[libs] Simplify unchecked_{shl,shr}

[scottmcm]

There's no need for the const_eval_select dance here. And while I originally wrote the .try_into().unwrap_unchecked() implementation here, it's kinda a mess in MIR -- this new one is substantially simpler, as shown by the old one being above the inlining threshold but the new one being below it in the mir-opt/inline/unchecked_shifts tests.

We don't need u32::checked_shl doing a dance through both Result and Option 🙃

[rustbot]

r? @Mark-Simulacrum

(rustbot has picked a reviewer for you, use r? to override)

[Mark-Simulacrum]

r=me unless we want to rerun perf, but probably not needed.

[scottmcm]

The unchecked shift methods are unstable and look like they're never used in the compiler, so it seems highly unlikely that this would materially affect perf -- TBH I'm more interested in the library code cleanup (less const_eval_select) than the MIR changes.

@bors r=Mark-Simulacrum

[bors]

📌 Commit 3ec4eed has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3ec4eed with merge 8d1fa47...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 8d1fa47 to master...

[rust-timer]

Finished benchmarking commit (8d1fa47): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.7%	[-0.7%, -0.6%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.7%	[-0.7%, -0.6%]	2

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)	-	-	0
Regressions ❌ (secondary)	3.7%	[3.7%, 3.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.7%	[-2.7%, -2.7%]	1
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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)	0.0%	[0.0%, 0.0%]	4
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	4

Bootstrap: 656.978s -> 653.833s (-0.48%)