Fix non-deterministic encoding of syntax contexts by aerooneqq · Pull Request #161450 · rust-lang/rust

aerooneqq · 2026-08-21T10:20:19Z

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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 · 2026-09-03T12:23:21Z

@bors try @rust-timer queue

…ing, r=<try> Fix non-deterministic encoding of syntax contexts

rust-bors · 2026-09-03T13:23:29Z

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

rust-timer · 2026-09-03T14:14:37Z

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%)

…ing, r=<try> Fix non-deterministic encoding of syntax contexts

rust-bors · 2026-09-13T13:12:58Z

☀️ Try build successful (CI)
Build commit: 39a7792 (39a77924d69cd79ad293cefa28ac22d76ff9fa69)
Base parent: 98fd715 (98fd715edd3a0a5aa8f2041d7e20a3143b0fc0d3)

rust-timer · 2026-09-13T13:57:27Z

Finished benchmarking commit (39a7792): 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)	0.3%	[0.1%, 0.5%]	29
Regressions ❌ (secondary)	0.3%	[0.1%, 0.5%]	18
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.1%, 0.5%]	29

Max RSS (memory usage)

Results (primary -0.6%, secondary -0.6%)

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%	[3.0%, 4.2%]	3
Improvements ✅ (primary)	-0.6%	[-0.7%, -0.5%]	2
Improvements ✅ (secondary)	-4.7%	[-8.2%, -2.7%]	3
All ❌✅ (primary)	-0.6%	[-0.7%, -0.5%]	2

Cycles

Results (secondary -5.1%)

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)	3.1%	[3.1%, 3.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-9.2%	[-15.3%, -3.1%]	2
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.0%, secondary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.1%]	26
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.1%]	26

Bootstrap: 492.1s -> 494.686s (0.53%)
Artifact size: 406.87 MiB -> 406.94 MiB (0.02%)

petrochenkov · 2026-09-14T09:39:57Z

@bors try @rust-timer queue

…ing, r=<try> Fix non-deterministic encoding of syntax contexts

rust-bors · 2026-09-14T10:39:24Z

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

rust-timer · 2026-09-14T11:48:55Z

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 · 2026-09-15T11:47:22Z

@rustbot ready

petrochenkov · 2026-09-15T13:05:46Z

+
+impl Default for HygieneEncodeContext {
+    fn default() -> HygieneEncodeContext {
+        Self {


Suggested change

Self {

HygieneEncodeContext {

petrochenkov · 2026-09-15T13:05:55Z

+            serialized_expns: Default::default(),
+            latest_expns: Default::default(),
+            // Zero is taken by root syntax context.
+            encoding_indices: FxHashMap::from_iter(std::iter::once((SyntaxContext::root(), 0))),


Suggested change

encoding_indices: FxHashMap::from_iter(std::iter::once((SyntaxContext::root(), 0))),

encoding_indices: FxHashMap::from_iter(iter::once((SyntaxContext::root(), 0))),

petrochenkov · 2026-09-15T13:11:52Z

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

rust-bors · 2026-09-15T13:11:55Z

✌️ @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 · 2026-09-15T13:11:56Z

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

rustbot added A-run-make Area: port run-make Makefiles to rmake.rs S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 21, 2026

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rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Aug 21, 2026

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aerooneqq force-pushed the syntax-ctxt-non-deterministic-encoding branch from b07d315 to 3f750ec Compare August 21, 2026 10:33

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aerooneqq mentioned this pull request Aug 25, 2026

Output deterministic AllocIds when emitting MIR #161768

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petrochenkov mentioned this pull request Sep 1, 2026

rustc: Support --reproducible option for requesting reproducible output #160448

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aerooneqq force-pushed the syntax-ctxt-non-deterministic-encoding branch from a34d24f to 3868148 Compare September 2, 2026 08:27

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Promoting Parallel Front End rust-lang/goals#121

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Auto merge of #161450 - aerooneqq:syntax-ctxt-non-deterministic-encod…

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…ing, r=<try> Fix non-deterministic encoding of syntax contexts

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39a7792

…ing, r=<try> Fix non-deterministic encoding of syntax contexts

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…ing, r=<try> Fix non-deterministic encoding of syntax contexts

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aerooneqq force-pushed the syntax-ctxt-non-deterministic-encoding branch from 658483a to e6d2012 Compare September 15, 2026 11:45

Fix non-deterministic encoding of syntax contexts

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	encoding_indices: FxHashMap::from_iter(std::iter::once((SyntaxContext::root(), 0))),
	encoding_indices: FxHashMap::from_iter(iter::once((SyntaxContext::root(), 0))),

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rust-timer commented Sep 3, 2026

Overall result: ❌ regressions - please read:

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rust-timer commented Sep 13, 2026

Overall result: ❌ regressions - please read:

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Overall result: ❌✅ regressions and improvements - please read:

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