Remove redundant information in `rustc_abi::Variants` by moulins · Pull Request #151742 · rust-lang/rust

moulins · 2026-01-27T14:26:28Z

Follow-up to #151040; partially addresses #113988.

Replaces the nested LayoutData in Variants::Multiple by a new, smaller VariantLayout struct, and adjust LayoutData::for_variantand the layout algorithm in consequence.
This PR is best reviewed commit-by-commit.

rustbot · 2026-01-27T14:26:32Z

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

rustbot · 2026-01-27T14:26:35Z

r? @wesleywiser

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moulins · 2026-01-27T14:28:30Z

+#[derive(PartialEq, Eq, Hash, Clone, Debug)]
+#[cfg_attr(feature = "nightly", derive(HashStable_Generic))]
+pub struct VariantLayout<FieldIdx: Idx> {
+    pub size: Size,


Can't be removed, as it is used by the variant_size_differences lint.

moulins · 2026-01-27T14:31:15Z

+#[cfg_attr(feature = "nightly", derive(HashStable_Generic))]
+pub struct VariantLayout<FieldIdx: Idx> {
+    pub size: Size,
+    pub backend_repr: BackendRepr,


I don't know enough about codegen to confidently say if it requires accurate reprs for enum variants, so I've left this field for now.

Enum variants should not need their own BackendRepr, but it is probably a good idea to leave this cleanup for a future PR.

moulins · 2026-01-27T14:33:53Z

+    pub size: Size,
+    pub backend_repr: BackendRepr,
+    pub field_offsets: IndexVec<FieldIdx, Size>,
+    fields_in_memory_order: IndexVec<u32, FieldIdx>,


Note: this field could be removed and recomputed from the field offsets in Layout::from_variant; it's unclear whether this is worth it.

moulins · 2026-01-27T14:35:01Z

+                // Remove discriminant values of the other variants from the largest niche. This assumes
+                // that the largest niche, when it exists, always corresponds to the enum discriminant.


This assumption was already implicitly made by the original code.

moulins · 2026-01-27T14:38:24Z

                let min = valid_range.start.min(valid_range.end);
                let min = tag.size(cx).truncate(min);

                let max = valid_range.start.max(valid_range.end);
                let max = tag.size(cx).truncate(max);


I think this logic is broken for valid ranges wrapping around uN::MAX? In any case, fixing this is out-of-scope of the PR, so I've left it as-is.

Maybe leave a FIXME comment?

Kobzol · 2026-01-28T08:03:09Z

@bors try @rust-timer queue

Remove redundant information in `rustc_abi::Variants`

rust-bors · 2026-01-28T10:17:01Z

☀️ Try build successful (CI)
Build commit: 968e542 (968e5428d360caed1ceef0036386d8f8959a91bf, parent: e96bb7e44fbcc23c1e6009e8d0ee8ab208668fb4)

rust-timer · 2026-01-28T10:58:24Z

Finished benchmarking commit (968e542): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in 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)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.3%, -0.0%]	6
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 1.4%, secondary -2.8%)

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

	mean	range	count
Regressions ❌ (primary)	3.8%	[1.6%, 5.7%]	4
Regressions ❌ (secondary)	1.9%	[0.5%, 3.4%]	3
Improvements ✅ (primary)	-1.7%	[-2.0%, -1.4%]	3
Improvements ✅ (secondary)	-4.7%	[-5.6%, -2.4%]	7
All ❌✅ (primary)	1.4%	[-2.0%, 5.7%]	7

Cycles

Results (secondary 3.7%)

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)	4.9%	[3.9%, 5.5%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-	-	0

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)	0.0%	[0.0%, 0.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	1

Bootstrap: 470.891s -> 473.88s (0.63%)
Artifact size: 383.44 MiB -> 383.70 MiB (0.07%)

apiraino · 2026-03-26T12:34:01Z

r? compiler

cc @makai410 since you approved #151040?

makai410 · 2026-03-26T13:36:06Z

cc @workingjubilee IIRC you were involved in the related discussion on discord?

chenyukang · 2026-03-26T13:47:37Z

@rustbot reroll

jieyouxu · 2026-03-27T10:24:08Z

I'm finding another compiler reviewer for this PR.

moulins · 2026-04-29T16:15:25Z

I can't reproduce this on my machine (linux, x86) with ./x --stage 2 test src/tools/rust-analyzer, so I don't have a clue to what's going on here :(

saethlin · 2026-05-04T00:27:29Z

The job that failed at least the last time above sets -Zrandomize-layout:

rust/src/ci/docker/host-x86_64/x86_64-gnu-llvm-21/Dockerfile

Line 54 in ad3a598

--set rust.randomize-layout=true \

In general when trying to reproduce a CI failure, it is good to run the CI scripts. Yes they can be very slow because they don't reuse your normal build artifacts, but they pull in all the configuration. You should be able to run CI locally, the docs for how to do that are here right now: https://rustc-dev-guide.rust-lang.org/tests/ci.html#docker

JonathanBrouwer · 2026-05-14T09:52:16Z

FYI the ci error was spurious, so feel free to retry when the open comment is resolved

For now, this is a 1-to-1 copy of `LayoutData`, but this will change.

