Fix non-deterministic encoding of syntax contexts

[aerooneqq]

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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.

[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%)

[rust-bors]

☀️ Try build successful (CI)
Build commit: 75e938b (75e938be4f6790138aa93ac7dcb1baf800c9fc3b)
Base parent: b722b55 (b722b5567eab6912db411110a2dcd7995b2d72cf)

[rust-timer]

Finished benchmarking commit (75e938b): comparison URL.

Overall result: ❌✅ regressions and improvements - 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)	0.1%	[0.1%, 0.1%]	1
Regressions ❌ (secondary)	0.4%	[0.3%, 0.5%]	4
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	1
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.2%]	4
All ❌✅ (primary)	-0.0%	[-0.2%, 0.1%]	2

Max RSS (memory usage)

Results (primary -1.2%, secondary -1.0%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.9%	[3.0%, 4.9%]	3
Improvements ✅ (primary)	-1.2%	[-1.8%, -0.6%]	2
Improvements ✅ (secondary)	-3.9%	[-8.1%, -1.6%]	5
All ❌✅ (primary)	-1.2%	[-1.8%, -0.6%]	2

Cycles

Results (primary -1.2%)

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

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.6%, 0.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.1%	[-3.1%, -3.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.2%	[-3.1%, 0.6%]	2

Binary size

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

Bootstrap: 498.957s -> 493.438s (-1.11%)
Artifact size: 406.94 MiB -> 406.96 MiB (0.00%)

[aerooneqq]

@rustbot ready

[petrochenkov]

r=me with the nits addressed.
@bors delegate+
@rustbot author

[rust-bors]

✌️ @aerooneqq, you can now approve this pull request!

If @petrochenkov told you to "r=me" after making some further change, then please make that change and post @bors r=petrochenkov.

View changes since this delegation.

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[aerooneqq]

@bors r=petrochenkov

[rust-bors]

📌 Commit 2e35677 has been approved by petrochenkov

It is now in the queue for this repository.

[JonathanBrouwer]

@bors p=6 scheduling

[rust-bors]

☀️ Test successful - CI
Approved by: petrochenkov
Duration: 3h 9m 26s
Pushing 2b9a7be to main...

[github-actions]

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 215a8af (parent) -> 2b9a7be (this PR)

Test differences

Show 22 test diffs

22 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 2b9a7be8551a6497a952d87d7dbf38484e5bfa5a --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

1. test-x86_64-gnu-gcc-core-tests: 9m 19s -> 17m 48s (+91.1%)
2. test-x86_64-gnu-llvm-21-3: 1h 26m -> 2h 3m (+42.8%)
3. test-x86_64-gnu-llvm-21-1: 44m 26s -> 1h 2m (+40.9%)
4. dist-powerpc64le-linux-musl: 1h 9m -> 1h 36m (+38.3%)
5. test-x86_64-gnu-nopt: 1h 48m -> 2h 30m (+38.1%)
6. test-x86_64-gnu-stdlib-semver-check: 11m 18s -> 15m 26s (+36.6%)
7. test-x86_64-gnu-llvm-21-2: 1h 16m -> 1h 43m (+35.8%)
8. test-various: 1h 36m -> 2h 10m (+35.8%)
9. test-pr-check-1: 30m 33s -> 40m 46s (+33.4%)
10. dist-riscv64-linux-gnu: 1h 32m -> 1h 2m (-32.5%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

[rust-timer]

Finished benchmarking commit (2b9a7be): comparison URL.

Overall result: ❌ regressions - please read:

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

• If the regression was expected or you think it can be justified,
  please write a comment with sufficient written justification, and add
  @rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
• If you think that you know of a way to resolve the regression, try to create
  a new PR with a fix for the regression.
• If you do not understand the regression or you think that it is just noise,
  you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
  were already notified of this PR).

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

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)	0.5%	[0.5%, 0.5%]	1
Regressions ❌ (secondary)	0.5%	[0.3%, 0.6%]	8
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[0.5%, 0.5%]	1

Max RSS (memory usage)

Results (primary -1.5%, secondary -2.1%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.5%	[2.1%, 5.5%]	3
Improvements ✅ (primary)	-1.5%	[-2.7%, -0.7%]	6
Improvements ✅ (secondary)	-5.5%	[-8.3%, -2.1%]	5
All ❌✅ (primary)	-1.5%	[-2.7%, -0.7%]	6

Cycles

Results (primary -3.0%, secondary 5.9%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	5.9%	[5.9%, 5.9%]	1
Improvements ✅ (primary)	-3.0%	[-3.0%, -3.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.0%	[-3.0%, -3.0%]	1

Binary size

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

Bootstrap: 498.703s -> 496.542s (-0.43%)
Artifact size: 406.94 MiB -> 406.87 MiB (-0.02%)