Summary

Fix a false pinned-borrow diagnostic in pin_ergonomics borrow checking for valid T: Unpin coercions involving Pin<&mut T>::as_mut().

The fix distinguishes user-written &pin borrows, which create persistent pinned-place facts, from compiler-generated pinned adjustments and coercions, which should not create long-lived pin facts.

What changed

• Added mir::PinBorrowKind to distinguish Persistent and Transient pinned borrows.
• Marked explicit HIR &pin mut place and &pin const place lowering as PinBorrowKind::Persistent.
• Marked compiler-generated AutoBorrow::Pin lowering as PinBorrowKind::Transient.
• Changed the Pins dataflow so it records long-lived pin facts only for persistent pinned borrows.
• Kept transient pinned borrows as ordinary live borrows while they are in scope; they no longer leave behind expired persistent pin facts.
• Added focused UI coverage for:
  • simple T: Unpin coercion round trips,
  • the Pin<&mut T>::get_mut() followed by as_mut() regression path,
  • projected-place pinning with &pin mut pair.0.

Why

tests/ui/pin-ergonomics/pin-coercion.rs rejected valid T: Unpin coercion code around Pin<&mut T>::as_mut() with:

cannot borrow `x` as mutable because it is pinned

The previous dataflow treated every BorrowKind::Pinned borrow as a long-lived pin fact. That included compiler-generated pinned adjustments used for method receivers and coercions. Those transient adjustments should not make later valid T: Unpin coercion code look like it is mutably borrowing an already pinned place.

This is not a broad Unpin filter: explicit user &pin borrows of Unpin values still create persistent pin facts and remain protected until reassignment.

Tests

Passed:

./x test tests/ui/pin-ergonomics/pin-coercion-unpin-roundtrip.rs --stage 1
./x test tests/ui/pin-ergonomics/pin-coercion-get-mut-then-as-mut.rs --stage 1
./x test tests/ui/pin-ergonomics/pin-coercion.rs --stage 1
./x test tests/ui/pin-ergonomics/borrow.rs --stage 1
./x test tests/ui/pin-ergonomics/borrow-unpin.rs --stage 1
./x test tests/ui/pin-ergonomics/pin-pattern.rs --stage 1
./x test tests/ui/pin-ergonomics/pattern-matching.rs --stage 1
./x test tests/ui/pin-ergonomics/pin-coercion-check.rs --stage 1
./x test tests/ui/pin-ergonomics/borrow-pinned-projection.rs --stage 1
git diff --check
./x fmt --check

Notes for reviewers

The main review point is whether every explicit user &pin path is represented as PinBorrowKind::Persistent, and whether any explicit user pinning path reaches MIR only through AutoBorrow::Pin.

Transient pinned borrows are still treated as normal live borrows; the change is only that they no longer create long-lived expired pin facts in the Pins dataflow.

Related

• Implement borrowck for &pin mut|const $place rust-lang/rust#153693
• Tracking issue for pin ergonomics rust-lang/rust#130494