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Fix pin ergonomics borrowck for transient pinned borrows #1

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Merged
frank-king merged 1 commit into from
Apr 28, 2026
Merged

Fix pin ergonomics borrowck for transient pinned borrows #1

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4 changes: 2 additions & 2 deletions compiler/rustc_borrowck/src/borrow_set.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -124,8 +124,8 @@ impl<'tcx> fmt::Display for BorrowData<'tcx> {
mir::BorrowKind::Mut {
kind: mir::MutBorrowKind::Default | mir::MutBorrowKind::TwoPhaseBorrow,
} => "mut ",
mir::BorrowKind::Pinned(mir::Mutability::Not) => "pin const ",
mir::BorrowKind::Pinned(mir::Mutability::Mut) => "pin mut ",
mir::BorrowKind::Pinned(mir::Mutability::Not, _) => "pin const ",
mir::BorrowKind::Pinned(mir::Mutability::Mut, _) => "pin mut ",
};
write!(w, "&{:?} {}{:?}", self.region, kind, self.borrowed_place)
}
Expand Down
7 changes: 4 additions & 3 deletions compiler/rustc_borrowck/src/dataflow.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -757,9 +757,10 @@ impl<'tcx> rustc_mir_dataflow::Analysis<'tcx> for Pins<'_, 'tcx> {
mir::StatementKind::Assign(box (lhs, rhs)) => {
self.kill_pins_on_place(state, *lhs);

// Check if this is a pinned borrow
if let mir::Rvalue::Ref(_, mir::BorrowKind::Pinned(_), place) = rhs {
// Generate the pin
// Only user-written `&pin` borrows create long-lived pin facts.
if let mir::Rvalue::Ref(_, mir::BorrowKind::Pinned(_, kind), place) = rhs
&& matches!(*kind, mir::PinBorrowKind::Persistent)
{
self.gen_pins_on_place(state, *place);
}
}
Expand Down
26 changes: 14 additions & 12 deletions compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1773,9 +1773,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
(
BorrowKind::Shared
| BorrowKind::Fake(FakeBorrowKind::Deep)
| BorrowKind::Pinned(mir::Mutability::Not),
| BorrowKind::Pinned(mir::Mutability::Not, _),
BorrowKind::Mut { kind: MutBorrowKind::Default | MutBorrowKind::TwoPhaseBorrow }
| BorrowKind::Pinned(mir::Mutability::Mut),
| BorrowKind::Pinned(mir::Mutability::Mut, _),
) => {
first_borrow_desc = "mutable ";
let mut err = self.cannot_reborrow_already_borrowed(
Expand All @@ -1800,10 +1800,10 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
}
(
BorrowKind::Mut { kind: MutBorrowKind::Default | MutBorrowKind::TwoPhaseBorrow }
| BorrowKind::Pinned(mir::Mutability::Mut),
| BorrowKind::Pinned(mir::Mutability::Mut, _),
BorrowKind::Shared
| BorrowKind::Fake(FakeBorrowKind::Deep)
| BorrowKind::Pinned(mir::Mutability::Not),
| BorrowKind::Pinned(mir::Mutability::Not, _),
) => {
first_borrow_desc = "immutable ";
let mut err = self.cannot_reborrow_already_borrowed(
Expand Down Expand Up @@ -1835,9 +1835,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {

(
BorrowKind::Mut { kind: MutBorrowKind::Default | MutBorrowKind::TwoPhaseBorrow }
| BorrowKind::Pinned(mir::Mutability::Mut),
| BorrowKind::Pinned(mir::Mutability::Mut, _),
BorrowKind::Mut { kind: MutBorrowKind::Default | MutBorrowKind::TwoPhaseBorrow }
| BorrowKind::Pinned(mir::Mutability::Mut),
| BorrowKind::Pinned(mir::Mutability::Mut, _),
) => {
first_borrow_desc = "first ";
let mut err = self.cannot_mutably_borrow_multiply(
Expand Down Expand Up @@ -1877,7 +1877,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
}

