const-eval: fix and re-enable pointer fragment support #148259

RalfJung · 2025-10-29T17:00:42Z

The pointer fragment support from #144081 got disabled due to #146291. This brings it back. To fix the issue, the per-byte provenance fragment tracking tracks both the provenance and raw address of the full pointer, so we can ensure that only fragments that are truly part of the same pointer are being merged.

r? @oli-obk
Cc @theemathas
Fixes rust-lang/const-eval#72 again.
Also fixes #147959.

@traviscross I assume this won't need another t-lang FCP since it already got FCP'd in #144081?

rustbot · 2025-10-29T17:00:45Z

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

The Miri subtree was changed

cc @rust-lang/miri

rustbot · 2025-10-29T17:00:48Z

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

traviscross · 2025-10-29T19:17:54Z

I assume this won't need another t-lang FCP since it already got FCP'd in #144081?

As I commented a moment ago on an unrelated matter in #145954 (comment):

As a procedural matter, though the FCP in ... did originally cover what's here, the fact that it was merged without that puts the ball back in our court, in the same way that if one of our stabilizations is reverted, we then need to re-FCP the restabilization. In this way, our FCPs represent a "point-in-time" decision to stabilize something.

We discussed that procedural analysis in the meeting and it seemed right to people.

In that light, we would FCP the restabilization here. It should be an easy call for us given the earlier decision.

traviscross · 2025-10-29T19:21:17Z

This seems right to me. I propose we restabilize this.

@rfcbot fcp merge

rust-rfcbot · 2025-10-29T19:21:19Z

Team member @traviscross 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!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

compiler/rustc_middle/src/mir/interpret/allocation/provenance_map.rs

RalfJung · 2025-10-29T19:41:01Z

compiler/rustc_middle/src/mir/interpret/pointer.rs

    fn get_alloc_id(self) -> Option<AllocId>;
-
-    /// Defines the 'join' of provenance: what happens when doing a pointer load and different bytes have different provenance.
-    fn join(left: Self, right: Self) -> Option<Self>;


I'm getting rid of the join function because the code that used it anyway had to special-case wildcards, and then it became easier to just have the entire wildcard logic there rather than having some of it inside join.

compiler/rustc_const_eval/src/interpret/memory.rs

theemathas · 2025-10-30T01:41:27Z

Do we need tests for #147959?

RalfJung · 2025-10-30T07:02:20Z

Do we need tests for #147959?

Good point, I added that.

rust-timer · 2025-11-09T14:23:55Z

Finished benchmarking commit (2b58ae9): 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)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 0.6%)

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

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.6%, 0.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.6%, 0.6%]	1

Cycles

Results (secondary -3.4%)

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)	-3.4%	[-3.4%, -3.4%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 474.157s -> 476.71s (0.54%)
Artifact size: 390.96 MiB -> 391.05 MiB (0.02%)

rustbot · 2025-11-15T09:12:54Z

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.

compiler/rustc_const_eval/src/interpret/place.rs

rust-rfcbot · 2025-11-15T16:12:23Z

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.

This will be merged soon.

RalfJung · 2025-11-16T07:04:46Z

@bors r=oli-obk

bors · 2025-11-16T07:04:49Z

📌 Commit 324e47b has been approved by oli-obk

It is now in the queue for this repository.

bors · 2025-11-16T07:09:49Z

⌛ Testing commit 324e47b with merge e1a2ec6...

bors · 2025-11-16T10:18:33Z

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing e1a2ec6 to main...

github-actions · 2025-11-16T10:21:47Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 67c4cf3 (parent) -> e1a2ec6 (this PR)

Test differences

Show 104 test diffs

Stage 1

[ui] tests/ui/consts/const-eval/ptr_fragments.rs: ignore (ignored always (disabled due to In consteval, mixing pointers to the same allocation but with different offsets gives results that depend on a static's address. #146291)) -> pass (J0)
[ui] tests/ui/consts/const-eval/ptr_fragments_in_final.rs: ignore (ignored always (disabled due to In consteval, mixing pointers to the same allocation but with different offsets gives results that depend on a static's address. #146291)) -> pass (J0)
[ui] tests/ui/consts/const-eval/read_partial_ptr.rs: ignore (ignored always (disabled due to In consteval, mixing pointers to the same allocation but with different offsets gives results that depend on a static's address. #146291)) -> pass (J0)

Stage 2

[ui] tests/ui/consts/const-eval/ptr_fragments.rs: ignore (ignored always (disabled due to In consteval, mixing pointers to the same allocation but with different offsets gives results that depend on a static's address. #146291)) -> pass (J1)
[ui] tests/ui/consts/const-eval/ptr_fragments_in_final.rs: ignore (ignored always (disabled due to In consteval, mixing pointers to the same allocation but with different offsets gives results that depend on a static's address. #146291)) -> pass (J1)
[ui] tests/ui/consts/const-eval/read_partial_ptr.rs: ignore (ignored always (disabled due to In consteval, mixing pointers to the same allocation but with different offsets gives results that depend on a static's address. #146291)) -> pass (J1)

