add #[rustc_no_writable] to slice::get_unchecked_mut by quiode · Pull Request #157202 · rust-lang/rust

quiode · 2026-05-31T15:16:02Z

This PR adds the #[rustc_no_writable] attribute introduced in #155207 to the slice::get_unchecked_mut function.

Two library functions already received this attribute, as they were known to cause problems with the llvm writable attribute and tree borrows. Since that PR, I ran Miri on the 30'000 most downloaded crates to see what kind of code is now UB under Tree Borrows + implicit writes, using the detection implemented in rust-lang/miri#4947. Adding this attribute to ignore checking for this function reduced the new UB introduced by more than 75%, meaning that instead of 19000 tests in 1700 crates having a difference, now only 3500 in 350 crates show a difference (measurement still running).

rustbot · 2026-05-31T15:16:06Z

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: libs
libs expanded to 7 candidates
Random selection from Mark-Simulacrum, jhpratt

quiode · 2026-05-31T15:16:21Z

r? @RalfJung

rustbot · 2026-05-31T15:16:26Z

RalfJung is not on the review rotation at the moment.
They may take a while to respond.

Mark-Simulacrum · 2026-05-31T19:02:29Z

@@ -681,6 +681,7 @@ impl<T> [T] {
 #[must_use]
 #[track_caller]
 #[rustc_const_unstable(feature = "const_index", issue = "143775")]
+ #[rustc_no_writable]


Should this get an annotation of some kind (e.g., "FIXME: #xxx")? I assume the intent is not to guarantee that we don't have compiler-added writes of the full slice, maybe with the long-term intent being that users move to using raw pointer functions where they need that (https://doc.rust-lang.org/nightly/std/primitive.pointer.html#method.get_unchecked_mut)?
View changes since the review

Personally I think this is fine to guarantee. But for now the attribute is experimental anyway.

RalfJung · 2026-06-01T09:28:24Z

@rustbot author

rustbot · 2026-06-01T09:28:29Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

rustbot · 2026-06-07T13:41:49Z

The Miri subtree was changed

cc @rust-lang/miri

quiode · 2026-06-07T13:57:45Z

The proposed test wasn't exactly the pattern I tried to fix (and also still fails with the attribute). It was more something like this: https://github.com/quiode/rust/blob/ignore-list/src/tools/miri/tests/fail/tree_borrows/implicit_writes/libc_recv.rs or https://github.com/quiode/rust/blob/ignore-list/src/tools/miri/tests/fail-dep/tree_borrows/implicit_writes/crates/fs-read.rs. I tried to replicate this in the new test, which fails without the attribute but passes with it.

@rustbot ready

RalfJung · 2026-06-12T08:55:05Z

I think it is the same pattern, but we'd need to decorate more functions... that pattern should ideally work. (https://rust.godbolt.org/z/TreEYqfW8)

But meanwhile we can land this.
@bors squash

* add #[rustc_no_writable] to slice::get_unchecked_mut * add #[rustc_no_writable] to slice::get_mut * add unchecked_mut miri test

rust-bors · 2026-06-12T08:55:25Z

🔨 3 commits were squashed into 42d7bf1.

RalfJung · 2026-06-12T08:55:56Z

@bors r+ rollup

rust-bors · 2026-06-12T08:55:59Z

📌 Commit 42d7bf1 has been approved by RalfJung

It is now in the queue for this repository.

RalfJung · 2026-06-12T09:07:05Z

It looks like if I run that test with your branch, there's no UB reported? So it does actually seem to help?

EDIT: Never mind I tested the wrong thing.