(
BorrowKind::Mut { .. } | BorrowKind::Pinned(mir::Mutability::Mut),
BorrowKind::Mut { .. } | BorrowKind::Pinned(mir::Mutability::Mut, _),
BorrowKind::Fake(FakeBorrowKind::Shallow),
) => {
if let Some(immutable_section_description) =
Expand Down Expand Up @@ -1944,7 +1944,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
(
BorrowKind::Shared
| BorrowKind::Fake(FakeBorrowKind::Deep)
| BorrowKind::Pinned(mir::Mutability::Not),
| BorrowKind::Pinned(mir::Mutability::Not, _),
BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture },
) => {
first_borrow_desc = "first ";
Expand All @@ -1962,7 +1962,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
}

(
BorrowKind::Mut { .. } | BorrowKind::Pinned(mir::Mutability::Mut),
BorrowKind::Mut { .. } | BorrowKind::Pinned(mir::Mutability::Mut, _),
BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture },
) => {
first_borrow_desc = "first ";
Expand All @@ -1982,15 +1982,17 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
(
BorrowKind::Shared
| BorrowKind::Fake(FakeBorrowKind::Deep)
| BorrowKind::Pinned(mir::Mutability::Not),
BorrowKind::Shared | BorrowKind::Fake(_) | BorrowKind::Pinned(mir::Mutability::Not),
| BorrowKind::Pinned(mir::Mutability::Not, _),
BorrowKind::Shared
| BorrowKind::Fake(_)
| BorrowKind::Pinned(mir::Mutability::Not, _),
)
| (
BorrowKind::Fake(FakeBorrowKind::Shallow),
BorrowKind::Mut { .. }
| BorrowKind::Shared
| BorrowKind::Fake(_)
| BorrowKind::Pinned(_),
| BorrowKind::Pinned(..),
) => {
unreachable!()
}
Expand Down
2 changes: 2 additions & 0 deletions compiler/rustc_borrowck/src/diagnostics/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -869,11 +869,13 @@ impl UseSpans<'_> {
| rustc_middle::mir::BorrowKind::Fake(_)
| rustc_middle::mir::BorrowKind::Pinned(
rustc_middle::mir::Mutability::Not,
_,
) => CaptureVarKind::Immut { kind_span: capture_kind_span },

rustc_middle::mir::BorrowKind::Mut { .. }
| rustc_middle::mir::BorrowKind::Pinned(
rustc_middle::mir::Mutability::Mut,
_,
) => CaptureVarKind::Mut { kind_span: capture_kind_span },
},
None => CaptureVarKind::Move { kind_span: capture_kind_span },
Expand Down
47 changes: 28 additions & 19 deletions compiler/rustc_borrowck/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1328,16 +1328,20 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {

(
Read(_),
BorrowKind::Shared | BorrowKind::Fake(_) | BorrowKind::Pinned(Mutability::Not),
BorrowKind::Shared
| BorrowKind::Fake(_)
| BorrowKind::Pinned(Mutability::Not, _),
)
| (
Read(ReadKind::Borrow(BorrowKind::Fake(FakeBorrowKind::Shallow))),
BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut),
BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut, _),
) => ControlFlow::Continue(()),

(
Reservation(_),
BorrowKind::Fake(_) | BorrowKind::Shared | BorrowKind::Pinned(Mutability::Not),
BorrowKind::Fake(_)
| BorrowKind::Shared
| BorrowKind::Pinned(Mutability::Not, _),
) => {
// This used to be a future compatibility warning (to be
// disallowed on NLL). See rust-lang/rust#56254
Expand All @@ -1350,13 +1354,13 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
}

// Ignore the expired borrow (pinnedness never conflicts with a read)
(Read(_), BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut))
(Read(_), BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut, _))
if !borrows_in_scope.contains(borrow_index) =>
{
ControlFlow::Continue(())
}

