Improve &pin reference-pattern suggestions

[P8L1]

This fills in the pin_ergonomics FIXME in borrow_pat_suggestion by making the diagnostic prefix account for both Pinnedness and Mutability.

Previously, the type-position suggestion path used ordinary reference spelling, which can spell & and &mut but cannot correctly spell pinned reference-pattern suggestions such as &pin const and &pin mut.

This is a diagnostic-only change. It adds focused UI coverage for both pinned const and pinned mut reference-pattern suggestions.

[rustbot]

r? @mu001999

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[mu001999]

r? Kivooeo

[Kivooeo]

two things i think should be improved

1. currently it make suggestion to a code, that requires pin_ergonomics feature to work, so imo this suggestion should only emits when this feature is enabled (i don't know if there is a way to check so)
2. i don't sure if this code only works on nightly or not? this suggestion have zero meaning on stable

so, if we can't check 1 point then i'd say to wait on stabilization of this feature, and then fix that fixme

[Kivooeo]

@rustbot authot

[P8L1]

two things i think should be improved

1. currently it make suggestion to a code, that requires pin_ergonomics feature to work, so imo this suggestion should only emits when this feature is enabled (i don't know if there is a way to check so)
2. i don't sure if this code only works on nightly or not? this suggestion have zero meaning on stable

so, if we can't check 1 point then i'd say to wait on stabilization of this feature, and then fix that fixme

Thanks for the suggestion. I gated the type-position &pin suggestion on pin_ergonomics, so crates without the feature fall back to the existing non-pin reference suggestion instead.