ptr::replace: make calls on ZST null ptr not UB by RalfJung · Pull Request #149169 · rust-lang/rust

RalfJung · 2025-11-21T07:56:28Z

See #138351 for context.

We made ptr::read and ptr::write not UB on ZST null pointers. This does the same with ptr::replace. Since we're just adding a branch on a constant, this should come at no runtime cost.

rustbot · 2025-11-21T07:56:32Z

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library/core/src/ptr/mod.rs

RalfJung · 2025-12-07T08:57:28Z

@rust-lang/libs any thoughts on this?
Independent of what exactly we decide in #138351, this seems like a basically free way to make replace have less UB and be more consistent with read and write.

RalfJung · 2026-02-14T13:01:15Z

r? libs

Amanieu · 2026-02-14T17:56:15Z

@rfcbot merge libs-api

rust-rfcbot · 2026-02-14T17:56:17Z

Team member @Amanieu has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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rust-rfcbot · 2026-02-14T19:31:42Z

🔔 This is now entering its final comment period, as per the review above. 🔔

Mark-Simulacrum · 2026-02-14T19:33:26Z

r=me with FCP complete

@rustbot author

the8472 · 2026-02-14T20:00:49Z

This doesn't seem like a API change to me, other than by implication of how #138351 gets addressed and what we might have to tell users about it, but it looks like #152615 is the one that's trying to update the documentation for that.

RalfJung · 2026-02-14T21:58:51Z

Yeah technically this PR just changes the implementation, the API already got changed a while ago.

library/core/src/ptr/mod.rs

rustbot · 2026-02-19T07:39:12Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

rust-rfcbot · 2026-02-24T19:35:08Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

RalfJung · 2026-02-24T20:41:09Z

@bors r=Mark-Simulacrum

rust-bors · 2026-02-24T20:41:12Z

📌 Commit 2484cfe has been approved by Mark-Simulacrum

It is now in the queue for this repository.

…ark-Simulacrum ptr::replace: make calls on ZST null ptr not UB See rust-lang#138351 for context. We made `ptr::read` and `ptr::write` not UB on ZST null pointers. This does the same with `ptr::replace`. Since we're just adding a branch on a constant, this should come at no runtime cost.

Rollup of 8 pull requests Successful merges: - #149169 (ptr::replace: make calls on ZST null ptr not UB) - #150562 (Fix doc link used in suggestion for pinning self) - #152679 (rustc_expand: improve diagnostics for non-repeatable metavars) - #153017 (Implement debuginfo for unsafe binder types) - #152868 (delete some very old trivial `Box` tests) - #152922 (rustc_public: Make fields that shouldn't be exposed visible only in `rustc_public`) - #153029 (Rename `rustc::pass_by_value` lint as `rustc::disallowed_pass_by_ref`.) - #153051 (Migration of `LintDiagnostic` - part 3)

Rollup of 12 pull requests Successful merges: - #149169 (ptr::replace: make calls on ZST null ptr not UB) - #150562 (Fix doc link used in suggestion for pinning self) - #152418 (`BTreeMap::merge` optimized) - #152679 (rustc_expand: improve diagnostics for non-repeatable metavars) - #152952 (mGCA: improve ogca diagnostic message ) - #152977 (Fix relative path handling for --extern-html-root-url) - #153017 (Implement debuginfo for unsafe binder types) - #152868 (delete some very old trivial `Box` tests) - #152922 (rustc_public: Make fields that shouldn't be exposed visible only in `rustc_public`) - #153032 (Fix attribute parser and kind names.) - #153051 (Migration of `LintDiagnostic` - part 3) - #153060 (Give a better error when updating a submodule fails)

Rollup merge of #149169 - RalfJung:replace-zst-null-ptr, r=Mark-Simulacrum ptr::replace: make calls on ZST null ptr not UB See #138351 for context. We made `ptr::read` and `ptr::write` not UB on ZST null pointers. This does the same with `ptr::replace`. Since we're just adding a branch on a constant, this should come at no runtime cost.

Kobzol · 2026-02-25T11:23:05Z

@rust-timer build 6e1951f

For #153074.

rust-timer · 2026-02-25T11:23:08Z

Missing artifact for sha 6e1951f11c05f55619bc18afac75d8827ca127a9 (https://ci-artifacts.rust-lang.org/rustc-builds/6e1951f11c05f55619bc18afac75d8827ca127a9/rustc-nightly-x86_64-unknown-linux-gnu.tar.xz); not built yet, try again later.

JonathanBrouwer · 2026-02-25T12:21:17Z

@rust-timer build 6e1951f

rust-timer · 2026-02-25T14:08:19Z

Finished benchmarking commit (6e1951f): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.5%	[0.5%, 0.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -1.0%, secondary -4.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.1%	[2.5%, 3.7%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.8%	[-4.2%, -3.6%]	3
Improvements ✅ (secondary)	-4.1%	[-4.1%, -4.1%]	1
All ❌✅ (primary)	-1.0%	[-4.2%, 3.7%]	5

Cycles

Results (secondary -3.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.5%	[-3.5%, -3.5%]	1
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 482.035s -> 480.395s (-0.34%)
Artifact size: 397.81 MiB -> 397.78 MiB (-0.01%)

JonathanBrouwer · 2026-02-25T15:38:25Z

Noise.

rustbot assigned ibraheemdev Nov 21, 2025

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 Nov 21, 2025

RalfJung mentioned this pull request Nov 21, 2025

core::ptr::replace implementation is unsound since Rust 1.80 #138351

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programmerjake reviewed Nov 21, 2025

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RalfJung force-pushed the replace-zst-null-ptr branch from bb81fb2 to 43aa6a7 Compare November 21, 2025 12:07

rustbot assigned Mark-Simulacrum and unassigned ibraheemdev Feb 14, 2026

rust-rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Feb 14, 2026

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rust-rfcbot added the final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. label Feb 14, 2026

rust-rfcbot removed the proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. label Feb 14, 2026

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 Feb 14, 2026

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ptr::replace: make calls on ZST null ptr not UB

2484cfe

RalfJung force-pushed the replace-zst-null-ptr branch from 43aa6a7 to 2484cfe Compare February 19, 2026 07:39

rust-rfcbot added the finished-final-comment-period The final comment period is finished for this PR / Issue. label Feb 24, 2026

rust-rfcbot added to-announce Announce this issue on triage meeting and removed final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. labels Feb 24, 2026

rustbot mentioned this pull request Feb 24, 2026

Tracking issue for release notes of #149169: ptr::replace: make calls on ZST null ptr not UB #153056

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rust-bors bot 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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 24, 2026

GuillaumeGomez mentioned this pull request Feb 24, 2026

Rollup of 8 pull requests #153059

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jhpratt mentioned this pull request Feb 25, 2026

Rollup of 12 pull requests #153074

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rust-bors bot merged commit 651db87 into rust-lang:main Feb 25, 2026
11 checks passed

rustbot added this to the 1.95.0 milestone Feb 25, 2026

RalfJung mentioned this pull request Feb 25, 2026

refactor 'valid for read/write' definition: exclude null #152615

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Overall result: ❌ regressions - no action needed

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