rustc_abi: Add `LayoutData::is_variant_uninhabited` method - #160398

moulins · 2026-08-02T19:18:36Z

This is a cheaper alternative to TyAndLayout::for_variant(_, idx).is_uninhabited(), which avoids the extra work done by TyAndLayout::for_variant.

EDIT: Let me expand on the motivation from this comment:

TyAndLayout::for_variant is a curious API: the resulting layout doesn't represent a Rust type in the strict sense (not even pattern_type!(Enum is Variant { .. }); instead, it only describes the "variant-specific" fields, and fields shared between variants (e.g. the enum tag or coroutine captures) are entirely missing.

As such, I believe alternative APIs should be used instead to clarify intent, when possible. This PR deals with the simplest cases, and a future PR will add TyAndLayout::variant_field(self, cx, variant_idx, field_idx) to deal with the next-most common case of only needing a variant layout for field lookups.

rustbot · 2026-08-02T19:18:39Z

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

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

cc @bjorn3

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

rustbot · 2026-08-02T19:18:41Z

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hanna-kruppe · 2026-08-02T20:07:34Z

@bors try @rust-timer queue

rustc_abi: Add `LayoutData::is_variant_uninhabited` method

rust-bors · 2026-08-02T22:13:12Z

☀️ Try build successful (CI)
Build commit: 4e9517e (4e9517ed78b554a781024590a80c4637001a0a3d)
Base parent: 28c66af (28c66af27578a28bc8cad1c5957be4b1e2e7fb8b)

rust-timer · 2026-08-02T23:05:28Z

Finished benchmarking commit (4e9517e): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (primary 0.4%, secondary 0.5%)

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

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.4%, 0.4%]	1
Regressions ❌ (secondary)	0.5%	[0.4%, 0.6%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[0.4%, 0.4%]	1

Cycles

Results (primary -0.2%, secondary -0.4%)

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

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.5%, 1.0%]	3
Regressions ❌ (secondary)	0.9%	[0.5%, 1.5%]	6
Improvements ✅ (primary)	-0.6%	[-1.0%, -0.5%]	6
Improvements ✅ (secondary)	-1.6%	[-4.8%, -0.4%]	6
All ❌✅ (primary)	-0.2%	[-1.0%, 1.0%]	9

Binary size

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

Bootstrap: 490.376s -> 489.668s (-0.14%)
Artifact size: 390.46 MiB -> 390.37 MiB (-0.02%)

mu001999 · 2026-08-03T05:52:05Z

@rustbot reroll

oli-obk · 2026-08-03T13:44:32Z

Considering there's no measurable impact, is it worth having two ways to do the same thing?

moulins · 2026-08-03T14:35:58Z

Well, I won't bury the lede: I find the concept of TyAndLayout::for_variant deeply suspect (it doesn't even cleanly represent the layout of a potential pattern_type!(foo is Enum::Variant { .. })) and I think it should be used as little as possible.

I'm also investigating providing a fn variant_field(self, cx, variant_idx, field_idx) method to lookup a variant-specific field directly, which should cover most of the other use-cases of TyAndLayout::for_variant.

oli-obk · 2026-08-04T10:19:01Z

good motivation :D Please add a doc comment to for_variant and update the main PR description with that motivation

r=me with that

This is a cheaper alternative to `TyAndLayout::for_variant(_, idx).is_uninhabited()`

rustbot · 2026-08-05T13:39:36Z

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.

moulins · 2026-08-05T15:33:17Z

@rustbot review

oli-obk

@bors r+ rollup

View changes since this review

rust-bors · 2026-08-06T06:42:41Z

📌 Commit 187cf5d has been approved by oli-obk

It is now in the queue for this repository.

…ted, r=oli-obk rustc_abi: Add `LayoutData::is_variant_uninhabited` method This is a cheaper alternative to `TyAndLayout::for_variant(_, idx).is_uninhabited()`, which avoids the extra work done by `TyAndLayout::for_variant`. EDIT: Let me expand on the motivation from this [comment](rust-lang#160398 (comment)): `TyAndLayout::for_variant` is a curious API: the resulting layout doesn't represent a Rust type in the strict sense (not even `pattern_type!(Enum is Variant { .. })`; instead, it only describes the "variant-specific" fields, and fields shared between variants (e.g. the enum tag or coroutine captures) are entirely missing. As such, I believe alternative APIs should be used instead to clarify intent, when possible. This PR deals with the simplest cases, and a future PR will add `TyAndLayout::variant_field(self, cx, variant_idx, field_idx)` to deal with the next-most common case of only needing a variant layout for field lookups.

