Correct and document semantics of `yield` terminator by cjgillot · Pull Request #156875 · rust-lang/rust

cjgillot · 2026-05-24T12:17:00Z

The current implementation of dataflow for the yield terminator confuses the drop target with unwinding. Which is wrong, in particular when implementing async drops.

r? @RalfJung

rustbot · 2026-05-24T12:17:03Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

This PR changes MIR

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

rustbot · 2026-05-24T12:17:06Z

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

RalfJung

I'm afraid I don't know nearly enough about async lowering to review this.

@rustbot reroll
(Or maybe you know a good reviewer?)

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adwinwhite · 2026-05-24T22:42:24Z

@rustbot reroll

oli-obk

r=me with doc review changes applied.

I was thinking and playing a bit with writing a run test, but it's not really worth it. It's an extreme corner case.

A mir-opt test would still be useful showing how the unwinding targets change now

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rustbot · 2026-05-26T10:21:32Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

cjgillot · 2026-05-29T20:10:44Z

I was thinking and playing a bit with writing a run test, but it's not really worth it. It's an extreme corner case.

A mir-opt test would still be useful showing how the unwinding targets change now

The only difference I could see in tests is the size of async-drop coroutine state. On the current (wrong) semantics, I got a size increase when trying to change async drop expansion: the resume argument was (wrongly) saved in coroutine state.

cjgillot · 2026-05-30T09:20:41Z

@bors r=oli-obk

rust-bors · 2026-05-30T09:20:44Z

📌 Commit 9ef4f52 has been approved by oli-obk

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 1. This pull request will be tested once the tree is reopened.

…uwer Rollup of 12 pull requests Successful merges: - #154591 (Remove `will_cache_on_disk_for_key_fn`) - #156672 (Misc improvements to coroutine transform code) - #157027 (HIR ty lowering: Move some things into submodules) - #157051 (Allow two object files for a single CGU in CompiledModule) - #157100 (Some more per owner things) - #153497 (Use `trait_object_dummy_self` more & heavily fix+update related docs) - #155638 (Fix tupled closure signature in `AsyncFn` arg mismatch diagnostic) - #156826 (style: Clarify nullary call and `()` no-break rule applies past max width) - #157004 (Remove unused functions in `value_analysis.rs`) - #157032 (Fixed more &x ->&mut x suggestions) - #157033 (Note irrefutable while let in loop type errors) - #157139 (compiler: `ops::RangeInclusive` → `range::RangeInclusive`) Failed merges: - #156875 (Correct and document semantics of `yield` terminator)

rustbot · 2026-05-30T14:46:31Z

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.

cjgillot · 2026-05-30T14:50:07Z

@bors r=oli-obk

rust-bors · 2026-05-30T14:50:10Z

📌 Commit b542677 has been approved by oli-obk

It is now in the queue for this repository.

…uwer Rollup of 8 pull requests Successful merges: - #156863 (Make hint::cold_path #[cold] so that it works even if the MIR inliner can't inline it) - #156875 (Correct and document semantics of `yield` terminator) - #157115 ([rustdoc] Fix foreign items macro expansion) - #157150 (Revert "drop derive helpers during ast lowering" ) - #156887 (Rename `-Zdebuginfo-for-profiling` switch) - #157039 (rustdoc: correctly propagate cfgs for glob reexports) - #157125 (Rewrite the `#[repr]` attribute parser) - #157154 (Revert workaround used to select the gcc codegen in the coretests CI)

@RalfJung

Rollup merge of #156875 - cjgillot:yield-drop-resume, r=oli-obk Correct and document semantics of `yield` terminator The current implementation of dataflow for the yield terminator confuses the `drop` target with unwinding. Which is wrong, in particular when implementing async drops. r? @RalfJung

JonathanBrouwer · 2026-05-31T07:22:43Z

@rust-timer build bb4f119

rust-timer · 2026-05-31T08:17:53Z

Finished benchmarking commit (bb4f119): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@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)	0.2%	[0.1%, 0.2%]	16
Regressions ❌ (secondary)	0.5%	[0.1%, 2.4%]	18
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.1%, 0.2%]	16

Max RSS (memory usage)

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

Cycles

Results (secondary 6.7%)

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)	6.7%	[6.7%, 6.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 515.367s -> 513.684s (-0.33%)
Artifact size: 400.82 MiB -> 400.81 MiB (-0.00%)

Kobzol · 2026-05-31T08:48:19Z

(thanks Jonathan!)

Looks like this PR caused a mild regression in check builds. Maybe because of the new Box? Is there some common number of return statements, maybe that could be a SmallVec?

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 May 24, 2026

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cjgillot force-pushed the yield-drop-resume branch from c2fa5ba to 4141c4c Compare May 24, 2026 12:27

RalfJung reviewed May 24, 2026

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rustbot assigned adwinwhite and unassigned RalfJung May 24, 2026

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RalfJung reviewed May 26, 2026

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Comment thread compiler/rustc_middle/src/mir/syntax.rs Outdated

oli-obk requested changes May 26, 2026

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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 May 26, 2026

cjgillot force-pushed the yield-drop-resume branch from ce3102b to 9ef4f52 Compare May 29, 2026 20:24

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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 May 30, 2026

JonathanBrouwer mentioned this pull request May 30, 2026

Rollup of 12 pull requests #157149

Merged

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cjgillot added 3 commits May 30, 2026 14:25

Yield drop assigns to the resume_arg.

cef7bd7

Correct semantics around return place.

3f96a45

Review nits.

b542677

cjgillot force-pushed the yield-drop-resume branch from 9ef4f52 to b542677 Compare May 30, 2026 14:46

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 May 30, 2026

JonathanBrouwer mentioned this pull request May 30, 2026

Rollup of 8 pull requests #157161

Merged

cjgillot mentioned this pull request May 30, 2026

Change type of async context parameter after state transform. #157166

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rust-bors Bot merged commit 95a2369 into rust-lang:main May 30, 2026
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rustbot added this to the 1.98.0 milestone May 30, 2026

cjgillot deleted the yield-drop-resume branch May 30, 2026 21:15

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rustbot added the perf-regression Performance regression. label May 31, 2026

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rust-timer commented May 31, 2026

Overall result: ❌ regressions - please read:

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