Underscore-prefixed bindings are explicitly allowed to be unused

[chenyukang]

Fixes #149889

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

r? @fee1-dead

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[fee1-dead]

@bors r+ rollup

[rust-bors]

📌 Commit ea93fb5 has been approved by fee1-dead

It is now in the queue for this repository.

[panstromek]

Investigating regression from #151291, as_str is potentially expensive and there's not many other candidates in that PR.

@rust-timer build a4ae3dd

[rust-timer]

Finished benchmarking commit (a4ae3dd): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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%	[0.2%, 0.3%]	3
Regressions ❌ (secondary)	0.1%	[0.0%, 0.4%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	3
All ❌✅ (primary)	0.2%	[0.2%, 0.3%]	3

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)	1.8%	[1.0%, 2.6%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.9%	[-5.5%, -1.7%]	8
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 4.0%)

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)	5.7%	[2.0%, 10.5%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.8%	[-2.8%, -2.8%]	1
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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	14
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	6
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	14

Bootstrap: 474.856s -> 470.984s (-0.82%)
Artifact size: 383.43 MiB -> 383.47 MiB (0.01%)

[chenyukang]

I think the check should be moved to a new place here:
https://github.com/chenyukang/rust/blob/ea93fb548c7c6e838b35893f3f9cc2ed3011aaea/compiler/rustc_mir_transform/src/liveness.rs#L1121

so that it will only check when unused is reported, and will not affect normal code path.
I fired a new PR to fix the performance issue?

[panstromek]

Nice, but this still doesn't explain all regressions in that rollup. I'll check other PRs, too.