Require #[pin_v2] for direct &pin borrows of ADTs

[P8L1]

Summary

Require direct user-written &pin borrows of ADTs to use #[pin_v2].

The check runs during HIR type checking for explicit hir::BorrowKind::Pin, so it
rejects direct &pin mut / &pin const borrow expressions on non-#[pin_v2]
ADTs without affecting compiler-generated AutoBorrow::Pin adjustments or
coercions.

What changed

• Added a direct-borrow diagnostic for attempting to directly pin an ADT that is
  not #[pin_v2].
• Added a typeck check in FnCtxt::check_expr_addr_of for explicit user-written
  &pin borrows.
• Reused AdtDef::is_pin_project() as the source of truth for #[pin_v2].
• Updated existing pin ergonomics tests that should continue reaching borrowck by
  marking their local ADTs #[pin_v2].
• Added focused UI coverage for direct &pin mut and &pin const borrows of a
  non-#[pin_v2] ADT.

Why

The active pin ergonomics borrowck PR still had an unchecked task to forbid
&pin borrows for ADTs without #[pin_v2].

Pattern projection already rejects non-#[pin_v2] ADTs in related cases, but
direct user-written &pin mut foo and &pin const foo borrow expressions were
still accepted.

This PR enforces that direct rule while avoiding compiler-generated transient pin
adjustments.

Tests

Passed:

./x test tests/ui/pin-ergonomics/direct-borrow-requires-pin-v2.rs --stage 1 --test-args --force-rerun
./x test tests/ui/pin-ergonomics/borrow.rs --stage 1 --test-args --force-rerun
./x test tests/ui/pin-ergonomics/borrow-unpin.rs --stage 1 --test-args --force-rerun
./x test tests/ui/pin-ergonomics/borrow-pinned-projection.rs --stage 1 --test-args --force-rerun
./x test tests/ui/pin-ergonomics/user-type-projection.rs --stage 1 --test-args --force-rerun
./x test tests/ui/pin-ergonomics/pin_v2-attr.rs --stage 1
./x test tests/ui/pin-ergonomics/impl-unpin.rs --stage 1
./x test tests/ui/pin-ergonomics/pattern-matching.rs --stage 1
./x test tests/ui/pin-ergonomics/pattern-matching-mix-deref-pattern.rs --stage 1
./x test tests/ui/pin-ergonomics/pin-coercion.rs --stage 1
./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
git diff --check
./x fmt --check

Notes for reviewers

The main review questions are:

• Should generic type parameters remain accepted by this direct ADT-only rule?
• Should direct &pin of the Pin ADT itself have any special exception, or
  should the literal ADT rule apply?
• Is the machine-applicable #[pin_v2] suggestion appropriate here, matching
  the existing projection diagnostic style?

Related

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

[P8L1]

The CI failure appears to come from src/ci/scripts/verify-channel.sh because this PR targets the feature/pin-borrowck branch rather than the normal Rust main branch.

The focused local checks passed and are listed in the PR body. This PR is intended as a patch against the active pin-borrowck feature branch for rust-lang#153693, not as an independent PR directly to main.

[frank-king]

Grateful for your help! Let's merge it first and see if the original PR can pass the CI.

[P8L1]

I saw the CI failure in https://github.com/rust-lang/rust/actions/runs/25086407095/job/73502863053 after my merged helper PR.

I’m reproducing the exact failing pin-coercion.rs path against the current feature/pin-borrowck branch now, including the stage-2 test path, before making any further changes.

If this was caused by my patch, I’ll follow up with a focused fix PR. Sorry for the churn.