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Shrink no-op drop elaboration#157491

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cjgillot wants to merge 4 commits into
rust-lang:mainfrom
cjgillot:elaborate-drop-chain
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Shrink no-op drop elaboration#157491
cjgillot wants to merge 4 commits into
rust-lang:mainfrom
cjgillot:elaborate-drop-chain

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@cjgillot cjgillot commented Jun 5, 2026

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.

cjgillot added 4 commits June 5, 2026 15:59
@cjgillot
Add test.
b01f63e
@cjgillot
Filter out dead drops from drop ladder elaboration.
a73a4e6 
These would become simple gotos anyway, so save the cost of creating such
blocks.
@cjgillot
Remove silly goto blocks.
3e86cd2 
We are not modifying these blocks afterwards, so no need to create clean
ones each time.

Drive-by fix for async drops with enums.
@cjgillot
Skip switch if all targets are the same.
c7f866c
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rustbot commented Jun 5, 2026

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

@rustbot 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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rustbot commented Jun 5, 2026

r? @mejrs

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cjgillot commented Jun 5, 2026

@bors try @rust-timer queue

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@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 5, 2026
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rust-bors Bot pushed a commit that referenced this pull request Jun 5, 2026
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Auto merge of #157491 - cjgillot:elaborate-drop-chain, r=<try>
0f76dc7 
Shrink no-op drop elaboration
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rust-bors Bot commented Jun 5, 2026

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

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rust-timer commented Jun 5, 2026

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%)

@rustbot 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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saethlin commented Jun 5, 2026

r? me

@rustbot rustbot assigned saethlin and unassigned mejrs Jun 5, 2026
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