Shrink no-op drop elaboration by cjgillot · Pull Request #157491 · rust-lang/rust

cjgillot · 2026-06-05T16:39:16Z

Drop elaboration can be very verbose, especially when locals are move-from by parts. This happens a lot with ? desugaring for instance.

I start from the example given in #157463 and attempt to shrink the generated code.

The remaining empty goto blocks come from resetting drop flags. The cost/benefit of the refactor was less interesting.

rustbot · 2026-06-05T16:39:19Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

rustbot · 2026-06-05T16:39:21Z

r? @mejrs

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cjgillot · 2026-06-05T16:39:31Z

@bors try @rust-timer queue

Shrink no-op drop elaboration

rust-bors · 2026-06-05T18:55:09Z

☀️ Try build successful (CI)
Build commit: 0f76dc7 (0f76dc763580d699bfbe02f2ca1ca32d8db54243, parent: 39ec825c441918a4ae623a9b8840d1ccd446dcff)

rust-timer · 2026-06-05T19:38:43Z

Finished benchmarking commit (0f76dc7): comparison URL.

Overall result: ❌✅ regressions and improvements - 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)	0.7%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	0.3%	[0.2%, 0.4%]	3
Improvements ✅ (primary)	-0.7%	[-1.2%, -0.1%]	10
Improvements ✅ (secondary)	-0.7%	[-1.2%, -0.3%]	10
All ❌✅ (primary)	-0.5%	[-1.2%, 0.7%]	11

Max RSS (memory usage)

Results (primary 1.0%, secondary 0.8%)

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

	mean	range	count
Regressions ❌ (primary)	2.7%	[1.8%, 3.7%]	2
Regressions ❌ (secondary)	3.4%	[3.4%, 3.4%]	1
Improvements ✅ (primary)	-2.4%	[-2.4%, -2.4%]	1
Improvements ✅ (secondary)	-0.6%	[-0.6%, -0.5%]	2
All ❌✅ (primary)	1.0%	[-2.4%, 3.7%]	3

Cycles

Results (secondary -1.4%)

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)	2.7%	[2.7%, 2.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.8%	[-4.0%, -2.1%]	3
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.0%, secondary -0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.2%, 0.5%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.1%, -0.0%]	26
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	38
All ❌✅ (primary)	-0.0%	[-0.1%, 0.5%]	28

Bootstrap: 514.669s -> 516.17s (0.29%)
Artifact size: 400.66 MiB -> 400.87 MiB (0.05%)

saethlin · 2026-06-05T21:36:44Z

r? me

These would become simple gotos anyway, so save the cost of creating such blocks.

We are not modifying these blocks afterwards, so no need to create clean ones each time. Drive-by fix for async drops with enums.

rustbot · 2026-07-05T03:19:41Z

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.

saethlin · 2026-07-07T00:26:30Z

@bors r+

rust-bors · 2026-07-07T00:26:32Z

📌 Commit 80705e8 has been approved by saethlin

It is now in the queue for this repository.

rust-bors · 2026-07-08T06:27:16Z

☀️ Test successful - CI
Approved by: saethlin
Duration: 3h 8m 2s
Pushing 99576cf to main...

github-actions · 2026-07-08T06:30:39Z

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 5e91de6 (parent) -> 99576cf (this PR)

Test differences

Show 6 test diffs

Stage 1

[mir-opt] tests/mir-opt/building/fallible_struct_drop.rs: [missing] -> pass (J0)

Stage 2

[mir-opt] tests/mir-opt/building/fallible_struct_drop.rs: [missing] -> pass (J1)

Additionally, 4 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 99576cf2ebc870c305225e9856b9423228872c56 --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

arm-android: 1h 47m -> 1h 3m (-40.4%)
x86_64-gnu-gcc-core-tests: 13m 51s -> 8m 45s (-36.9%)
dist-arm-linux-gnueabi: 1h 1m -> 1h 23m (+35.2%)
x86_64-gnu-gcc: 1h 11m -> 48m 44s (-31.6%)
x86_64-mingw-1: 2h 9m -> 2h 46m (+28.9%)
x86_64-msvc-1: 2h 5m -> 2h 29m (+19.5%)
pr-check-1: 30m 26s -> 25m 34s (-16.0%)
dist-x86_64-linux-alt: 2h 19m -> 1h 57m (-15.8%)
x86_64-gnu-nopt: 2h 26m -> 2h 3m (-15.4%)
x86_64-gnu-tools: 1h 3m -> 54m 30s (-13.9%)