…fns, r=RalfJung add #[rustc_no_writable] to slice::get_unchecked_mut This PR adds the `#[rustc_no_writable]` attribute introduced in rust-lang#155207 to the `slice::get_unchecked_mut` function. Two library functions already received this attribute, as they were known to cause problems with the llvm writable attribute and tree borrows. Since that PR, I ran Miri on the 30'000 most downloaded crates to see what kind of code is now UB under Tree Borrows + implicit writes, using the detection implemented in rust-lang/miri#4947. Adding this attribute to ignore checking for this function reduced the new UB introduced by more than 75%, meaning that instead of 19000 tests in 1700 crates having a difference, now only 3500 in 350 crates show a difference (measurement still running).

…uwer Rollup of 17 pull requests Successful merges: - #156950 (Staticlib rename internal symbols) - #157322 (test pre-stabilization items on CI) - #157490 (Add field-wise CoerceShared reborrow tests) - #157655 (Make Share::share final and improve docs) - #157688 (Create experimental test job `aarch64-apple-macos-26` for evaluating `macos-26` runner images) - #157796 (rustdoc: Some more lazy formatting) - #157069 (Test that you can't implement Unpin for a compiler-generated future using TAIT) - #157202 (add #[rustc_no_writable] to slice::get_unchecked_mut) - #157622 (Disable retagging for variadic arguments in const-eval) - #157684 (-Zassumptions-on-binders: insert empty assumptions when entering binders in the solver) - #157695 (Extend capabilities of `TypeFoldable_Generic`) - #157752 (Rename `errors.rs` file to `diagnostics.rs` (6/N)) - #157766 (interpret: avoid computing layout of sized raw pointee) - #157785 (fuchsia: Support AddressSanitizer on riscv64gc-unknown-fuchsia) - #157795 (revert 157013) - #157798 (Prevent approving PRs that wait for Crater or formal decisions) - #157803 (Rename `errors.rs` file to `diagnostics.rs` (7/N))

…fns, r=RalfJung add #[rustc_no_writable] to slice::get_unchecked_mut This PR adds the `#[rustc_no_writable]` attribute introduced in rust-lang#155207 to the `slice::get_unchecked_mut` function. Two library functions already received this attribute, as they were known to cause problems with the llvm writable attribute and tree borrows. Since that PR, I ran Miri on the 30'000 most downloaded crates to see what kind of code is now UB under Tree Borrows + implicit writes, using the detection implemented in rust-lang/miri#4947. Adding this attribute to ignore checking for this function reduced the new UB introduced by more than 75%, meaning that instead of 19000 tests in 1700 crates having a difference, now only 3500 in 350 crates show a difference (measurement still running).

…uwer Rollup of 23 pull requests Successful merges: - #144220 (Add powerpc64-unknown-linux-gnuelfv2 target) - #153238 (debuginfo: slices are DW_TAG_array_type's) - #157112 (Update aarch64-unknown-freebsd target description) - #157322 (test pre-stabilization items on CI) - #157348 (Don't track cwd for `-Zremap-cwd-prefix` in incremental compilation) - #157490 (Add field-wise CoerceShared reborrow tests) - #157655 (Make Share::share final and improve docs) - #157672 (Region inference: Simplify initialisation of region values) - #157680 (Require `#[pin_v2]` for explicit pin-projection patterns) - #157688 (Create experimental test job `aarch64-apple-macos-26` for evaluating `macos-26` runner images) - #157796 (rustdoc: Some more lazy formatting) - #157818 (miri subtree update) - #157069 (Test that you can't implement Unpin for a compiler-generated future using TAIT) - #157079 (Don't recover `&raw EXPR` as a missing comma) - #157202 (add #[rustc_no_writable] to slice::get_unchecked_mut) - #157622 (Disable retagging for variadic arguments in const-eval) - #157684 (-Zassumptions-on-binders: insert empty assumptions when entering binders in the solver) - #157695 (Extend capabilities of `TypeFoldable_Generic`) - #157766 (interpret: avoid computing layout of sized raw pointee) - #157785 (fuchsia: Support AddressSanitizer on riscv64gc-unknown-fuchsia) - #157795 (revert 157013) - #157798 (Prevent approving PRs that wait for Crater or formal decisions) - #157803 (Rename `errors.rs` file to `diagnostics.rs` (7/N)) Failed merges: - #157752 (Rename `errors.rs` file to `diagnostics.rs` (6/N))

