Stop emitting UbChecks on every Vec→Slice

[scottmcm]

Spotted this in #148766's test changes. It doesn't seem like this ubcheck would catch anything useful; let's see if skipping it helps perf. (After all, this is inside every [] on a vec, among other things.)

[scottmcm]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: d3405d7 (d3405d79c1b6f5617f04715f0b349c565c6a743d, parent: 000ccd651d6dfeab13f7703d92a5fd7a9ff7510f)

[rust-timer]

Finished benchmarking commit (d3405d7): 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)	1.6%	[0.6%, 2.4%]	4
Regressions ❌ (secondary)	0.4%	[0.0%, 2.3%]	7
Improvements ✅ (primary)	-1.4%	[-6.0%, -0.1%]	28
Improvements ✅ (secondary)	-0.7%	[-2.2%, -0.1%]	7
All ❌✅ (primary)	-1.0%	[-6.0%, 2.4%]	32

Max RSS (memory usage)

Results (primary 1.5%, secondary 1.9%)

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

	mean	range	count
Regressions ❌ (primary)	3.8%	[1.3%, 6.7%]	6
Regressions ❌ (secondary)	3.3%	[0.9%, 6.1%]	14
Improvements ✅ (primary)	-5.4%	[-7.2%, -3.5%]	2
Improvements ✅ (secondary)	-1.9%	[-2.6%, -1.5%]	5
All ❌✅ (primary)	1.5%	[-7.2%, 6.7%]	8

Cycles

Results (primary -1.0%, secondary -1.9%)

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

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.3%, 2.8%]	2
Regressions ❌ (secondary)	2.3%	[2.1%, 2.8%]	3
Improvements ✅ (primary)	-2.4%	[-2.7%, -2.3%]	5
Improvements ✅ (secondary)	-4.0%	[-6.5%, -2.1%]	6
All ❌✅ (primary)	-1.0%	[-2.7%, 2.8%]	7

Binary size

Results (primary -0.2%, secondary -0.5%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 1.2%]	11
Regressions ❌ (secondary)	0.6%	[0.0%, 1.2%]	2
Improvements ✅ (primary)	-0.3%	[-0.9%, -0.0%]	64
Improvements ✅ (secondary)	-0.6%	[-3.6%, -0.0%]	20
All ❌✅ (primary)	-0.2%	[-0.9%, 1.2%]	75

Bootstrap: 481.34s -> 483.129s (0.37%)
Artifact size: 390.37 MiB -> 390.63 MiB (0.07%)