Disallow accesses through an Index projection when a sibling ConstantIndex projection has been moved out of by beepster4096 · Pull Request #160780 · rust-lang/rust

beepster4096 · 2026-08-08T22:21:13Z

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I did not use an LLM to create a change in this PR.
I used an LLM to create a change in this PR, and I have explained below how it was used.

Fixes #160525

Refactored the LookupResult enum returned by MovePathLookup::find:
- The LookupResult::Parent(None) case is now a separate variant, LookupResult::None
- LookupResult::Parent has an additional field containing the variant immediately following the parent move path.
Borrowck's check_if_full_path_is_moved now checks that new field and if it is PlaceElem::Index, it makes sure there are no uninit descendants of the indexed array's move path (which can only be ConstantIndex and descendants of those.)
- move_path_closest_to is removed and inlined into check_if_full_path_is_moved.

…ons to be overlapping in some cases

rustbot · 2026-08-08T22:21:16Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

rustbot · 2026-08-08T22:21:18Z

r? @oli-obk

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cjgillot · 2026-08-09T00:44:07Z

+        mpi: MovePathIndex,
+
+        /// The PlaceElem in the place immediately projecting from the parent move path.
+        next_elem: PlaceElem<'tcx>,


Should this be a ProjectionKind?

View changes since the review

Yeah, makes more sense considering the variant is the only info used right now

cjgillot · 2026-08-09T00:44:39Z

+            // Subslice and ConstantIndex projections of slices also overlap siblings,
+            // but the parent slice will never have a move path
+            // Subslice projections of arrays are specifically checked in `check_if_subslice_element_is_moved`
+            LookupResult::Parent { mpi, next_elem: PlaceElem::Index(..) } => self


Could we match exhaustively on next_elem?

View changes since the review

…haustively on it

oli-obk · 2026-08-13T15:11:52Z

While this is a soundness fix where we're def landing it, let's still crater it and inform any potentially broken crates.

@bors try

Disallow accesses through an Index projection when a sibling ConstantIndex projection has been moved out of

rust-bors · 2026-08-13T16:14:59Z

☀️ Try build successful (CI)
Build commit: 47574b8 (47574b82ff3fe9b0a45ff65a4fe189e295ecb748)
Base parent: ba28ff7 (ba28ff76f353a722f31c4f3dd2ac4e437d36411b)

oli-obk · 2026-08-13T18:30:00Z

@craterbot check

craterbot · 2026-08-13T18:30:34Z

👌 Experiment pr-160780 created and queued.
🤖 Automatically detected try build 47574b8
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

theemathas · 2026-09-03T13:58:48Z

@craterbot cancel

See #162233

craterbot · 2026-09-03T13:59:07Z

🗑️ Experiment pr-160780 deleted!

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

theemathas · 2026-09-03T13:59:51Z

Note: In the crater rollup, I added this small change to make the code compile. Hopefully that's fine. 09cc812 (#162233)

theemathas · 2026-09-06T02:16:52Z

@craterbot check crates=https://crater-reports.s3.amazonaws.com/pr-162233-crater-rollup/retry-regressed-list.txt p=1

craterbot · 2026-09-06T02:17:00Z

👌 Experiment pr-160780 created and queued.
🤖 Automatically detected try build 47574b8
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-09-08T02:04:36Z

🚧 Experiment pr-160780 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-09-08T10:05:53Z

🎉 Experiment pr-160780 is completed!
📊 0 regressed and 0 fixed (7794 total)
📊 725 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-160780/retry-regressed-list.txt ↩

oli-obk

sad that it needed another special casing of the index projection. But after digging around in your branch for a bit I think this is indeed the right solution

@bors r+

View changes since this review

rust-bors · 2026-09-10T13:51:10Z

📋 This PR cannot be approved because it currently has the following label: needs-fcp.

oli-obk · 2026-09-11T10:35:13Z

Ah darn it. Could have started the FCP while crater was running. oh well.

@rfcbot fcp merge types

This PR fixes an unsoundness in borrowck where we didn't realize index operations can overlap with slice patterns under certain circumstances (they both need to be within a single expression, otherwise we notice the local is uninit).

The other projections are mutually exclusive (e.g. can't have a field projection and a deref projection on the same type/local), so they don't run into this situation.

rust-rfcbot · 2026-09-11T10:35:16Z

@oli-obk 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!

See this document for info about what commands tagged team members can give me.

fix borrowck not considering sibling index and constantindex projecti…

364f9fd

…ons to be overlapping in some cases

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. labels Aug 8, 2026

rustbot assigned oli-obk Aug 8, 2026

beepster4096 mentioned this pull request Aug 9, 2026

simplify MoveDataBuilder::move_path_for with new UnDerefer::co_iter_projections #160784

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cjgillot reviewed Aug 9, 2026

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use ProjectionKind instead of PlaceElem for LookupResult and match ex…

e2ab037

…haustively on it

oli-obk assigned cjgillot and oli-obk and unassigned oli-obk Aug 13, 2026

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rust-bors Bot pushed a commit that referenced this pull request Aug 13, 2026

Auto merge of #160780 - beepster4096:overlapping_move_paths, r=<try>

47574b8

Disallow accesses through an Index projection when a sibling ConstantIndex projection has been moved out of

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 13, 2026

theemathas mentioned this pull request Sep 3, 2026

Crater rollup #162233

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craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Sep 3, 2026

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 6, 2026

craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Sep 8, 2026

oli-obk approved these changes Sep 10, 2026

View reviewed changes

oli-obk added S-waiting-on-fcp Status: PR is in FCP and is awaiting for FCP to complete. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 11, 2026

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