Additionally, 98 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard e1a2ec605124d9f9b0095435faef70bd4bd3f128 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

pr-check-1: 1457.4s -> 1861.0s (+27.7%)
dist-apple-various: 3226.9s -> 3756.0s (+16.4%)
aarch64-apple: 7549.0s -> 8269.3s (+9.5%)
dist-powerpc-linux: 5057.5s -> 5503.1s (+8.8%)
aarch64-gnu-llvm-20-1: 3431.9s -> 3726.3s (+8.6%)
tidy: 153.1s -> 165.1s (+7.8%)
x86_64-gnu-gcc: 3422.9s -> 3176.7s (-7.2%)
x86_64-gnu-aux: 7054.1s -> 7540.8s (+6.9%)
dist-x86_64-mingw: 8601.6s -> 9191.5s (+6.9%)
x86_64-gnu-tools: 3772.2s -> 3515.2s (-6.8%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2025-11-16T11:38:55Z

Finished benchmarking commit (e1a2ec6): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -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
Improvements ✅ (primary)	-3.0%	[-3.0%, -3.0%]	1
Improvements ✅ (secondary)	-0.3%	[-0.4%, -0.2%]	8
All ❌✅ (primary)	-3.0%	[-3.0%, -3.0%]	1

Max RSS (memory usage)

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

Cycles

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

Binary size

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

Bootstrap: 473.917s -> 473.313s (-0.13%)
Artifact size: 388.65 MiB -> 388.72 MiB (0.02%)

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Oct 29, 2025

rustbot assigned oli-obk Oct 29, 2025

RalfJung force-pushed the const-ptr-fragment branch from 9058e28 to 4cce5fe Compare October 29, 2025 17:40

RalfJung added I-lang-nominated Nominated for discussion during a lang team meeting. I-lang-easy-decision Issue: The decision needed by the team is conjectured to be easy; this does not imply nomination labels Oct 29, 2025

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 Oct 29, 2025

traviscross added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Oct 29, 2025

RalfJung commented Oct 29, 2025

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compiler/rustc_middle/src/mir/interpret/allocation/provenance_map.rs Outdated Show resolved Hide resolved

RalfJung force-pushed the const-ptr-fragment branch 2 times, most recently from 4e023d0 to 739d035 Compare October 29, 2025 19:38

RalfJung commented Oct 29, 2025

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RalfJung force-pushed the const-ptr-fragment branch from 739d035 to 32b18d6 Compare October 29, 2025 19:45

theemathas reviewed Oct 30, 2025

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compiler/rustc_const_eval/src/interpret/memory.rs Outdated Show resolved Hide resolved

RalfJung force-pushed the const-ptr-fragment branch from 32b18d6 to bfad332 Compare October 30, 2025 07:04

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rustbot removed perf-regression Performance regression. S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Nov 9, 2025

RalfJung added 2 commits November 15, 2025 10:09

const-eval: fix and re-enable pointer fragment support

907fd85

when writing a scalar pair, always reset the entire destination range

324e47b

RalfJung force-pushed the const-ptr-fragment branch from 5d5d229 to 324e47b Compare November 15, 2025 09:12

oli-obk reviewed Nov 15, 2025

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oli-obk approved these changes Nov 15, 2025

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rust-rfcbot added finished-final-comment-period The final comment period is finished for this PR / Issue. and removed final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. labels Nov 15, 2025

rust-rfcbot added the to-announce Announce this issue on triage meeting label Nov 15, 2025

rustbot mentioned this pull request Nov 15, 2025

Tracking issue for release notes of #148259: const-eval: fix and re-enable pointer fragment support #148976

Open

bors 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 Nov 16, 2025

traviscross mentioned this pull request Nov 16, 2025

Remove restriction on dereferencing pointers in const rust-lang/reference#2090

Merged

bors added the merged-by-bors This PR was explicitly merged by bors. label Nov 16, 2025

bors merged commit e1a2ec6 into rust-lang:main Nov 16, 2025
12 checks passed

rustbot added this to the 1.93.0 milestone Nov 16, 2025

traviscross mentioned this pull request Nov 16, 2025

const-eval: always do mem-to-mem copies if there might be padding involved #148967

Open

RalfJung deleted the const-ptr-fragment branch November 16, 2025 14:29

fneddy mentioned this pull request Nov 18, 2025

fix the fragment_in_dst_padding_gets_overwritten test on s390x #149056

Merged

const-eval: fix and re-enable pointer fragment support #148259

const-eval: fix and re-enable pointer fragment support #148259

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Overall result: ❌ regressions - no action needed

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Test differences

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Overall result: ✅ improvements - no action needed

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