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

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[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.