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Gate direct pin-borrow ADT diagnostic on !Unpin#4

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frank-king merged 1 commit intofrank-king:feature/pin-borrowckfrom
P8L1:gate-pin-borrow-diagnostic-on-unpin
May 1, 2026
Merged

Gate direct pin-borrow ADT diagnostic on !Unpin#4
frank-king merged 1 commit intofrank-king:feature/pin-borrowckfrom
P8L1:gate-pin-borrow-diagnostic-on-unpin

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@P8L1 P8L1 commented Apr 30, 2026

Follow-up to #2.

The merged direct &pin borrow diagnostic was too strict for ADTs that are Unpin.

For T: Unpin, &pin mut T and &mut T are mutually coercible, so this diagnostic should only fire when the ADT is known to be !Unpin.

This updates the typeck check to keep the diagnostic limited to explicit user-written &pin borrows, but suppress it unless the ADT is known/proven to be !Unpin.

Tests now cover:

  • non-#[pin_v2] Unpin ADTs being accepted,
  • non-#[pin_v2] !Unpin ADTs being rejected,
  • #[pin_v2] !Unpin ADTs being accepted by this check,
  • &pin mut and &pin const,
  • generic/inference cases being accepted conservatively.

Tested with:

.\x.ps1 test tests/ui/pin-ergonomics/direct-borrow-requires-pin-v2.rs --stage 1 --test-args --force-rerun
.\x.ps1 test tests/pretty/pin-ergonomics.rs --stage 1 --test-args --force-rerun
.\x.ps1 test tests/ui/pin-ergonomics/borrow.rs --stage 1 --test-args --force-rerun
.\x.ps1 test tests/ui/pin-ergonomics/borrow-unpin.rs --stage 1 --test-args --force-rerun
.\x.ps1 test tests/ui/pin-ergonomics/borrow-pinned-projection.rs --stage 1 --test-args --force-rerun
.\x.ps1 test tidy --bless
.\x.ps1 fmt --check
git diff --check
.\x.ps1 test tests/ui/pin-ergonomics --stage 1 --test-args --force-rerun
.\x.ps1 test tests/ui/pin-ergonomics/direct-borrow-requires-pin-v2.rs --stage 2 --test-args --force-rerun

@P8L1 P8L1 mentioned this pull request Apr 30, 2026
Merged
@P8L1 P8L1 force-pushed the gate-pin-borrow-diagnostic-on-unpin branch from 99b01b8 to 965d40f Compare May 1, 2026 10:21
@frank-king frank-king merged commit edeac5e into frank-king:feature/pin-borrowck May 1, 2026
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2 participants

@P8L1 @frank-king