…uwer Rollup of 17 pull requests Successful merges: - #159530 (Cap socket send length to c_int::MAX on Apple targets) - #159506 (Allow associated const equality constraints with GCA) - #160006 (Account for desugaring in method call move errors) - #160415 (Split `aarch64-apple{,-macos-26}` => `aarch64-apple{,-macos-26}-{1,2}` jobs) - #160464 (fix: Check the fallback map before queueing child in `visible_parent_map` breadth-first search) - #160555 (Split `apply_primary_terminator_effect`) - #160592 (Suggest if-let chain continuation on unclosed delimiter) - #160600 (Avoid the std DLL copy alongside rustc) - #160156 (check_consts: exhaustively match on CastKind) - #160211 (Rename `#[unroll]` => `#[rustc_unroll]` to mitigate nameres ambiguity) - #160304 (Add tests for fixed new solver issues) - #160398 (rustc_abi: Add `LayoutData::is_variant_uninhabited` method) - #160546 (Update error message in documentation comments) - #160568 (Use `VisitorResult` helper macros) - #160571 (Add regression test for array type recovery in generic arguments) - #160588 (add a test showing polonius alpha is not a subset of datalog polonius) - #160617 (Add a suggestion to MissingUnsafeOnExtern diagnostic)

Rollup merge of #160398 - moulins:layout-is-variant-uninhabited, r=oli-obk rustc_abi: Add `LayoutData::is_variant_uninhabited` method This is a cheaper alternative to `TyAndLayout::for_variant(_, idx).is_uninhabited()`, which avoids the extra work done by `TyAndLayout::for_variant`. EDIT: Let me expand on the motivation from this [comment](#160398 (comment)): `TyAndLayout::for_variant` is a curious API: the resulting layout doesn't represent a Rust type in the strict sense (not even `pattern_type!(Enum is Variant { .. })`; instead, it only describes the "variant-specific" fields, and fields shared between variants (e.g. the enum tag or coroutine captures) are entirely missing. As such, I believe alternative APIs should be used instead to clarify intent, when possible. This PR deals with the simplest cases, and a future PR will add `TyAndLayout::variant_field(self, cx, variant_idx, field_idx)` to deal with the next-most common case of only needing a variant layout for field lookups.

JonathanBrouwer · 2026-08-06T20:11:18Z

@rust-timer build b4a4124

rust-timer · 2026-08-07T07:00:45Z

Finished benchmarking commit (b4a4124): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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
Improvements ✅ (secondary)	-0.4%	[-0.5%, -0.2%]	7
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -6.8%)

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)	-	-	0
Improvements ✅ (primary)	-6.8%	[-6.8%, -6.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-6.8%	[-6.8%, -6.8%]	1

Cycles

Results (primary 2.3%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.0%, 2.5%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.3%	[2.0%, 2.5%]	2

Binary size

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

Bootstrap: 458.35s -> 460.051s (0.37%)
Artifact size: 398.64 MiB -> 398.69 MiB (0.01%)

…i-obk rustc_abi: Add `LayoutData::is_variant_uninhabited` method This is a cheaper alternative to `TyAndLayout::for_variant(_, idx).is_uninhabited()`, which avoids the extra work done by `TyAndLayout::for_variant`. EDIT: Let me expand on the motivation from this [comment](rust-lang/rust#160398 (comment)): `TyAndLayout::for_variant` is a curious API: the resulting layout doesn't represent a Rust type in the strict sense (not even `pattern_type!(Enum is Variant { .. })`; instead, it only describes the "variant-specific" fields, and fields shared between variants (e.g. the enum tag or coroutine captures) are entirely missing. As such, I believe alternative APIs should be used instead to clarify intent, when possible. This PR deals with the simplest cases, and a future PR will add `TyAndLayout::variant_field(self, cx, variant_idx, field_idx)` to deal with the next-most common case of only needing a variant layout for field lookups.

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Aug 2, 2026

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rust-bors Bot pushed a commit that referenced this pull request Aug 2, 2026

Auto merge of #160398 - moulins:layout-is-variant-uninhabited, r=<try>

4e9517e

rustc_abi: Add `LayoutData::is_variant_uninhabited` method

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 2, 2026

rustbot assigned oli-obk and unassigned mu001999 Aug 3, 2026

oli-obk 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 Aug 4, 2026

moulins added 2 commits August 5, 2026 15:38

rustc_abi: Add LayoutData::is_variant_uninhabited method

fee8922

This is a cheaper alternative to `TyAndLayout::for_variant(_, idx).is_uninhabited()`

Add a doc comment to TyAndLayout discouraging its uses when possible

187cf5d

moulins force-pushed the layout-is-variant-uninhabited branch from 01aa949 to 187cf5d Compare August 5, 2026 13:39

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 Aug 5, 2026

oli-obk approved these changes Aug 6, 2026

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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-review Status: Awaiting review from the assignee but also interested parties. labels Aug 6, 2026

JonathanBrouwer mentioned this pull request Aug 6, 2026

Rollup of 13 pull requests #160609

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JonathanBrouwer mentioned this pull request Aug 6, 2026

Rollup of 13 pull requests #160610

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Rollup of 17 pull requests #160615

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Rollup of 16 pull requests #160623

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Rollup of 17 pull requests #160627

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rust-bors Bot merged commit 592096b into rust-lang:main Aug 6, 2026
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