Report precondition violation for `<usize as SliceIndex>::get_unchecked` in const-eval by bardiharborow · Pull Request #161616 · rust-lang/rust

bardiharborow · 2026-08-23T17:55:18Z

The precondition check was gated on check_language_ub, which is disabled in const-eval and Miri, on the grounds that the assume below it is language UB that the interpreter will catch anyway. It does catch it, but only as "assume called with false", which says nothing about what the caller did wrong.

This PR instead gates the check on check_library_ub, matching get_unchecked_mut, so that the interpreter reports the violated precondition.

Fixes #161611

@rustbot label +A-const-eval +A-diagnostics +A-slice +T-libs +requires-nightly

rustbot · 2026-08-23T17:55:23Z

r? @wesleywiser

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teor2345

Thanks, looks like a straightforward fix!

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rustbot · 2026-09-05T03:25:31Z

Any special-casing of Miri in the standard library requires review.

cc @rust-lang/miri

RalfJung · 2026-09-05T07:33:27Z

@bors squash

…ed` in const-eval * Add a test for the diagnostics on `<usize as SliceIndex>::get_unchecked` in const-eval * Report a precondition violation for `<usize as SliceIndex>::get_unchecked` in const-eval The precondition check was gated on `check_language_ub`, which is disabled in const-eval and Miri, on the grounds that the `assume` below it is language UB that the interpreter will catch anyway. It does catch it, but only as "`assume` called with `false`", which says nothing about what the caller did wrong. This commit instead gates the check on `check_library_ub`, matching `get_unchecked_mut`, so that the interpreter reports the violated precondition. * fixup! Report a precondition violation for `<usize as SliceIndex>::get_unchecked` in const-eval * tweak comment Co-authored-by: Ralf Jung <post@ralfj.de>

rust-bors · 2026-09-05T07:33:49Z

🔨 4 commits were squashed into 3974528.

RalfJung · 2026-09-05T07:34:41Z

Makes sense, thanks :)
@bors r+ rollup

rust-bors · 2026-09-05T07:34:44Z

📌 Commit 3974528 has been approved by RalfJung

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 10. This pull request will be tested once the tree is reopened.

Reason for tree closure: Linux x64 EC2 instances are not starting

…precondition-const-eval, r=RalfJung Report precondition violation for `<usize as SliceIndex>::get_unchecked` in const-eval The precondition check was gated on `check_language_ub`, which is disabled in const-eval and Miri, on the grounds that the `assume` below it is language UB that the interpreter will catch anyway. It does catch it, but only as "`assume` called with `false`", which says nothing about what the caller did wrong. This PR instead gates the check on `check_library_ub`, matching `get_unchecked_mut`, so that the interpreter reports the violated precondition. Fixes rust-lang#161611

Rollup of 14 pull requests Successful merges: - #162324 (miri subtree update) - #162170 (bootstrap: use target's LLVM libdir when cross-compiling) - #158312 (Adds support for AArch64 SVE to inline assembly) - #159792 (A more readable debug map for IndexMaps) - #160745 (make closures act like MaybeDangling) - #161263 (break rustc_expand-rustc_middle dependency) - #161895 (std::sys::pal::sgx: fix mismatched alloc/free alignment) - #161940 (Promote `wasm32-wasip3` to a tier 2 target) - #161397 (coverage: Tidy tests and add some new ones) - #161616 (Report precondition violation for `<usize as SliceIndex>::get_unchecked` in const-eval) - #162248 (Add regression test for unsized const parameter default ICE) - #162250 (Fix hashing of span end columns in incremental compilation) - #162265 (cargotest: add lockfiles) - #162318 (bootstrap: Fix broken path for `./x doc compiler/rustc --open`)

Rollup merge of #161616 - bardiharborow:get-unchecked-index-precondition-const-eval, r=RalfJung Report precondition violation for `<usize as SliceIndex>::get_unchecked` in const-eval The precondition check was gated on `check_language_ub`, which is disabled in const-eval and Miri, on the grounds that the `assume` below it is language UB that the interpreter will catch anyway. It does catch it, but only as "`assume` called with `false`", which says nothing about what the caller did wrong. This PR instead gates the check on `check_library_ub`, matching `get_unchecked_mut`, so that the interpreter reports the violated precondition. Fixes #161611

rust-timer · 2026-09-06T05:11:09Z

Note

This PR was benchmarked as part of triage of its containing rollup: triage URL.

Finished benchmarking commit (bee0ce4): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (secondary -0.3%)

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)	5.7%	[5.7%, 5.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.4%	[-6.4%, -6.4%]	1
All ❌✅ (primary)	-	-	0

Cycles

This perf run didn't have relevant results for this metric.

Binary size

Results (primary 0.0%, secondary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.1%]	11
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.1%]	11

Bootstrap: missing data
Artifact size: 403.33 MiB -> 403.36 MiB (0.01%)

rustbot assigned wesleywiser Aug 23, 2026

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 Aug 23, 2026

bardiharborow changed the title ~~Report precondition violations for <usize as SliceIndex>::get_unchecked in const-eval~~ Report precondition violation for <usize as SliceIndex>::get_unchecked in const-eval Aug 23, 2026

teor2345 approved these changes Aug 24, 2026

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workingjubilee reviewed Aug 24, 2026

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Comment thread library/core/src/slice/index.rs Outdated

RalfJung reviewed Sep 5, 2026

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rust-bors Bot force-pushed the get-unchecked-index-precondition-const-eval branch from 2a40480 to 3974528 Compare September 5, 2026 07:33

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-review Status: Awaiting review from the assignee but also interested parties. labels Sep 5, 2026

JonathanBrouwer mentioned this pull request Sep 5, 2026

Rollup of 14 pull requests #162327

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Zalathar mentioned this pull request Sep 5, 2026

Rollup of 14 pull requests #162333

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rust-bors Bot merged commit bdedfdd into rust-lang:main Sep 5, 2026
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rustbot added this to the 1.100.0 milestone Sep 5, 2026

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Report precondition violation for `<usize as SliceIndex>::get_unchecked` in const-eval - #161616

Report precondition violation for `<usize as SliceIndex>::get_unchecked` in const-eval#161616
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