…uwer Rollup of 23 pull requests Successful merges: - rust-lang/rust#144220 (Add powerpc64-unknown-linux-gnuelfv2 target) - rust-lang/rust#153238 (debuginfo: slices are DW_TAG_array_type's) - rust-lang/rust#157112 (Update aarch64-unknown-freebsd target description) - rust-lang/rust#157322 (test pre-stabilization items on CI) - rust-lang/rust#157348 (Don't track cwd for `-Zremap-cwd-prefix` in incremental compilation) - rust-lang/rust#157490 (Add field-wise CoerceShared reborrow tests) - rust-lang/rust#157655 (Make Share::share final and improve docs) - rust-lang/rust#157672 (Region inference: Simplify initialisation of region values) - rust-lang/rust#157680 (Require `#[pin_v2]` for explicit pin-projection patterns) - rust-lang/rust#157688 (Create experimental test job `aarch64-apple-macos-26` for evaluating `macos-26` runner images) - rust-lang/rust#157796 (rustdoc: Some more lazy formatting) - rust-lang/rust#157818 (miri subtree update) - rust-lang/rust#157069 (Test that you can't implement Unpin for a compiler-generated future using TAIT) - rust-lang/rust#157079 (Don't recover `&raw EXPR` as a missing comma) - rust-lang/rust#157202 (add #[rustc_no_writable] to slice::get_unchecked_mut) - rust-lang/rust#157622 (Disable retagging for variadic arguments in const-eval) - rust-lang/rust#157684 (-Zassumptions-on-binders: insert empty assumptions when entering binders in the solver) - rust-lang/rust#157695 (Extend capabilities of `TypeFoldable_Generic`) - rust-lang/rust#157766 (interpret: avoid computing layout of sized raw pointee) - rust-lang/rust#157785 (fuchsia: Support AddressSanitizer on riscv64gc-unknown-fuchsia) - rust-lang/rust#157795 (revert 157013) - rust-lang/rust#157798 (Prevent approving PRs that wait for Crater or formal decisions) - rust-lang/rust#157803 (Rename `errors.rs` file to `diagnostics.rs` (7/N)) Failed merges: - rust-lang/rust#157752 (Rename `errors.rs` file to `diagnostics.rs` (6/N))

@RalfJung

…, r=RalfJung add rustc_no_writable to mem::forget and structs it uses This builds upon rust-lang#155207 and is similar to rust-lang#157202. It adds the `#[rustc_no_writable]` attribute to `mem::forget` and the `MaybeDangling` and `ManuallyDrop`. This makes Miri with Tree Borrows and implicit writes no longer report UB for a test in `derive_more`. As the pattern itself is quite unclean, and I have not seen `mem::forget` to cause trouble before with implicit writes, I'm not sure how much sense it makes to add it to `mem::forget`. The test works unter Tree Borrows, but fails already for Stacked Borrows. Thus I would be happy for some guidance if the attribute makes sense here @RalfJung @JoJoDeveloping.

@RalfJung

Rollup merge of #159181 - quiode:mem--forget-implicit-writes, r=RalfJung add rustc_no_writable to mem::forget and structs it uses This builds upon #155207 and is similar to #157202. It adds the `#[rustc_no_writable]` attribute to `mem::forget` and the `MaybeDangling` and `ManuallyDrop`. This makes Miri with Tree Borrows and implicit writes no longer report UB for a test in `derive_more`. As the pattern itself is quite unclean, and I have not seen `mem::forget` to cause trouble before with implicit writes, I'm not sure how much sense it makes to add it to `mem::forget`. The test works unter Tree Borrows, but fails already for Stacked Borrows. Thus I would be happy for some guidance if the attribute makes sense here @RalfJung @JoJoDeveloping.