(Read(kind), BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut)) => {
(Read(kind), BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut, _)) => {
// Reading from mere reservations of mutable-borrows is OK.
if !is_active(this.dominators(), borrow, location) {
assert!(borrow.kind.is_two_phase_borrow());
Expand Down Expand Up @@ -1393,7 +1397,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
ControlFlow::Continue(())
}
// Mutable (pinned) borrow doesn't conflict with an expired borrow
WriteKind::MutableBorrow(BorrowKind::Pinned(Mutability::Mut)) => {
WriteKind::MutableBorrow(BorrowKind::Pinned(Mutability::Mut, _)) => {
ControlFlow::Continue(())
}
// Mutable (but non-pinned) borrow conflicts with an earlier pinned borrow
Expand Down Expand Up @@ -1550,12 +1554,16 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
BorrowKind::Fake(FakeBorrowKind::Shallow) => {
(Shallow(Some(ArtificialField::FakeBorrow)), Read(ReadKind::Borrow(bk)))
}
BorrowKind::Pinned(Mutability::Not) => {
(Shallow(None), Read(ReadKind::Borrow(BorrowKind::Pinned(Mutability::Not))))
}
BorrowKind::Pinned(Mutability::Mut) => (
BorrowKind::Pinned(Mutability::Not, pin_kind) => (
Shallow(None),
Read(ReadKind::Borrow(BorrowKind::Pinned(Mutability::Not, pin_kind))),
),
BorrowKind::Pinned(Mutability::Mut, pin_kind) => (
Shallow(None),
Write(WriteKind::MutableBorrow(BorrowKind::Pinned(Mutability::Mut))),
Write(WriteKind::MutableBorrow(BorrowKind::Pinned(
Mutability::Mut,
pin_kind,
))),
),
BorrowKind::Shared | BorrowKind::Fake(FakeBorrowKind::Deep) => {
(Deep, Read(ReadKind::Borrow(bk)))
Expand Down Expand Up @@ -1922,9 +1930,10 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {

// only mutable borrows should be 2-phase
assert!(match borrow.kind {
BorrowKind::Shared | BorrowKind::Fake(_) | BorrowKind::Pinned(Mutability::Not) =>
false,
BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut) => true,
BorrowKind::Shared
| BorrowKind::Fake(_)
| BorrowKind::Pinned(Mutability::Not, _) => false,
BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut, _) => true,
});

