Avoid a UB check in Layout::for_value

[scottmcm]

Trying another thought for what might have been the thing that helped in #152867

[scottmcm]

@bors try @rust-timer queue

[rust-bors]

💥 Test timed out after 21600s

[scottmcm]

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 6942fca (6942fcac84b6b1ed24f9154bdc6d7ff76a8ec2ac, parent: f2b291d902bfde7d7f209fc3a64908134bcef201)

[rust-timer]

Finished benchmarking commit (6942fca): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.9%	[-1.8%, -0.2%]	6
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.6%, secondary -2.7%)

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

	mean	range	count
Regressions ❌ (primary)	3.6%	[1.5%, 6.5%]	5
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.7%	[-2.7%, -2.7%]	1
Improvements ✅ (secondary)	-2.7%	[-3.1%, -2.2%]	4
All ❌✅ (primary)	2.6%	[-2.7%, 6.5%]	6

Cycles

Results (primary 2.3%, secondary 27.2%)

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

	mean	range	count
Regressions ❌ (primary)	4.4%	[2.3%, 8.7%]	5
Regressions ❌ (secondary)	27.2%	[17.6%, 37.1%]	6
Improvements ✅ (primary)	-2.7%	[-3.5%, -2.0%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.3%	[-3.5%, 8.7%]	7

Binary size

Results (primary 0.1%, secondary -0.6%)

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

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.1%, 0.6%]	7
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.5%, -0.1%]	9
Improvements ✅ (secondary)	-0.6%	[-1.4%, -0.1%]	14
All ❌✅ (primary)	0.1%	[-0.5%, 0.6%]	16

Bootstrap: 497.331s -> 498.466s (0.23%)
Artifact size: 395.05 MiB -> 397.19 MiB (0.54%)