UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuracy by RalfJung · Pull Request #157658 · rust-lang/rust

RalfJung · 2026-06-09T10:53:15Z

Addresses parts of #157544.

The second commit fixes a slight inaccuracy in the aliasing model section.

@kpreid I hope I interpreted your feedback correctly, and this would be helpful?

rustbot · 2026-06-09T10:53:19Z

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RalfJung · 2026-06-09T10:55:37Z

 /// to an `&T`, then the data in `T` must remain immutable (modulo any `UnsafeCell` data found
 /// within `T`, of course) until that reference's lifetime expires. Similarly, if you create a
-/// `&mut T` reference that is released to safe code, then you must not access the data within the
+/// `&mut T` reference, then you must not access the data within the


The existing docs seemed to imply that if you don't release the &mut to safe code, the rules don't apply. That's incorrect, these rules always apply.
View changes since the review

kpreid

@kpreid I hope I interpreted your feedback correctly, and this would be helpful?

Yes, this is a significant improvement.

View changes since this review

Co-authored-by: Kevin Reid <kpreid@switchb.org>

joboet

👍
r=me with the nits addressed.

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joboet · 2026-06-11T13:16:15Z

- /// This can be cast to a pointer of any kind. When creating references, you must uphold the
- /// aliasing rules; see [the type-level docs][UnsafeCell#aliasing-rules] for more discussion and
- /// caveats.
+ /// This can be cast to a pointer of any kind. When creating (shared or mutable) references, you


I'd remove the parentheses, for me they just add unnecessary nesting.
Suggested change
/// This can be cast to a pointer of any kind. When creating (shared or mutable) references, you
/// This can be cast to a pointer of any kind. When creating shared or mutable references, you

This was deliberate. We could arguably just say "when creating references" since the kind of reference does not matter -- the parenthetical just clarifies that yes we mean both kinds of references.

Mmmh, fair enough...

joboet · 2026-06-11T17:37:02Z

@bors r+ rollup

rust-bors · 2026-06-11T17:37:05Z

📌 Commit c40fe47 has been approved by joboet

It is now in the queue for this repository.

@kpreid

UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuracy Addresses parts of rust-lang#157544. The second commit fixes a slight inaccuracy in the aliasing model section. @kpreid I hope I interpreted your feedback correctly, and this would be helpful?

…uwer Rollup of 23 pull requests Successful merges: - #157716 (update Enzyme, June'26) - #149793 (Add inline asm support for amdgpu) - #152852 (Remove driver_lint_caps) - #155299 (make repr_transparent_non_zst_fields a hard error) - #155439 (Enable Cargo's new build-dir layout) - #157612 (Add a test where subtyping inhibits coercion.) - #157626 (Autogenerate unstable compiler flag stubs for unstable-book) - #157667 (Rename typing modes to better describe real usage) - #156212 (Additionally gate negative bounds behind new `-Zinternal-testing-features`) - #157342 (Reduce verbosity of cycle errors when possible) - #157366 (Add a regression test for an unconstrained TransmuteFrom ICE) - #157459 (rustc_target: callconv: powerpc64: Remove unreachable fallback code path) - #157658 (UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuracy) - #157698 (Remove an unnecessary cloning) - #157699 (Arg splat experiment - hir FnDecl impl) - #157713 (resolve: Remove exported imports from `maybe_unused_trait_imports`) - #157722 (Move create_scope_map to rustc_codegen_ssa.) - #157725 (Keep generic suggestion for macro-expanded missing-type items) - #157733 (Remove old FIXMEs about nocapture attribute) - #157737 (Reorganize `tests/ui/issues` [7/N]) - #157746 (supports_c_variadic_definitions: extend checklist for new targets) - #157763 (Move unused target expression error to appropriate place and rename it) - #157768 (codegen_ssa: peel trans. wrappers on scalable vecs)

@kpreid

UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuracy Addresses parts of rust-lang#157544. The second commit fixes a slight inaccuracy in the aliasing model section. @kpreid I hope I interpreted your feedback correctly, and this would be helpful?

…uwer Rollup of 23 pull requests Successful merges: - #157716 (update Enzyme, June'26) - #149793 (Add inline asm support for amdgpu) - #155299 (make repr_transparent_non_zst_fields a hard error) - #155439 (Enable Cargo's new build-dir layout) - #157612 (Add a test where subtyping inhibits coercion.) - #157626 (Autogenerate unstable compiler flag stubs for unstable-book) - #157667 (Rename typing modes to better describe real usage) - #149749 (Make `BorrowedBuf` and `BorrowedCursor` generic over the data) - #156212 (Additionally gate negative bounds behind new `-Zinternal-testing-features`) - #157342 (Reduce verbosity of cycle errors when possible) - #157366 (Add a regression test for an unconstrained TransmuteFrom ICE) - #157459 (rustc_target: callconv: powerpc64: Remove unreachable fallback code path) - #157658 (UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuracy) - #157698 (Remove an unnecessary cloning) - #157699 (Arg splat experiment - hir FnDecl impl) - #157713 (resolve: Remove exported imports from `maybe_unused_trait_imports`) - #157722 (Move create_scope_map to rustc_codegen_ssa.) - #157725 (Keep generic suggestion for macro-expanded missing-type items) - #157733 (Remove old FIXMEs about nocapture attribute) - #157737 (Reorganize `tests/ui/issues` [7/N]) - #157746 (supports_c_variadic_definitions: extend checklist for new targets) - #157763 (Move unused target expression error to appropriate place and rename it) - #157768 (codegen_ssa: peel trans. wrappers on scalable vecs)

