Shrink no-op drop elaboration

[cjgillot]

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]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

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[cjgillot]

@bors try @rust-timer queue

[rust-bors]

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

[rust-timer]

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]

r? me