only call polymorphic array iter drop machinery when the type requires it

[the8472]

I saw a bunch of dead, empty <[core::mem::maybe_uninit::MaybeUninit<T>; N] as core::array::iter::iter_inner::PartialDrop>::partial_drop functions when compiling with more than 1 CGU.

Let's see if we can help optimizations to eliminate stuff earlier.

r? ghost

[the8472]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 7a5c0c2 (7a5c0c26f80ea669941a7c760c9d0e4ebbfc8a23, parent: 2cb4e7dce84fdebc0279159f1082f92b99299d87)

[rust-timer]

Finished benchmarking commit (7a5c0c2): 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)	0.6%	[0.6%, 0.6%]	1
Regressions ❌ (secondary)	0.9%	[0.8%, 1.0%]	6
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	1
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	2
All ❌✅ (primary)	0.3%	[-0.1%, 0.6%]	2

Max RSS (memory usage)

Results (primary 0.7%, secondary 2.3%)

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

	mean	range	count
Regressions ❌ (primary)	3.0%	[0.7%, 5.1%]	4
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-4.0%	[-5.6%, -2.4%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[-5.6%, 5.1%]	6

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

Results (primary -0.1%, secondary 0.1%)

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%, 0.1%]	6
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-0.2%	[-0.5%, -0.0%]	14
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-0.5%, 0.1%]	20

Bootstrap: 471.593s -> 470.766s (-0.18%)
Artifact size: 388.35 MiB -> 388.32 MiB (-0.01%)

[the8472]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 61c5b78 (61c5b789d1ab11492050b0106aac2e724cf4a4d9, parent: e2c96cc06bdbdbc6f59c7551194d6a742260d6ff)

[the8472]

First perf run looks better, going with that one. Binary sizes are down on average, including rustc artifact sizes.
The wg-grammar instruction regression... the history looks like it's just oscillating? So maybe noise.

r? @scottmcm

[scottmcm]

Is there any way we can have a test for the original problem? This feels like the kind of change that a well-meaning person could easily undo later without realizing it.

[the8472]

On godbolt it only reproduces when linking a lib crate and then disassembling it. Using emit-asm doesn't work because it forces 1CGU. https://rust.godbolt.org/z/78fMrGGxc

I could try writing a run-make test but seems a bit overkill.

[the8472]

But I can add a comment.