Rollup of 24 pull requests Successful merges: - #157716 (update Enzyme, June'26) - #149793 (Add inline asm support for amdgpu) - #155299 (make repr_transparent_non_zst_fields a hard error) - #157612 (Add a test where subtyping inhibits coercion.) - #157626 (Autogenerate unstable compiler flag stubs for unstable-book) - #157667 (Rename typing modes to better describe real usage) - #149749 (Make `BorrowedBuf` and `BorrowedCursor` generic over the data) - #155113 (Ensure Send/Sync impl for std::process::CommandArgs) - #156212 (Additionally gate negative bounds behind new `-Zinternal-testing-features`) - #157342 (Reduce verbosity of cycle errors when possible) - #157366 (Add a regression test for an unconstrained TransmuteFrom ICE) - #157459 (rustc_target: callconv: powerpc64: Remove unreachable fallback code path) - #157658 (UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuracy) - #157698 (Remove an unnecessary cloning) - #157699 (Arg splat experiment - hir FnDecl impl) - #157713 (resolve: Remove exported imports from `maybe_unused_trait_imports`) - #157722 (Move create_scope_map to rustc_codegen_ssa.) - #157723 (Move uninhabited unreachable code lint to rustc_mir_transform) - #157725 (Keep generic suggestion for macro-expanded missing-type items) - #157733 (Remove old FIXMEs about nocapture attribute) - #157737 (Reorganize `tests/ui/issues` [7/N]) - #157746 (supports_c_variadic_definitions: extend checklist for new targets) - #157763 (Move unused target expression error to appropriate place and rename it) - #157768 (codegen_ssa: peel trans. wrappers on scalable vecs)

@kpreid

Rollup merge of #157658 - RalfJung:unsafe-cell-docs, r=joboet UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuracy Addresses parts of #157544. The second commit fixes a slight inaccuracy in the aliasing model section. @kpreid I hope I interpreted your feedback correctly, and this would be helpful?

Rollup of 24 pull requests Successful merges: - rust-lang/rust#157716 (update Enzyme, June'26) - rust-lang/rust#149793 (Add inline asm support for amdgpu) - rust-lang/rust#155299 (make repr_transparent_non_zst_fields a hard error) - rust-lang/rust#157612 (Add a test where subtyping inhibits coercion.) - rust-lang/rust#157626 (Autogenerate unstable compiler flag stubs for unstable-book) - rust-lang/rust#157667 (Rename typing modes to better describe real usage) - rust-lang/rust#149749 (Make `BorrowedBuf` and `BorrowedCursor` generic over the data) - rust-lang/rust#155113 (Ensure Send/Sync impl for std::process::CommandArgs) - rust-lang/rust#156212 (Additionally gate negative bounds behind new `-Zinternal-testing-features`) - rust-lang/rust#157342 (Reduce verbosity of cycle errors when possible) - rust-lang/rust#157366 (Add a regression test for an unconstrained TransmuteFrom ICE) - rust-lang/rust#157459 (rustc_target: callconv: powerpc64: Remove unreachable fallback code path) - rust-lang/rust#157658 (UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuracy) - rust-lang/rust#157698 (Remove an unnecessary cloning) - rust-lang/rust#157699 (Arg splat experiment - hir FnDecl impl) - rust-lang/rust#157713 (resolve: Remove exported imports from `maybe_unused_trait_imports`) - rust-lang/rust#157722 (Move create_scope_map to rustc_codegen_ssa.) - rust-lang/rust#157723 (Move uninhabited unreachable code lint to rustc_mir_transform) - rust-lang/rust#157725 (Keep generic suggestion for macro-expanded missing-type items) - rust-lang/rust#157733 (Remove old FIXMEs about nocapture attribute) - rust-lang/rust#157737 (Reorganize `tests/ui/issues` [7/N]) - rust-lang/rust#157746 (supports_c_variadic_definitions: extend checklist for new targets) - rust-lang/rust#157763 (Move unused target expression error to appropriate place and rename it) - rust-lang/rust#157768 (codegen_ssa: peel trans. wrappers on scalable vecs)

UnsafeCell: mention shared-ref-to-interior case in type-level docs
6e38878

rustbot assigned joboet Jun 9, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jun 9, 2026

fix subtle issue in UnsafeCell aliasing model docs
d0d453c

RalfJung commented Jun 9, 2026
View reviewed changes

RalfJung changed the title ~~UnsafeCell: mention shared-ref-to-interior case in type-level docs~~UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuractJun 9, 2026

RalfJung changed the title ~~UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuract~~UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuracyJun 9, 2026

kpreid approved these changes Jun 9, 2026
View reviewed changes

Comment threadlibrary/core/src/cell.rs Outdated

fix typo
44a1ba5
Co-authored-by: Kevin Reid <kpreid@switchb.org>

joboet approved these changes Jun 11, 2026
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an → a
c40fe47

rust-borsBot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 11, 2026

JonathanBrouwer mentioned this pull request Jun 11, 2026
Rollup of 23 pull requests #157784
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JonathanBrouwer mentioned this pull request Jun 11, 2026
Rollup of 23 pull requests #157790
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jhpratt mentioned this pull request Jun 12, 2026
Rollup of 24 pull requests #157794
Merged

rust-borsBot merged commit abe85f4 into rust-lang:mainJun 12, 2026
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rustbot added this to the 1.98.0 milestone Jun 12, 2026

RalfJung deleted the unsafe-cell-docs branch June 21, 2026 18:51

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UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuracy - #157658

UnsafeCell: mention shared-ref-to-interior case, fix aliasing model inaccuracy#157658
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	/// This can be cast to a pointer of any kind. When creating (shared or mutable) references, you
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