Use unchecked_sub in str indexing

[saethlin]

#108763 applied this logic to indexing for slices, but of course str has its own separate impl.

Found this by skimming over the codegen for https://github.com/oxidecomputer/hubris/; their dist builds enable overflow checks so the lack of unchecked_sub was producing an impossible-to-hit overflow check and also inhibiting some inlining.

r? scottmcm

[workingjubilee]

Does this get different codegen if you just use the public stdlib unchecked_sub?

[scottmcm]

It gets a boatload more MIR, or at least it will once #121571 lands.

[scottmcm]

(We really need a "look, nobody cares about the parameter debug info from this method" for things like that and i32:PartialOrd and …)

[workingjubilee]

Ah okay! Cool then.

[saethlin]

No, but it doubles up on the precondition check.

[scottmcm]

Ah, good catch!
@bors r+ rollup=iffy (codegen tests aren't tested in CI, AFAIK)

[bors]

📌 Commit 712aab7 has been approved by scottmcm

It is now in the queue for this repository.

[bors]

⌛ Testing commit 712aab7 with merge 4e431fa...

[bors]

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing 4e431fa to master...

[rust-timer]

Finished benchmarking commit (4e431fa): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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)	0.9%	[0.9%, 0.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.9%, -0.4%]	7
All ❌✅ (primary)	0.9%	[0.9%, 0.9%]	1

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)	-	-	0
Improvements ✅ (primary)	-1.3%	[-1.3%, -1.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.3%	[-1.3%, -1.3%]	1

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
Regressions ❌ (secondary)	2.9%	[2.9%, 2.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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)	0.5%	[0.1%, 0.9%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.4%, -0.0%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.4%, 0.9%]	9

Bootstrap: 667.899s -> 667.449s (-0.07%)
Artifact size: 318.21 MiB -> 318.47 MiB (0.08%)

[pnkfelix]

• this is an improvement to the code for str::get_unchecked when overflow checks are enabled; its calling a compiler-intrinsic directly now.
• it really doesn't make any sense that it caused any regression at all. (Perhaps this change is causing a change to inlining decisions, at least for cargo?)
• marking as triaged.

@rustbot label: +perf-regression-triaged