self.access_place(
Expand Down Expand Up @@ -2501,7 +2510,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
| WriteKind::StorageDeadOrDrop
| WriteKind::MutableBorrow(BorrowKind::Shared)
| WriteKind::MutableBorrow(BorrowKind::Fake(_))
| WriteKind::MutableBorrow(BorrowKind::Pinned(Mutability::Mut)),
| WriteKind::MutableBorrow(BorrowKind::Pinned(Mutability::Mut, _)),
) => {
if self.is_mutable(place.as_ref(), is_local_mutation_allowed).is_err()
&& !self.has_buffered_diags()
Expand Down Expand Up @@ -2529,18 +2538,18 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
BorrowKind::Mut { .. }
| BorrowKind::Shared
| BorrowKind::Fake(_)
| BorrowKind::Pinned(Mutability::Not),
| BorrowKind::Pinned(Mutability::Not, _),
)
| ReadKind::Copy,
) => {
// Access authorized
return false;
}
Reservation(WriteKind::MutableBorrow(BorrowKind::Pinned(_))) => {
Reservation(WriteKind::MutableBorrow(BorrowKind::Pinned(..))) => {
span_bug!(span, "invalid reservation with a pinned borrow kind: {:?}", kind);
}
Write(WriteKind::MutableBorrow(BorrowKind::Pinned(Mutability::Not)))
| Read(ReadKind::Borrow(BorrowKind::Pinned(Mutability::Mut))) => {
Write(WriteKind::MutableBorrow(BorrowKind::Pinned(Mutability::Not, _)))
| Read(ReadKind::Borrow(BorrowKind::Pinned(Mutability::Mut, _))) => {
span_bug!(
span,
"invalid read with a pinned mutable borrow kind or write with an immutable pinned borrow kind: {:?}",
Expand Down
28 changes: 17 additions & 11 deletions compiler/rustc_borrowck/src/polonius/legacy/loan_invalidations.rs
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Expand Up @@ -262,12 +262,17 @@ impl<'a, 'tcx> LoanInvalidationsGenerator<'a, 'tcx> {
BorrowKind::Shared | BorrowKind::Fake(FakeBorrowKind::Deep) => {
(Deep, Read(ReadKind::Borrow(bk)))
}
BorrowKind::Pinned(Mutability::Not) => {
(Deep, Read(ReadKind::Borrow(BorrowKind::Pinned(Mutability::Not))))
}
BorrowKind::Pinned(Mutability::Mut) => {
(Deep, Write(WriteKind::MutableBorrow(BorrowKind::Pinned(Mutability::Mut))))
}
BorrowKind::Pinned(Mutability::Not, pin_kind) => (
Deep,
Read(ReadKind::Borrow(BorrowKind::Pinned(Mutability::Not, pin_kind))),
),
BorrowKind::Pinned(Mutability::Mut, pin_kind) => (
Deep,
Write(WriteKind::MutableBorrow(BorrowKind::Pinned(
Mutability::Mut,
pin_kind,
))),
),
BorrowKind::Mut { .. } => {
let wk = WriteKind::MutableBorrow(bk);
if bk.is_two_phase_borrow() {
Expand Down Expand Up @@ -383,7 +388,7 @@ impl<'a, 'tcx> LoanInvalidationsGenerator<'a, 'tcx> {
Read(_),
BorrowKind::Fake(_)
| BorrowKind::Shared
| BorrowKind::Pinned(Mutability::Not),
| BorrowKind::Pinned(Mutability::Not, _),
)
| (
Read(ReadKind::Borrow(BorrowKind::Fake(FakeBorrowKind::Shallow))),
Expand All @@ -392,7 +397,7 @@ impl<'a, 'tcx> LoanInvalidationsGenerator<'a, 'tcx> {
// Reads don't invalidate shared or shallow borrows
}

(Read(_), BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut)) => {
(Read(_), BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut, _)) => {
// Reading from mere reservations of mutable-borrows is OK.
if !is_active(this.dominators, borrow, location) {
// If the borrow isn't active yet, reads don't invalidate it
Expand Down Expand Up @@ -433,9 +438,10 @@ impl<'a, 'tcx> LoanInvalidationsGenerator<'a, 'tcx> {

// only mutable borrows should be 2-phase
assert!(match borrow.kind {
BorrowKind::Shared | BorrowKind::Fake(_) | BorrowKind::Pinned(Mutability::Not) =>
false,
BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut) => true,
BorrowKind::Shared
| BorrowKind::Fake(_)
| BorrowKind::Pinned(Mutability::Not, _) => false,
BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut, _) => true,
});

self.access_place(
Expand Down
8 changes: 6 additions & 2 deletions compiler/rustc_const_eval/src/check_consts/check.rs
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Expand Up @@ -582,7 +582,11 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
}
}

Rvalue::Ref(_, BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut), place)
Rvalue::Ref(
_,
BorrowKind::Mut { .. } | BorrowKind::Pinned(Mutability::Mut, _),
place,
)
| Rvalue::RawPtr(RawPtrKind::Mut, place) => {
// Inside mutable statics, we allow arbitrary mutable references.
// We've allowed `static mut FOO = &mut [elements];` for a long time (the exact
Expand All @@ -598,7 +602,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {