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 · 2026-07-08T07:14:07Z

Finished benchmarking commit (99576cf): 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)	-0.7%	[-1.2%, -0.3%]	10
Improvements ✅ (secondary)	-0.7%	[-1.1%, -0.3%]	10
All ❌✅ (primary)	-0.7%	[-1.2%, -0.3%]	10

Max RSS (memory usage)

Results (primary -2.0%, secondary 0.8%)

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)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-2.0%	[-6.1%, -0.5%]	5
Improvements ✅ (secondary)	-0.8%	[-0.8%, -0.8%]	1
All ❌✅ (primary)	-2.0%	[-6.1%, -0.5%]	5

Cycles

Results (primary -2.7%)

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)	-	-	0
Improvements ✅ (primary)	-2.7%	[-3.2%, -2.1%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.7%	[-3.2%, -2.1%]	2

Binary size

Results (primary -0.1%, secondary -0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	5
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-0.1%	[-0.3%, -0.0%]	13
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	8
All ❌✅ (primary)	-0.1%	[-0.3%, 0.2%]	18

Bootstrap: 487.812s -> 488.508s (0.14%)
Artifact size: 388.49 MiB -> 388.95 MiB (0.12%)

@saethlin

…aethlin Drop elaboration: Only create a reset block if there are flags to reset. Follow-up to rust-lang#157491 I'm not totally convinced this is worth the effort. The generated pattern is trivially cleaned-up by SimplifyCfg, so we should not need to bother. r? @saethlin since you reviewed the earlier PR

@saethlin

…aethlin Drop elaboration: Only create a reset block if there are flags to reset. Follow-up to rust-lang#157491 I'm not totally convinced this is worth the effort. The generated pattern is trivially cleaned-up by SimplifyCfg, so we should not need to bother. r? @saethlin since you reviewed the earlier PR

@saethlin

…aethlin Drop elaboration: Only create a reset block if there are flags to reset. Follow-up to rust-lang#157491 I'm not totally convinced this is worth the effort. The generated pattern is trivially cleaned-up by SimplifyCfg, so we should not need to bother. r? @saethlin since you reviewed the earlier PR

@saethlin

Drop elaboration: Only create a reset block if there are flags to reset. Follow-up to #157491 I'm not totally convinced this is worth the effort. The generated pattern is trivially cleaned-up by SimplifyCfg, so we should not need to bother. r? @saethlin since you reviewed the earlier PR

@saethlin

…aethlin Drop elaboration: Only create a reset block if there are flags to reset. Follow-up to rust-lang#157491 I'm not totally convinced this is worth the effort. The generated pattern is trivially cleaned-up by SimplifyCfg, so we should not need to bother. r? @saethlin since you reviewed the earlier PR

@saethlin

Rollup merge of #159195 - cjgillot:elaborate-drop-flags, r=saethlin Drop elaboration: Only create a reset block if there are flags to reset. Follow-up to #157491 I'm not totally convinced this is worth the effort. The generated pattern is trivially cleaned-up by SimplifyCfg, so we should not need to bother. r? @saethlin since you reviewed the earlier PR

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 Jun 5, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jun 5, 2026

Auto merge of #157491 - cjgillot:elaborate-drop-chain, r=<try>

0f76dc7

Shrink no-op drop elaboration

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jun 5, 2026

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traviscross mentioned this pull request Jun 9, 2026

Drop elaboration emits O(N^2) cleanup for partially-initialized aggregate literals #157463

Open

cjgillot added 4 commits July 4, 2026 23:21

Add test.

1e1ee30

Filter out dead drops from drop ladder elaboration.

ae84529

These would become simple gotos anyway, so save the cost of creating such blocks.

Remove silly goto blocks.

e92eba1

We are not modifying these blocks afterwards, so no need to create clean ones each time. Drive-by fix for async drops with enums.

Skip switch if all targets are the same.

80705e8

cjgillot force-pushed the elaborate-drop-chain branch from c7f866c to 80705e8 Compare July 5, 2026 03:19

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 Jul 7, 2026

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rust-bors Bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jul 8, 2026

rust-bors Bot merged commit 99576cf into rust-lang:main Jul 8, 2026
14 checks passed

rustbot added this to the 1.99.0 milestone Jul 8, 2026

rustbot removed the perf-regression Performance regression. label Jul 8, 2026

cjgillot deleted the elaborate-drop-chain branch July 8, 2026 10:50

cjgillot mentioned this pull request Jul 12, 2026

Drop elaboration: Only create a reset block if there are flags to reset. #159195

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rust-timer commented Jun 5, 2026

Overall result: ❌✅ regressions and improvements - please read:

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Test differences

Stage 1

Stage 2

Job duration changes

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rust-timer commented Jul 8, 2026

Overall result: ✅ improvements - no action needed

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