Fix non-deterministic encoding of syntax contexts

[aerooneqq]

Reiteration of #157409. Explanation can be found in comment for encoding_indices and in the issue.

Fixes #129094. Blocked by #161353.

r? @petrochenkov

No llms used.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[petrochenkov]

Note: here and below get_encoding_index takes the encoding_indices in every iteration of the loop, it could be more efficient to take the lock once.

View changes since the review

[petrochenkov]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 7e01adf (7e01adfdd3209974b2d32d2eb35d4173deaa540b)
Base parent: 4fcf397 (4fcf39725a9c99bd495d8c73af83628a256ff9a9)

[rust-timer]

Finished benchmarking commit (7e01adf): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@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)	0.6%	[0.1%, 1.9%]	72
Regressions ❌ (secondary)	0.6%	[0.1%, 1.7%]	50
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.1%, 1.9%]	72

Max RSS (memory usage)

Results (primary 1.0%, secondary -1.6%)

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

	mean	range	count
Regressions ❌ (primary)	1.0%	[1.0%, 1.0%]	1
Regressions ❌ (secondary)	3.7%	[2.6%, 4.9%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-4.6%, -1.4%]	5
All ❌✅ (primary)	1.0%	[1.0%, 1.0%]	1

Cycles

Results (primary -0.2%, secondary 4.5%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	4.5%	[4.5%, 4.5%]	1
Improvements ✅ (primary)	-2.7%	[-2.7%, -2.7%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.2%	[-2.7%, 2.3%]	2

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 476.065s -> 474.462s (-0.34%)
Artifact size: 401.18 MiB -> 401.32 MiB (0.03%)