Simplify Vec using iter::repeat_n

[scottmcm]

By using repeat_n instead of ExtendElement, we can delete a bunch more unsafe code from Vec.

All the codegen tests are passing (and I made one stricter to help be sure), so let's see if perf is also happy with it.

r? @ghost

Built atop #104435; will leave draft until I can rebase atop that.

[scottmcm]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 50b394a with merge 30c97e84bdbff017290f6d12db34df2f225359a8...

[bors]

☀️ Try build successful - checks-actions
Build commit: 30c97e84bdbff017290f6d12db34df2f225359a8 (30c97e84bdbff017290f6d12db34df2f225359a8)

[rust-timer]

Finished benchmarking commit (30c97e84bdbff017290f6d12db34df2f225359a8): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf +perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.2%, 0.7%]	24
Regressions ❌ (secondary)	2.3%	[0.2%, 5.5%]	15
Improvements ✅ (primary)	-0.8%	[-1.0%, -0.6%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[-1.0%, 0.7%]	26

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.8%	[0.4%, 3.8%]	5
Regressions ❌ (secondary)	1.7%	[1.2%, 2.5%]	3
Improvements ✅ (primary)	-4.6%	[-4.6%, -4.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.8%	[-4.6%, 3.8%]	6

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.8%	[0.7%, 0.9%]	3
Regressions ❌ (secondary)	5.0%	[4.5%, 5.8%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.3%	[-5.3%, -5.3%]	1
All ❌✅ (primary)	0.8%	[0.7%, 0.9%]	3

[the8472]

I'm not sure if it's a good idea to remove this. There's untapped optimization potential in the current implementation. E.g. it could use the zero-value check that vec initialization already has and then use Allocator::grow_zeroed.

[scottmcm]

@the8472 I agree that's a good thing to add, but I don't think it needs the ExtendWith trait, since we can't do that for ExtendFunc anyway. I could leave an private extend_element function without the trait and type, though, if you think that'd be helpful.

[scottmcm]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 78f2356 with merge d6ba54d913266c13b4ddf2eaf8b1b9b023df4caa...

[bors]

☀️ Try build successful - checks-actions
Build commit: d6ba54d913266c13b4ddf2eaf8b1b9b023df4caa (d6ba54d913266c13b4ddf2eaf8b1b9b023df4caa)

[rust-timer]

Finished benchmarking commit (d6ba54d913266c13b4ddf2eaf8b1b9b023df4caa): comparison URL.

Overall result: ❌ regressions - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf +perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.2%, 1.2%]	76
Regressions ❌ (secondary)	1.8%	[0.2%, 6.6%]	30
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	0.5%	[0.2%, 1.2%]	76

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.1%	[0.4%, 2.4%]	6
Regressions ❌ (secondary)	1.5%	[1.5%, 1.5%]	1
Improvements ✅ (primary)	-4.8%	[-4.8%, -4.8%]	1
Improvements ✅ (secondary)	-3.7%	[-5.3%, -2.1%]	2
All ❌✅ (primary)	0.3%	[-4.8%, 2.4%]	7

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.8%, 1.0%]	6
Regressions ❌ (secondary)	2.5%	[1.3%, 3.7%]	8
Improvements ✅ (primary)	-1.4%	[-1.9%, -0.9%]	9
Improvements ✅ (secondary)	-2.8%	[-5.2%, -2.1%]	13
All ❌✅ (primary)	-0.5%	[-1.9%, 1.0%]	15

[scottmcm]

Curious -- arguably good on cycles, but clearly bad on instructions.

I guess I should do this in smaller pieces, if at all.