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-log-analyzer · 2026-06-16T19:56:11Z

The job x86_64-gnu-pre-stabilization failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

---- [ui] tests/ui/stats/input-stats.rs stdout ----
Saved the actual stderr to `/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/stats/input-stats/input-stats.stderr`
diff of stderr:

33 ast-stats AssocItem                320 (NN.N%)             4            80
34 ast-stats - Fn                       160 (NN.N%)             2
35 ast-stats - Type                     160 (NN.N%)             2
- ast-stats FieldDef                 272 (NN.N%)             2           136
37 ast-stats Variant                  208 (NN.N%)             2           104
38 ast-stats Block                    192 (NN.N%)             6            32
+ ast-stats FieldDef                 176 (NN.N%)             2            88
39 ast-stats Stmt                     160 (NN.N%)             5            32
40 ast-stats - Let                       32 (NN.N%)             1
41 ast-stats - Semi                      32 (NN.N%)             1

57 ast-stats - AngleBracketed            40 (NN.N%)             1
58 ast-stats Crate                     40 (NN.N%)             1            40
59 ast-stats ----------------------------------------------------------------
- ast-stats Total                  7_624                   127
+ ast-stats Total                  7_528                   127
61 ast-stats ================================================================
62 hir-stats ================================================================
63 hir-stats HIR STATS: input_stats


The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args stats/input-stats.rs`

