Remap syntax contexts and expansion ids for det. metadata encoding by aerooneqq · Pull Request #163668 · rust-lang/rust

aerooneqq · 2026-10-02T15:49:21Z

#163495 which parallelizes AST -> HIR lowering breaks det. encoding of syntax contexts and expansions, however this PR with #162809 should fix the problem.

Now draft for perf and CI (dont look at the code). Remapping of those entities can cost us much, because unlike def ids there may be many expansions and contexts after we committed end of determinism.

r? @petrochenkov

petrochenkov · 2026-10-02T15:50:24Z

@bors try @rust-timer queue

…remapping, r=<try> Remap syntax contexts and expansion ids for det. metadata encoding

petrochenkov · 2026-10-02T15:59:46Z

@bors try cancel

rust-bors · 2026-10-02T15:59:49Z

Try build cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/37029803876

Hint: if you want to run another try build, you do not need to manually cancel the previous one. Just run @bors try and bors will cancel the previous build automatically.

petrochenkov · 2026-10-02T15:59:56Z

@bors try @rust-timer queue

…remapping, r=<try> Remap syntax contexts and expansion ids for det. metadata encoding

rust-bors · 2026-10-02T16:09:17Z

💔 Test for 19b2816 failed: CI. Failed jobs:

try - dist-aarch64-linux-quick (web logs, enhanced plaintext logs)
try - dist-x86_64-linux-quick (web logs, enhanced plaintext logs)

aerooneqq · 2026-10-05T14:37:35Z

@bors try @rust-timer queue

…remapping, r=<try> Remap syntax contexts and expansion ids for det. metadata encoding

rust-bors · 2026-10-05T15:37:35Z

☀️ Try build successful (CI)
Build commit: a7c6732 (a7c67326654d086b506b6f55d82af110f520b88b)
Base parent: 602727f (602727f26878dcf9eb2999e2f58e5df1269ff47e)

rust-timer · 2026-10-05T16:22:44Z

Finished benchmarking commit (a7c6732): 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)	0.7%	[0.3%, 1.2%]	7
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-1.2%, -0.1%]	57
Improvements ✅ (secondary)	-0.3%	[-0.7%, -0.1%]	28
All ❌✅ (primary)	-0.1%	[-1.2%, 1.2%]	64

Max RSS (memory usage)

Results (primary -2.9%, secondary -1.7%)

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

	mean	range	count
Regressions ❌ (primary)	3.1%	[3.1%, 3.1%]	1
Regressions ❌ (secondary)	2.6%	[1.6%, 6.6%]	9
Improvements ✅ (primary)	-3.0%	[-8.3%, -0.9%]	112
Improvements ✅ (secondary)	-3.1%	[-5.4%, -1.2%]	29
All ❌✅ (primary)	-2.9%	[-8.3%, 3.1%]	113

Cycles

Results (primary 11.8%, secondary 2.4%)

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

	mean	range	count
Regressions ❌ (primary)	11.8%	[2.3%, 27.3%]	15
Regressions ❌ (secondary)	3.8%	[3.0%, 4.5%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.4%	[-4.4%, -4.4%]	1
All ❌✅ (primary)	11.8%	[2.3%, 27.3%]	15

Binary size

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

Bootstrap: 490.152s -> 495.841s (1.16%)
Artifact size: 408.63 MiB -> 408.61 MiB (-0.00%)

aerooneqq · 2026-10-06T13:06:51Z

@bors try @rust-timer queue

…remapping, r=<try> Remap syntax contexts and expansion ids for det. metadata encoding

rust-bors · 2026-10-06T14:07:35Z

☀️ Try build successful (CI)
Build commit: 7bc5025 (7bc50256f104e1317a31182442df8ccc6f17b131)
Base parent: b57eb9a (b57eb9a5fd94f30697f51fc88fbe898e67c7aae6)

rust-timer · 2026-10-06T14:51:17Z

Finished benchmarking commit (7bc5025): 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
Regressions ❌ (secondary)	0.5%	[0.3%, 0.6%]	2
Improvements ✅ (primary)	-0.2%	[-0.4%, -0.2%]	32
Improvements ✅ (secondary)	-0.3%	[-0.7%, -0.1%]	28
All ❌✅ (primary)	-0.2%	[-0.4%, -0.2%]	32

Max RSS (memory usage)

Results (primary -1.1%, secondary -1.3%)

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)	3.3%	[2.7%, 3.8%]	3
Improvements ✅ (primary)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-3.3%	[-5.4%, -0.9%]	7
All ❌✅ (primary)	-1.1%	[-1.1%, -1.1%]	1

Cycles

Results (primary 2.3%, secondary -6.4%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.4%	[-14.6%, -3.8%]	8
All ❌✅ (primary)	2.3%	[2.3%, 2.3%]	1

Binary size

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

Bootstrap: 489.693s -> 489.628s (-0.01%)
Artifact size: 409.33 MiB -> 408.61 MiB (-0.18%)

aerooneqq · 2026-10-07T11:37:34Z

@bors try @rust-timer queue

…remapping, r=<try> Remap syntax contexts and expansion ids for det. metadata encoding

rust-bors · 2026-10-07T12:42:54Z

☀️ Try build successful (CI)
Build commit: 37acb80 (37acb80f7852dea4ca8afa0f9791fd8019455cfa)
Base parent: 8d1a764 (8d1a76430406c877b35d0b627e7f796dcf0dfeca)

rust-timer · 2026-10-07T15:23:42Z

Finished benchmarking commit (37acb80): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.4%, -0.1%]	28
Improvements ✅ (secondary)	-0.3%	[-0.7%, -0.1%]	23
All ❌✅ (primary)	-0.2%	[-0.4%, -0.1%]	28

Max RSS (memory usage)

Results (primary -0.6%, 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)	1.7%	[1.1%, 2.2%]	2
Regressions ❌ (secondary)	3.4%	[2.9%, 3.8%]	2
Improvements ✅ (primary)	-2.8%	[-4.4%, -1.2%]	2
Improvements ✅ (secondary)	-5.3%	[-5.3%, -5.3%]	2
All ❌✅ (primary)	-0.6%	[-4.4%, 2.2%]	4

Cycles

Results (secondary 0.8%)

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)	3.3%	[3.2%, 3.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.2%	[-4.2%, -4.2%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 489.893s -> 490.336s (0.09%)
Artifact size: 408.60 MiB -> 408.69 MiB (0.02%)

aerooneqq · 2026-10-08T10:03:57Z

+    }
+}
+
+pub struct SyntaxContextRemapper<'a>(&'a mut HygieneData);


This is done for the following reasons:

HygieneData is private and as I understood it should not be exposed outside of the crate

There are a lot of common things in remapping of syntax contexts and local expansion ids, they differ only in small details like hash calculation, those common things are placed in default functions of HygieneEntityRemapper trait

Those structs contain static functions to access remappers for syntax contexts and local expansion ids, as HygieneData is not accessible

Also we take the lock for the whole time of remapping calculation, so we do not acquire lock on every hash operation for example, it is ensured by &'a mut HygieneData (it can be a problem if we do it in parallel, meaning calculation of syntax contexts and local expansion remapping), however that is the problem with design of access to HygieneData as it always takes mutable lock, but after certain stage in compilation it should behave like a FreezeLock, as there should be only readonly accesses

I am not that sure about the design, so If you don't like it I can rework it somehow, but it seems to satisfy the conditions/thoughts above.

@rustbot ready

View changes since the review

rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Oct 2, 2026

rustbot assigned petrochenkov Oct 2, 2026

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Oct 2, 2026