@RalfJung

add rustc_no_writable to mem::forget and structs it uses This builds upon rust-lang/rust#155207 and is similar to rust-lang/rust#157202. It adds the `#[rustc_no_writable]` attribute to `mem::forget` and the `MaybeDangling` and `ManuallyDrop`. This makes Miri with Tree Borrows and implicit writes no longer report UB for a test in `derive_more`. As the pattern itself is quite unclean, and I have not seen `mem::forget` to cause trouble before with implicit writes, I'm not sure how much sense it makes to add it to `mem::forget`. The test works unter Tree Borrows, but fails already for Stacked Borrows. Thus I would be happy for some guidance if the attribute makes sense here @RalfJung @JoJoDeveloping.

rustbot assigned Mark-Simulacrum May 31, 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 May 31, 2026

rustbot assigned RalfJung and unassigned Mark-Simulacrum May 31, 2026

Mark-Simulacrum reviewed May 31, 2026
View reviewed changes

RalfJung reviewed Jun 1, 2026
View reviewed changes

Comment threadlibrary/core/src/slice/mod.rs

RalfJung reviewed Jun 1, 2026
View reviewed changes

Comment threadlibrary/core/src/slice/mod.rs

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 1, 2026

This comment has been minimized.
Sign in to view

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 7, 2026

This comment has been minimized.
Sign in to view

add #[rustc_no_writable] to slice::get_unchecked_mut
42d7bf1
* add #[rustc_no_writable] to slice::get_unchecked_mut * add #[rustc_no_writable] to slice::get_mut * add unchecked_mut miri test

rust-borsBotforce-pushed the add-rustc_no_writable-more-fns branch from 566bf0b to 42d7bf1Compare June 12, 2026 08:55

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 12, 2026

GuillaumeGomez mentioned this pull request Jun 12, 2026
Rollup of 9 pull requests #157804
Closed

JonathanBrouwer mentioned this pull request Jun 12, 2026
Rollup of 14 pull requests #157809
Closed

JonathanBrouwer mentioned this pull request Jun 12, 2026
Rollup of 17 pull requests #157815
Closed

RalfJung mentioned this pull request Jun 12, 2026
make more slice mutable ref getters rustc_no_writable #157816
Merged

This was referenced Jun 12, 2026
Rollup of 23 pull requests #157827
Closed
Rollup of 23 pull requests #157828
Merged

rust-borsBot merged commit 64eda74 into rust-lang:mainJun 12, 2026
12 checks passed

rustbot added this to the 1.98.0 milestone Jun 12, 2026

quiode mentioned this pull request Jul 12, 2026
add rustc_no_writable to mem::forget and structs it uses #159181
Merged

Uh oh!

Uh oh!

Conversation

quiode commented May 31, 2026• edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented May 31, 2026

Uh oh!

quiode commented May 31, 2026

Uh oh!

rustbot commented May 31, 2026

Uh oh!

Mark-SimulacrumMay 31, 2026• edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

RalfJungJun 1, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

RalfJung commented Jun 1, 2026

Uh oh!

rustbot commented Jun 1, 2026

Uh oh!

rustbot commented Jun 7, 2026

Uh oh!

This comment has been minimized.

quiode commented Jun 7, 2026

Uh oh!

RalfJung commented Jun 12, 2026• edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

This comment has been minimized.

rust-borsBot commented Jun 12, 2026

Uh oh!

RalfJung commented Jun 12, 2026

Uh oh!

rust-borsBot commented Jun 12, 2026

Uh oh!

RalfJung commented Jun 12, 2026• edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

quiode commented May 31, 2026•
edited by rustbot
Loading

Mark-SimulacrumMay 31, 2026•
edited by rustbot
Loading

RalfJung commented Jun 12, 2026•
edited
Loading

RalfJung commented Jun 12, 2026•
edited
Loading