Try to make FieldDef smaller by panstromek · Pull Request #157986 · rust-lang/rust

panstromek · 2026-06-16T19:15:33Z

Tries to mitigate the regression from #156824. We do this by Option Boxing together all the properties that are only used for unstable features.

fixes #157003

panstromek · 2026-06-16T19:15:50Z

@bors try @rust-timer queue

Try to make FieldDef smaller

rust-bors · 2026-06-16T21:36:11Z

☀️ Try build successful (CI)
Build commit: 289b567 (289b567a59d12dd1f7712d6ff110fbd73a56766d, parent: 89a99936d9e76a50e8df622e7242190841fd871b)

rust-timer · 2026-06-16T23:06:43Z

Finished benchmarking commit (289b567): 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
@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%, -0.2%]	10
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.1%]	7
All ❌✅ (primary)	-0.2%	[-0.4%, -0.2%]	10

Max RSS (memory usage)

Results (primary 6.4%, secondary 2.0%)

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

	mean	range	count
Regressions ❌ (primary)	6.4%	[2.4%, 10.6%]	3
Regressions ❌ (secondary)	5.9%	[5.9%, 5.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	6.4%	[2.4%, 10.6%]	3

Cycles

Results (primary 1.4%, secondary 4.9%)

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

	mean	range	count
Regressions ❌ (primary)	1.4%	[1.4%, 1.4%]	1
Regressions ❌ (secondary)	4.9%	[2.8%, 7.6%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.4%	[1.4%, 1.4%]	1

Binary size

Results (primary -0.0%)

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)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	4

Bootstrap: 522.035s -> 522.066s (0.01%)
Artifact size: 401.81 MiB -> 401.82 MiB (0.00%)

panstromek · 2026-07-12T11:54:56Z

I'll undraft this for preliminary review, because I want to get some second opinion. This helps to mitigate the perf regression, but feels somewhat arbitrary/ad-hoc to me? I'm not sure if this is the right approach.

Also, this exploits the fact that these features are unstable, and used rarely - as soon as the majority of fields in the ecosystem use at least one of those features (presumably the default value in most cases), this will no longer be an optimization.

rustbot · 2026-07-12T11:55:04Z

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

rustfmt is developed in its own repository. If possible, consider making this change to rust-lang/rustfmt instead.

cc @rust-lang/rustfmt

rustbot · 2026-07-12T11:55:06Z

r? @khyperia

rustbot has assigned @khyperia.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

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The reviewer was selected based on:

Owners of files modified in this PR: compiler
compiler expanded to 75 candidates
Random selection from 18 candidates

khyperia · 2026-07-13T07:42:16Z

apologies, I've fallen ill and probably won't get around to reviewing this for a while, so I'm rerolling

r? compiler

petrochenkov · 2026-07-13T14:45:41Z

We need to re-benchmark this after #158942, tokens in MutRestriction were clearly unnecessary.
@rustbot author

rustbot · 2026-07-13T14:45:45Z

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

rust-bors · 2026-07-18T23:48:58Z

☀️ Try build successful (CI)
Build commit: aee29e1 (aee29e16223a403e2e07ed5fef9f3772c1c4544a)
Base parent: eff8269 (eff8269f797067c30555e77f160ec84c0ed15cd9)

rust-timer · 2026-07-19T00:51:20Z

Finished benchmarking commit (aee29e1): 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)	-0.3%	[-0.3%, -0.2%]	14
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.3%	[-0.3%, -0.2%]	14

Max RSS (memory usage)

Results (secondary -1.5%)

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)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.3%	[-5.1%, -1.5%]	2
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 1.9%, secondary 2.7%)

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

	mean	range	count
Regressions ❌ (primary)	1.9%	[1.9%, 1.9%]	1
Regressions ❌ (secondary)	2.7%	[2.2%, 3.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.9%	[1.9%, 1.9%]	1

Binary size

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

Bootstrap: 489.126s -> 485.679s (-0.70%)
Artifact size: 389.52 MiB -> 389.46 MiB (-0.02%)

panstromek · 2026-07-19T06:43:11Z

@rustbot ready

petrochenkov · 2026-07-20T14:56:17Z

Some tests are also failing.
@rustbot author

rustbot · 2026-07-22T19:28:04Z

Changes to the size of AST and/or HIR nodes.

cc @nnethercote

rustbot · 2026-07-22T19:43:47Z

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.

panstromek · 2026-07-23T06:49:23Z

@rustbot ready

petrochenkov · 2026-07-23T12:11:17Z

r=me after squashing commits.
@rustbot author

panstromek · 2026-07-23T12:46:35Z

Thanks! I squashed the commits but I don't have rights to r= you. Sorry for the churn.

@rustbot ready

petrochenkov · 2026-07-23T12:57:33Z

@bors r+

rust-bors · 2026-07-23T12:57:36Z

📌 Commit 283ec3a has been approved by petrochenkov

It is now in the queue for this repository.

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 16, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jun 16, 2026

Auto merge of #157986 - panstromek:make-field-def-smaller, r=<try>

289b567

Try to make FieldDef smaller

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panstromek commented Jul 12, 2026

View reviewed changes

Comment thread compiler/rustc_ast/src/ast.rs Outdated

Comment thread compiler/rustc_ast/src/ast.rs Outdated

panstromek marked this pull request as ready for review July 12, 2026 11:55

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 12, 2026

rustbot assigned khyperia Jul 12, 2026

rustbot assigned petrochenkov and unassigned khyperia Jul 13, 2026

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petrochenkov reviewed Jul 20, 2026

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Comment thread compiler/rustc_ast/src/ast.rs Outdated

Comment thread compiler/rustc_ast_lowering/src/item.rs Outdated

Comment thread compiler/rustc_parse/src/parser/item.rs Outdated

Comment thread compiler/rustc_resolve/src/build_reduced_graph.rs Outdated

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 20, 2026

panstromek force-pushed the make-field-def-smaller branch from f94ea58 to ce94ff5 Compare July 22, 2026 19:43

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 23, 2026

Make FieldDef smaller

283ec3a

panstromek force-pushed the make-field-def-smaller branch from ce94ff5 to 283ec3a Compare July 23, 2026 12:44

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rust-timer commented Jun 16, 2026

Overall result: ✅ improvements - no action needed

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Overall result: ✅ improvements - no action needed

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