repr(transparent): do not consider repr(C) types to be 1-ZST #147185

RalfJung · 2025-09-30T11:45:09Z

Context: rust-lang/unsafe-code-guidelines#552

This experiments with a suggestion by @RustyYato to stop considering repr(C) types as 1-ZST for the purpose of repr(transparent). If we go with rust-lang/rfcs#3845 (or another approach for fixing repr(C)), they will anyway not be ZST on all targets any more, so this removes a portability hazard. Furthermore, zero-sized repr(C) structs may have to be treated as non-ZST for the win64 ABI (at least that's what gcc/clang do), so allowing them to be ignored in repr(transparent) types is not entirely coherent.

Turns out we already have an FCW for repr(transparent), namely #78586. This extends that lint to also check for repr(C).

rustbot · 2025-09-30T11:45:15Z

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RalfJung · 2025-09-30T11:48:11Z

@bors try

Repr c not zst

rust-bors · 2025-09-30T14:05:20Z

☀️ Try build successful (CI)
Build commit: 0de49ae (0de49ae9e81f9a1e7df6f0783824ce94ed18e8a9, parent: a2db9280539229a3b8a084a09886670a57bc7e9c)

petrochenkov · 2025-09-30T14:27:15Z

@craterbot check

craterbot · 2025-09-30T14:27:36Z

👌 Experiment pr-147185 created and queued.
🤖 Automatically detected try build 0de49ae
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-10-02T07:58:22Z

🚧 Experiment pr-147185 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-10-03T23:11:35Z

🎉 Experiment pr-147185 is completed!
📊 8 regressed and 3 fixed (708741 total)
📊 1547 spurious results on the retry-regessed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-147185/retry-regressed-list.txt ↩

RalfJung · 2025-10-05T11:53:37Z

This found

a few cases of abi_stable-0.9.3, an outdated crate version where the lint probably has been fixed in more recent versions (and I highly doubt there's any repr(C) here, this is going to be about the non-exhaustive part of the lint)
dynamic_graph-0.1.5, a crate that didn't have any updates in 4 years, here, involving a use of generativity::Id. I don't actually understand why this happens as the Id type is repr(transparent) itself... but nothing here is repr(C) so this is also almost certainly about the non-exhaustive part of the lint, not the repr(C) part.

EDIT: It later got confirmed that these crates indeed already trigger the lint before this PR.

RalfJung · 2025-10-05T14:22:36Z

@bors try

repr(transparent): do not consider repr(C) types to be 1-ZST

rust-bors · 2025-10-05T16:37:31Z

☀️ Try build successful (CI)
Build commit: e19b5cc (e19b5cc0f024358aaaecf2776a5291fa5a95b623, parent: e2c96cc06bdbdbc6f59c7551194d6a742260d6ff)

RalfJung · 2025-10-05T16:52:00Z

@craterbot check crates=https://crater-reports.s3.amazonaws.com/pr-147185/retry-regressed-list.txt p=1

craterbot · 2025-10-05T16:52:04Z

👌 Experiment pr-147185-1 created and queued.
🤖 Automatically detected try build e19b5cc
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-10-06T21:52:57Z

🚧 Experiment pr-147185-1 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-10-07T03:21:38Z

🎉 Experiment pr-147185-1 is completed!
📊 6 regressed and 0 fixed (1534 total)
📊 90 spurious results on the retry-regessed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-147185-1/retry-regressed-list.txt ↩

RalfJung · 2025-10-26T09:06:24Z

@petrochenkov @rustbot ready

compiler/rustc_hir_analysis/src/check/check.rs

petrochenkov · 2025-10-27T16:23:18Z

r=me with the outdated comment updated.
@rustbot author

RalfJung · 2025-10-27T18:25:35Z

Thanks!

@bors r=petrochenkov

bors · 2025-10-27T18:25:39Z

📌 Commit b9b29c4 has been approved by petrochenkov

It is now in the queue for this repository.

…enkov repr(transparent): do not consider repr(C) types to be 1-ZST Context: rust-lang/unsafe-code-guidelines#552 This experiments with a [suggestion](rust-lang/rfcs#3845 (comment)) by `@RustyYato` to stop considering repr(C) types as 1-ZST for the purpose of repr(transparent). If we go with rust-lang/rfcs#3845 (or another approach for fixing repr(C)), they will anyway not be ZST on all targets any more, so this removes a portability hazard. Furthermore, zero-sized repr(C) structs [may have to be treated](rust-lang/unsafe-code-guidelines#552 (comment)) as non-ZST for the win64 ABI (at least that's what gcc/clang do), so allowing them to be ignored in repr(transparent) types is not entirely coherent. Turns out we already have an FCW for repr(transparent), namely rust-lang#78586. This extends that lint to also check for repr(C).

