rustc_errors: use perfect hashing for character replacements

[GrigorenkoPV]

The correctness of code in #128200 relies on an array being sorted (so that it can be used in binary search later), which is currently enforced with // tidy-alphabetical (and characters being written in \u{XXXX} form), as well as lack of duplicate entries with conflicting keys, which is not currently enforced.

A const assert or a test can be added checking that (implemented in #128465).

But this PR tries to use perfect hashing instead.

The performance implications are unclear. Asymptotically it's faster, but in reality we should just benchmark. Plus if there are no significant performance wins, this entire things is probably not even worse the additional dependencies it brings.

UPD: funnily enough, there's a PR optimizing the binary search implementation (#128254) in the queue right now. So I guess we have to wait until that is merged too before benchmarking this.

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Failed to set assignee to ghost: invalid assignee

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[GrigorenkoPV]

r? @ghost

[rustbot]

Failed to set assignee to ghost: invalid assignee

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[rustbot]

Failed to set assignee to ghost: invalid assignee

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[GrigorenkoPV]

Failed to set assignee to ghost: invalid assignee

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So I guess it only works when it is present in the first version of the OP. Cool.

[workingjubilee]

was about to queue this and noticed that the other PR still hadn't landed...

[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
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cc @davidtwco, @wesleywiser

[GrigorenkoPV]

was about to queue this and noticed that the other PR still hadn't landed...

Now it has! Could you please queue it for benchmarking? I do not think I have enough rights to do it myself, nor do I really remember how it is done.

[estebank]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 4108ac4 with merge 1de00dc...

[bors]

☀️ Try build successful - checks-actions
Build commit: 1de00dc (1de00dcc88616d20eed9528aee78fef6370f7cf0)

[rust-timer]

Finished benchmarking commit (1de00dc): 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-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)	1.0%	[0.2%, 1.8%]	11
Regressions ❌ (secondary)	0.4%	[0.4%, 0.4%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.0%	[0.2%, 1.8%]	11

Max RSS (memory usage)

Results (primary 2.1%)

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)	2.1%	[2.1%, 2.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.1%	[2.1%, 2.1%]	1

Cycles

Results (primary -4.2%, secondary 0.5%)

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)	5.1%	[4.7%, 5.5%]	2
Improvements ✅ (primary)	-4.2%	[-4.2%, -4.2%]	1
Improvements ✅ (secondary)	-2.5%	[-2.8%, -2.3%]	3
All ❌✅ (primary)	-4.2%	[-4.2%, -4.2%]	1

Binary size

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

Bootstrap: 760.99s -> 765.201s (0.55%)
Artifact size: 336.87 MiB -> 336.89 MiB (0.01%)

[GrigorenkoPV]

Well, the regressions are on syn again (#128169 (comment)), and there are no improvements whatsoever. At least on the instruction count. Number of cycles seem to have gotten a bit better overall, but not significantly, so probably not worth all the additional dependencies.