'a mut Buffer<'a>) {
  |                         ++

After

help: consider introducing a new lifetime `'b` to distinguish the borrow duration from `'a`
  |
5 | pub fn foo<'b, 'a>(buffer: &'b mut Buffer<'a>) {
  |            +++              ++

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[mejrs]

Please drop d061dcb from this pr, you already have another pr open for that and this pr does not build upon it.

View changes since this review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[Dnreikronos]

Rebased to drop that commit. The PR now only contains the perpetual borrow fix on top of main.