It was always set to `Variants::Single`.

Enum variants always have `Arbitrary` layout, so the enum isn't needed.

This field is only used during layout calculations, so re-synthetized `LayoutData`s for enum variants can use a dummy value instead.

Reusing the alignment of the enclosing enum in `LayoutData::for_variant` doesn't appear to cause any issues.

rustbot · 2026-05-15T02:37:15Z

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.

saethlin · 2026-05-17T02:35:19Z

@bors r+

rust-bors · 2026-05-17T02:35:21Z

📌 Commit 75015f4 has been approved by saethlin

It is now in the queue for this repository.

Remove redundant information in `rustc_abi::Variants` Follow-up to rust-lang#151040; partially addresses rust-lang#113988. Replaces the nested `LayoutData` in `Variants::Multiple` by a new, smaller `VariantLayout` struct, and adjust `LayoutData::for_variant`and the layout algorithm in consequence. This PR is best reviewed commit-by-commit.

JonathanBrouwer · 2026-05-17T12:25:37Z

@bors try jobs=aarch64-apple

rust-bors · 2026-05-17T12:25:43Z

⌛ Trying commit 75015f4 with merge b6014dc…

To cancel the try build, run the command @bors try cancel.

Workflow: https://github.com/rust-lang/rust/actions/runs/25990793137

Remove redundant information in `rustc_abi::Variants` try-job: aarch64-apple

JonathanBrouwer · 2026-05-17T13:51:13Z

@bors try cancel

rust-bors · 2026-05-17T13:51:16Z

Try build cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/25990793137

Remove redundant information in `rustc_abi::Variants` Follow-up to rust-lang#151040; partially addresses rust-lang#113988. Replaces the nested `LayoutData` in `Variants::Multiple` by a new, smaller `VariantLayout` struct, and adjust `LayoutData::for_variant`and the layout algorithm in consequence. This PR is best reviewed commit-by-commit.

…uwer Rollup of 14 pull requests Successful merges: - #151742 (Remove redundant information in `rustc_abi::Variants`) - #151362 (Add interior-mutability suggestion to `static_mut_refs`) - #156121 (compiler: suggest `.collect()` when `String` is expected and `Iterator` is found) - #156208 (Emit retags in codegen to support BorrowSanitizer (part 1)) - #156596 (Split `LintExpectationId`s) - #156607 (ci: Update FreeBSD version to FreeBSD 14) - #156376 (suggest hex escapes for C-style escapes) - #156577 (Test EII UI tests with prefer-dynamic) - #156585 (explicit tail calls: ignore some tests on unsupported LLVM targets) - #156598 (Avoid rustfix suggestions for macro-expanded unused imports) - #156616 (rustdoc: add test case for `-Drustdoc::` and `--cap-lints`) - #156633 (Add regression test for issue #41261) - #156635 (rename unexpected_try_recover function) - #156636 (minor `rustc_mir_transform` cleanup)

rustbot added the A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. label Jan 27, 2026

rustbot assigned wesleywiser Jan 27, 2026

moulins commented Jan 27, 2026

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 28, 2026

rust-bors Bot pushed a commit that referenced this pull request Jan 28, 2026

Auto merge of #151742 - moulins:variant-layout, r=<try>

968e542

Remove redundant information in `rustc_abi::Variants`

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 28, 2026

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the8472 reviewed May 11, 2026

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Comment thread compiler/rustc_abi/src/layout/simple.rs

moulins force-pushed the variant-layout branch from 93db272 to aa122ce Compare May 15, 2026 02:07

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moulins added 5 commits May 15, 2026 04:34

Introduce VariantLayout struct

464dc07

For now, this is a 1-to-1 copy of `LayoutData`, but this will change.

Remove redundant VariantLayout.variants field

1162bdc

It was always set to `Variants::Single`.

Store FieldsShape::Arbitrary fields directly in VariantLayout

0569574

Enum variants always have `Arbitrary` layout, so the enum isn't needed.

Remove unused VariantLayout.randomization_seed field

3459964

This field is only used during layout calculations, so re-synthetized `LayoutData`s for enum variants can use a dummy value instead.

Remove per-variant alignment in VariantLayout

75015f4

Reusing the alignment of the enclosing enum in `LayoutData::for_variant` doesn't appear to cause any issues.

moulins force-pushed the variant-layout branch from aa122ce to 75015f4 Compare May 15, 2026 02:37

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 17, 2026

JonathanBrouwer mentioned this pull request May 17, 2026

Rollup of 11 pull requests #156658

Closed

rust-bors Bot pushed a commit that referenced this pull request May 17, 2026

Auto merge of #151742 - moulins:variant-layout, r=<try>

b6014dc

Remove redundant information in `rustc_abi::Variants` try-job: aarch64-apple

JonathanBrouwer mentioned this pull request May 17, 2026

Rollup of 14 pull requests #156670

Open

		// Remove discriminant values of the other variants from the largest niche. This assumes
		// that the largest niche, when it exists, always corresponds to the enum discriminant.

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Overall result: ✅ improvements - no action needed

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