Fix unused_parens for pinned reference patterns

[P8L1]

The unused_parens lint previously treated pinned shared-reference patterns like plain shared-reference patterns when deciding whether removing parentheses could change the meaning of a leading mut binding.

That was too conservative for pinned reference patterns such as &pin const (mut x). The pin const part belongs to the outer reference pattern, so removing the inner parentheses still leaves mut x as the subpattern.

This updates the ambiguity check so the leading-mut suppression only applies to plain shared reference patterns. Pinned reference patterns can now receive correct rustfix suggestions.

The patch also broadens the binding check from only plain mut bindings to all binding modes whose binding mutability is Mutability::Mut, covering forms such as mut ref pin const.

Tests cover:

• &pin const (...)
• &pin mut (...)
• mutable bindings under pinned refs
• mut _x @ _
• nested pinned reference patterns
• ref pin const / ref pin mut
• mut ref pin const / mut ref pin mut
• pinned or-patterns remaining suppressed
• plain shared &(mut ...) cases remaining suppressed

[rustbot]

r? @nnethercote

rustbot has assigned @nnethercote.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 73 candidates
• Random selection from 21 candidates

[Kivooeo]

same as here: #156087 (comment)

@rustbot author

[rustbot]

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

[P8L1]

same as here: #156087 (comment)

@rustbot author

I gated the pinned-pattern unused-parens rustfix path on pin_ergonomics, so it is only emitted for crates that opted into the feature. I also left a FIXME next to the gate saying to remove it once pin_ergonomics is stabilized.

[P8L1]

@rustbot ready

Addressed the review feedback by gating the pinned-pattern unused_parens rustfix path on pin_ergonomics, so it is only emitted for crates that opted into the feature.

I also added a FIXME next to the gate saying to remove it once pin_ergonomics is stabilized.

[P8L1]

r? @Kivooeo

[P8L1]

@Kivooeo Please review this as well

[Kivooeo]

it's been only two days since it's open, review may take up to two weeks

since i'm pretty busy with other stuff i'd ask you to wait, because it may take a little longer

[P8L1]

Hey @Kivooeo, just wanted to gently bump this since it’s been about two weeks.

I think I’ve addressed the previous feedback, but please let me know if there’s anything else you’d like changed.

Thanks!

[Kivooeo]

looks fine to me, but some nits

View changes since this review

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[P8L1]

@Kivooeo ready
I addressed all the nits and squashed to 1 commit