error: 1 errors occurred comparing output.
status: exit status: 0
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage1/bin/rustc" "/checkout/tests/ui/stats/input-stats.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage1" "--target=x86_64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/stats/input-stats" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--edition=2015" "-Zinput-stats"
stdout: none
--- stderr -------------------------------
ast-stats ================================================================
ast-stats POST EXPANSION AST STATS: input_stats
ast-stats Name                Accumulated Size         Count     Item Size
ast-stats ----------------------------------------------------------------
ast-stats Item                   1_672 (22.2%)            11           152
ast-stats - Enum                     152 ( 2.0%)             1
ast-stats - ExternCrate              152 ( 2.0%)             1
ast-stats - ForeignMod               152 ( 2.0%)             1
ast-stats - Impl                     152 ( 2.0%)             1
ast-stats - Trait                    152 ( 2.0%)             1
ast-stats - Fn                       304 ( 4.0%)             2
ast-stats - Use                      608 ( 8.1%)             4
ast-stats Ty                       896 (11.9%)            14            64
ast-stats - Ptr                       64 ( 0.9%)             1
ast-stats - Ref                       64 ( 0.9%)             1
ast-stats - ImplicitSelf             128 ( 1.7%)             2
ast-stats - Path                     640 ( 8.5%)            10
ast-stats PathSegment              864 (11.5%)            36            24
ast-stats Expr                     648 ( 8.6%)             9            72
ast-stats - InlineAsm                 72 ( 1.0%)             1
ast-stats - Match                     72 ( 1.0%)             1
ast-stats - Path                      72 ( 1.0%)             1
ast-stats - Struct                    72 ( 1.0%)             1
ast-stats - Lit                      144 ( 1.9%)             2
ast-stats - Block                    216 ( 2.9%)             3
ast-stats Pat                      560 ( 7.4%)             7            80
ast-stats - Struct                    80 ( 1.1%)             1
ast-stats - Wild                      80 ( 1.1%)             1
ast-stats - Ident                    400 ( 5.3%)             5
ast-stats GenericParam             480 ( 6.4%)             5            96
ast-stats GenericBound             352 ( 4.7%)             4            88
ast-stats - Trait                    352 ( 4.7%)             4
ast-stats AssocItem                320 ( 4.3%)             4            80
ast-stats - Fn                       160 ( 2.1%)             2
ast-stats - Type                     160 ( 2.1%)             2
ast-stats Variant                  208 ( 2.8%)             2           104
ast-stats Block                    192 ( 2.6%)             6            32
ast-stats FieldDef                 176 ( 2.3%)             2            88
ast-stats Stmt                     160 ( 2.1%)             5            32
ast-stats - Let                       32 ( 0.4%)             1
ast-stats - Semi                      32 ( 0.4%)             1
ast-stats - Expr                      96 ( 1.3%)             3
ast-stats Param                    160 ( 2.1%)             4            40
ast-stats Attribute                128 ( 1.7%)             4            32
ast-stats - DocComment                32 ( 0.4%)             1
ast-stats - Normal                    96 ( 1.3%)             3
ast-stats InlineAsm                120 ( 1.6%)             1           120
ast-stats FnDecl                   120 ( 1.6%)             5            24
ast-stats Local                     96 ( 1.3%)             1            96
ast-stats Arm                       96 ( 1.3%)             2            48
ast-stats ForeignItem               80 ( 1.1%)             1            80
ast-stats - Fn                        80 ( 1.1%)             1
ast-stats WherePredicate            72 ( 1.0%)             1            72
ast-stats - BoundPredicate            72 ( 1.0%)             1
ast-stats ExprField                 48 ( 0.6%)             1            48
ast-stats GenericArgs               40 ( 0.5%)             1            40
ast-stats - AngleBracketed            40 ( 0.5%)             1
ast-stats Crate                     40 ( 0.5%)             1            40
ast-stats ----------------------------------------------------------------
ast-stats Total                  7_528                   127
ast-stats ================================================================
hir-stats ================================================================
hir-stats HIR STATS: input_stats
hir-stats Name                Accumulated Size         Count     Item Size
hir-stats ----------------------------------------------------------------
hir-stats PathSegment            1_776 (20.7%)            37            48
hir-stats Path                   1_040 (12.1%)            26            40
hir-stats Item                     968 (11.3%)            11            88
hir-stats - Enum                      88 ( 1.0%)             1
hir-stats - ExternCrate               88 ( 1.0%)             1
hir-stats - ForeignMod                88 ( 1.0%)             1
hir-stats - Impl                      88 ( 1.0%)             1
hir-stats - Trait                     88 ( 1.0%)             1
hir-stats - Fn                       176 ( 2.1%)             2
hir-stats - Use                      352 ( 4.1%)             4
hir-stats Expr                     768 ( 9.0%)            12            64
hir-stats - InlineAsm                 64 ( 0.7%)             1
hir-stats - Match                     64 ( 0.7%)             1
hir-stats - Path                      64 ( 0.7%)             1
hir-stats - Struct                    64 ( 0.7%)             1
hir-stats - Lit                      128 ( 1.5%)             2
hir-stats - Block                    384 ( 4.5%)             6
hir-stats Ty                       720 ( 8.4%)            15            48
hir-stats - Ptr                       48 ( 0.6%)             1
hir-stats - Ref                       48 ( 0.6%)             1
hir-stats - Path                     624 ( 7.3%)            13
hir-stats Generics                 560 ( 6.5%)            10            56
hir-stats Pat                      400 ( 4.7%)             5            80
hir-stats - Struct                    80 ( 0.9%)             1
hir-stats - Wild                      80 ( 0.9%)             1
hir-stats - Binding                  240 ( 2.8%)             3
hir-stats GenericParam             400 ( 4.7%)             5            80
hir-stats Block                    288 ( 3.4%)             6            48
hir-stats GenericBound             256 ( 3.0%)             4            64
hir-stats - Trait                    256 ( 3.0%)             4
hir-stats Attribute                160 ( 1.9%)             4            40
hir-stats Variant                  144 ( 1.7%)             2            72
hir-stats GenericArgs              144 ( 1.7%)             3            48
hir-stats FieldDef                 128 ( 1.5%)             2            64
hir-stats FnDecl                   120 ( 1.4%)             3            40
hir-stats Stmt                      96 ( 1.1%)             3            32
hir-stats - Expr                      32 ( 0.4%)             1
hir-stats - Let                       32 ( 0.4%)             1
hir-stats - Semi                      32 ( 0.4%)             1
hir-stats Arm                       80 ( 0.9%)             2            40
hir-stats WherePredicate            72 ( 0.8%)             3            24
hir-stats - BoundPredicate            72 ( 0.8%)             3
hir-stats InlineAsm                 72 ( 0.8%)             1            72
hir-stats Body                      72 ( 0.8%)             3            24
hir-stats Param                     64 ( 0.7%)             2            32
hir-stats Local                     64 ( 0.7%)             1            64
hir-stats GenericArg                64 ( 0.7%)             4            16
hir-stats - Type                      16 ( 0.2%)             1
hir-stats - Lifetime                  48 ( 0.6%)             3
hir-stats ExprField                 40 ( 0.5%)             1            40
hir-stats Mod                       32 ( 0.4%)             1            32
hir-stats Lifetime                  28 ( 0.3%)             1            28
hir-stats TraitItemId                8 ( 0.1%)             2             4
hir-stats ImplItemId                 8 ( 0.1%)             2             4
hir-stats ForeignItemId              4 ( 0.0%)             1             4
hir-stats ----------------------------------------------------------------
hir-stats Total                  8_576                   172
hir-stats ================================================================
------------------------------------------

---- [ui] tests/ui/stats/input-stats.rs stdout end ----

failures:

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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 · 2026-07-12T11:47:14Z

    pub vis: Visibility,
-    pub mut_restriction: MutRestriction,
-    pub safety: Safety,
+    pub extras: Option<Box<(Safety, MutRestriction, Option<AnonConst>)>>,


I tried to put this into a separate struct instead of a tuple, but got a lot of compile errors in derive impls. I'm not sure what's the problem.

View changes since the review

The new structure will need to be registered somewhere in compiler\rustc_ast\src\visit.rs.

panstromek · 2026-07-12T11:48:24Z

+            None => &DEFAULT,
+        }
+    }
+    pub fn default(&self) -> Option<&AnonConst> {


We should maybe use different name, to avoid confusion with Default::default()

View changes since the review

default_val/default_value could work.

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.
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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.

panstromek added 2 commits June 16, 2026 20:22

add size assert for FieldDef

8a628ee

Make FieldDef smaller

d1237c3

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

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panstromek commented Jul 12, 2026

View reviewed changes

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

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 13, 2026

rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jul 13, 2026

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rust-timer commented Jun 16, 2026

Overall result: ✅ improvements - no action needed

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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