Turn ProjectionElem::Subtype into CastKind::Subtype #147055

beepster4096 · 2025-09-26T04:53:10Z

I noticed that drop elaboration can't, in general, handle ProjectionElem::SubType. It creates a disjoint move path that overlaps with other move paths. (Subslice does too, and I'm working on a different PR to make that special case less fragile.) If its skipped and treated as the same move path as its parent then MovePath.place has multiple possible projections. (It would probably make sense to remove all Subtype projections for the canonical place but it doesn't make sense to have this special case for a problem that doesn't actually occur in real MIR.)

The only reason this doesn't break is that Subtype is always the sole projection of the local its applied to. For the same reason, it works fine as a CastKind so I figured that makes more sense than documenting and validating this hidden invariant.

cc #112651, #133258

r? Icnr (bc you've been the main person dealing with Subtype it looks like)

rustbot · 2025-09-26T04:53:13Z

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

Some changes occurred to constck

cc @fee1-dead

rustbot · 2025-09-26T04:53:14Z

Failed to set assignee to Icnr: cannot assign: response: {"message":"Not Found","documentation_url":"https://docs.github.com/rest/issues/assignees#add-assignees-to-an-issue","status":"404"}

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

beepster4096 · 2025-09-26T04:54:55Z

r? lcnr (sorry, I got your name wrong 😅)

lcnr

pls fix clippy in CI, r=me after

View changes since this review

rustbot · 2025-09-26T19:19:37Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

beepster4096 · 2025-09-26T21:47:19Z

Hooray? I am a little scared because I have no idea how I actually fixed this. Maybe I accidentally turned off a check or something and this should keep ICEing..

also slightly minimized the test

rustbot · 2025-09-26T22:37:46Z

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

beepster4096 · 2025-09-26T22:39:10Z

Nope, it's not fixed, it just requires -Zvalidate-mir now.

beepster4096 · 2025-09-27T01:06:21Z

@rustbot ready

lcnr · 2025-09-30T09:18:26Z

tests/crashes/120016.rs

+        let f: F = 0u32;
+
        1
    }],


that change seems odd, why remove the async blocks here?

While investigating, I saw on the issue page that the async blocks are actually not part of the bug. I can split that into a different PR if you'd like.

lcnr · 2025-10-01T10:57:40Z

@bors r+ rollup=never

bors · 2025-10-01T10:57:42Z

📌 Commit 413f095 has been approved by lcnr

It is now in the queue for this repository.

bors · 2025-10-02T01:54:51Z

⌛ Testing commit 413f095 with merge 42b384e...

bors · 2025-10-02T04:58:22Z

☀️ Test successful - checks-actions
Approved by: lcnr
Pushing 42b384e to master...

github-actions · 2025-10-02T05:01:21Z

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 3369e82 (parent) -> 42b384e (this PR)

Test differences

Show 16 test diffs

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

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 42b384ec0dfcd528d99a4db0a337d9188a9eecaa --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

dist-apple-various: 3541.1s -> 4295.3s (21.3%)
aarch64-apple: 9515.5s -> 7806.2s (-18.0%)
dist-aarch64-msvc: 5631.1s -> 6621.4s (17.6%)
i686-gnu-2: 5432.2s -> 6358.4s (17.0%)
x86_64-gnu-nopt: 6845.9s -> 7854.8s (14.7%)
aarch64-gnu-debug: 3872.6s -> 4408.7s (13.8%)
i686-gnu-1: 7379.3s -> 8257.9s (11.9%)
dist-various-1: 4158.9s -> 3667.0s (-11.8%)
aarch64-msvc-2: 5289.0s -> 4702.4s (-11.1%)
x86_64-gnu-miri: 4447.4s -> 4924.7s (10.7%)

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 · 2025-10-02T06:22:33Z

Finished benchmarking commit (42b384e): comparison URL.