Rvalue::Ref(
_,
BorrowKind::Shared | BorrowKind::Fake(_) | BorrowKind::Pinned(Mutability::Not),
BorrowKind::Shared | BorrowKind::Fake(_) | BorrowKind::Pinned(Mutability::Not, _),
place,
)
| Rvalue::RawPtr(RawPtrKind::Const, place) => {
Expand Down
4 changes: 2 additions & 2 deletions compiler/rustc_const_eval/src/check_consts/resolver.rs
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Expand Up @@ -103,10 +103,10 @@ where

fn ref_allows_mutation(&self, kind: mir::BorrowKind, place: mir::Place<'tcx>) -> bool {
match kind {
mir::BorrowKind::Mut { .. } | mir::BorrowKind::Pinned(mir::Mutability::Mut) => true,
mir::BorrowKind::Mut { .. } | mir::BorrowKind::Pinned(mir::Mutability::Mut, _) => true,
mir::BorrowKind::Shared
| mir::BorrowKind::Fake(_)
| mir::BorrowKind::Pinned(mir::Mutability::Not) => {
| mir::BorrowKind::Pinned(mir::Mutability::Not, _) => {
self.shared_borrow_allows_mutation(place)
}
}
Expand Down
4 changes: 2 additions & 2 deletions compiler/rustc_middle/src/mir/pretty.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1117,8 +1117,8 @@ impl<'tcx> Debug for Rvalue<'tcx> {
BorrowKind::Fake(FakeBorrowKind::Deep) => "fake ",
BorrowKind::Fake(FakeBorrowKind::Shallow) => "fake shallow ",
BorrowKind::Mut { .. } => "mut ",
BorrowKind::Pinned(Mutability::Not) => "pin ",
BorrowKind::Pinned(Mutability::Mut) => "pin mut ",
BorrowKind::Pinned(Mutability::Not, _) => "pin ",
BorrowKind::Pinned(Mutability::Mut, _) => "pin mut ",
};

// When printing regions, add trailing space if necessary.
Expand Down
6 changes: 3 additions & 3 deletions compiler/rustc_middle/src/mir/statement.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -838,7 +838,7 @@ impl BorrowKind {
match *self {
BorrowKind::Shared | BorrowKind::Fake(_) => Mutability::Not,
BorrowKind::Mut { .. } => Mutability::Mut,
BorrowKind::Pinned(mutability) => mutability,
BorrowKind::Pinned(mutability, _) => mutability,
}
}

Expand All @@ -849,7 +849,7 @@ impl BorrowKind {
BorrowKind::Shared
| BorrowKind::Fake(_)
| BorrowKind::Mut { kind: MutBorrowKind::Default | MutBorrowKind::ClosureCapture }
| BorrowKind::Pinned(_) => false,
| BorrowKind::Pinned(..) => false,
BorrowKind::Mut { kind: MutBorrowKind::TwoPhaseBorrow } => true,
}
}
Expand All @@ -862,7 +862,7 @@ impl BorrowKind {
// We have no type corresponding to a shallow borrow, so use
// `&` as an approximation.
BorrowKind::Fake(_) => hir::Mutability::Not,
BorrowKind::Pinned(mutability) => mutability,
BorrowKind::Pinned(mutability, _) => mutability,
}
}
}
Expand Down
18 changes: 16 additions & 2 deletions compiler/rustc_middle/src/mir/syntax.rs
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Expand Up @@ -179,8 +179,22 @@ pub enum BorrowKind {
/// Data is mutable and not aliasable.
Mut { kind: MutBorrowKind },

/// Data is pinned and cannot be moved or mutably borrowed until dropped or reassigned.
Pinned(Mutability),
/// Data is pinned.
Pinned(Mutability, PinBorrowKind),
}

#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, TyEncodable, TyDecodable)]
#[derive(Hash, HashStable)]
pub enum PinBorrowKind {
/// A user-written `&pin` borrow.
///
/// The borrowed place stays pinned after this borrow ends, until reassignment.
Persistent,
/// A compiler-generated pinned borrow used for adjustments or coercions.
///
/// This remains a normal borrow while live, but does not create a persistent
/// pin fact after it ends.
Transient,
}

#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, TyEncodable, TyDecodable)]
Expand Down
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