Overall result: ❌ regressions - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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)	3.0%	[3.0%, 3.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.0%	[3.0%, 3.0%]	1

Max RSS (memory usage)

Results (secondary 2.8%)

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)	2.8%	[2.7%, 2.9%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 2.8%, secondary 2.5%)

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

	mean	range	count
Regressions ❌ (primary)	2.8%	[2.8%, 2.8%]	1
Regressions ❌ (secondary)	2.5%	[1.2%, 3.8%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.8%	[2.8%, 2.8%]	1

Binary size

Results (primary 1.1%)

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

	mean	range	count
Regressions ❌ (primary)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[1.1%, 1.1%]	1

Bootstrap: 472.027s -> 471.941s (-0.02%)
Artifact size: 387.76 MiB -> 387.76 MiB (0.00%)

Relevant upstream PR: - rust-lang/rust#147055 (Turn ProjectionElem::Subtype into CastKind::Subtype) Resolves: model-checking#4392

Relevant upstream PR: - rust-lang/rust#147055 (Turn ProjectionElem::Subtype into CastKind::Subtype) Resolves: #4392 By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

panstromek · 2025-10-06T14:56:13Z

perf triage:

clap_derive bimodal noise

@rustbot label: +perf-regression-triaged

…tion, r=lcnr Turn ProjectionElem::Subtype into CastKind::Subtype I noticed that drop elaboration can't, in general, handle `ProjectionElem::SubType`. It creates a disjoint move path that overlaps with other move paths. (`Subslice` does too, and I'm working on a different PR to make that special case less fragile.) If its skipped and treated as the same move path as its parent then `MovePath.place` has multiple possible projections. (It would probably make sense to remove all `Subtype` projections for the canonical place but it doesn't make sense to have this special case for a problem that doesn't actually occur in real MIR.) The only reason this doesn't break is that `Subtype` is always the sole projection of the local its applied to. For the same reason, it works fine as a `CastKind` so I figured that makes more sense than documenting and validating this hidden invariant. cc rust-lang#112651, rust-lang#133258 r? Icnr (bc you've been the main person dealing with `Subtype` it looks like)

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 Sep 26, 2025

rustbot assigned lcnr Sep 26, 2025

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beepster4096 changed the title ~~Turn ProjectionKind::Subtype into CastKind::Subtype~~ Turn ProjectionElem::Subtype into CastKind::Subtype Sep 26, 2025

beepster4096 force-pushed the subtype_is_not_a_projection branch from 37be44c to 0132f5e Compare September 26, 2025 07:29

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ProjectionElem::Subtype -> CastKind::Subtype

aa5a214

beepster4096 force-pushed the subtype_is_not_a_projection branch from 0132f5e to aa5a214 Compare September 26, 2025 08:27

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lcnr reviewed Sep 26, 2025

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lcnr 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 Sep 26, 2025

castkind::subtype in clippy

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rustbot added the T-clippy Relevant to the Clippy team. label Sep 26, 2025

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this ice now requires -Zvalidate-mir

413f095

also slightly minimized the test

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 Sep 27, 2025

lcnr reviewed Sep 30, 2025

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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 Oct 1, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Oct 2, 2025

bors merged commit 42b384e into rust-lang:master Oct 2, 2025
11 checks passed

rustbot added this to the 1.92.0 milestone Oct 2, 2025

bors mentioned this pull request Oct 2, 2025

GVN: Evaluate constants lazily. #146869

Merged

rustbot added the perf-regression Performance regression. label Oct 2, 2025

tautschnig added a commit to tautschnig/kani that referenced this pull request Oct 3, 2025

Upgrade Rust toolchain to 2025-10-03

fe45ace

Relevant upstream PR: - rust-lang/rust#147055 (Turn ProjectionElem::Subtype into CastKind::Subtype) Resolves: model-checking#4392

tautschnig mentioned this pull request Oct 3, 2025

Upgrade Rust toolchain to 2025-10-03 model-checking/kani#4393

Merged

github-actions bot mentioned this pull request Oct 3, 2025

Toolchain upgrade to nightly-2025-10-09 flux-rs/flux#1271

Merged

beepster4096 mentioned this pull request Oct 6, 2025

ice: type mismatch when copying #120016

Open

rustbot added the perf-regression-triaged The performance regression has been triaged. label Oct 6, 2025

Turn ProjectionElem::Subtype into CastKind::Subtype #147055

Turn ProjectionElem::Subtype into CastKind::Subtype #147055

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Test differences

Job duration changes

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rust-timer commented Oct 2, 2025

Overall result: ❌ regressions - please read the text below

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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panstromek commented Oct 6, 2025

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