…enkov repr(transparent): do not consider repr(C) types to be 1-ZST Context: rust-lang/unsafe-code-guidelines#552 This experiments with a [suggestion](rust-lang/rfcs#3845 (comment)) by ``@RustyYato`` to stop considering repr(C) types as 1-ZST for the purpose of repr(transparent). If we go with rust-lang/rfcs#3845 (or another approach for fixing repr(C)), they will anyway not be ZST on all targets any more, so this removes a portability hazard. Furthermore, zero-sized repr(C) structs [may have to be treated](rust-lang/unsafe-code-guidelines#552 (comment)) as non-ZST for the win64 ABI (at least that's what gcc/clang do), so allowing them to be ignored in repr(transparent) types is not entirely coherent. Turns out we already have an FCW for repr(transparent), namely rust-lang#78586. This extends that lint to also check for repr(C).

Rollup of 14 pull requests Successful merges: - #144936 (CFI: Fix types that implement Fn, FnMut, or FnOnce) - #147185 (repr(transparent): do not consider repr(C) types to be 1-ZST) - #147840 (Rework unsizing coercions in the new solver) - #147915 (Update target maintainers android.md) - #148013 (1.91.0 release notes) - #148044 (compiletest: show output in debug logging) - #148057 (tests/ui/sanitizer/hwaddress.rs: Run on aarch64 and remove cgu hack) - #148139 (Add `coverage` scope for controlling paths in code coverage) - #148154 (Add a mailmap entry) - #148158 (ci: loongarch64: use medium code model to avoid relocation overflows) - #148166 (Re-enable macro-stepping test for AArch64) - #148172 (rustc-dev-guide subtree update) - #148175 (Fix typos: duplicate words in comments) - #148186 (rustdoc-search: add an integration test for CCI) Failed merges: - #147935 (Add LLVM realtime sanitizer) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #147185 - RalfJung:repr-c-not-zst, r=petrochenkov repr(transparent): do not consider repr(C) types to be 1-ZST Context: rust-lang/unsafe-code-guidelines#552 This experiments with a [suggestion](rust-lang/rfcs#3845 (comment)) by ```@RustyYato``` to stop considering repr(C) types as 1-ZST for the purpose of repr(transparent). If we go with rust-lang/rfcs#3845 (or another approach for fixing repr(C)), they will anyway not be ZST on all targets any more, so this removes a portability hazard. Furthermore, zero-sized repr(C) structs [may have to be treated](rust-lang/unsafe-code-guidelines#552 (comment)) as non-ZST for the win64 ABI (at least that's what gcc/clang do), so allowing them to be ignored in repr(transparent) types is not entirely coherent. Turns out we already have an FCW for repr(transparent), namely #78586. This extends that lint to also check for repr(C).

Zalathar · 2025-10-28T23:52:24Z

This appears to be the cause of a few perf regressions: #148202 (comment)

RalfJung · 2025-10-29T07:23:23Z

Are you sure? This code only affects the repr(transparent) check, which shouldn't run that often -- and it only affects it in fairly trivial ways, too; def.repr().c() is a trivial flag check.

We do run that iterator multiple times, but that was already the case before this PR.

perf experiment for #147185

RalfJung · 2025-10-29T19:11:56Z

That regression is caused by the lint rename, which triggers the "lint has been renamed" lint in syn, and it takes some work to render those nice diagnostics we have.

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Auto merge of #147185 - RalfJung:repr-c-not-zst, r=<try>

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Repr c not zst

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RalfJung changed the title ~~Repr c not zst~~ repr(transparent): do not consider repr(C) types to be 1-ZST Sep 30, 2025

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Auto merge of #147185 - RalfJung:repr-c-not-zst, r=<try>

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Tracking issue for release notes of #147185: repr(transparent): do not consider repr(C) types to be 1-ZST #148119

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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 Oct 27, 2025

repr(transparent): do not consider repr(C) types to be 1-ZST

b9b29c4

RalfJung force-pushed the repr-c-not-zst branch from 091a9d5 to b9b29c4 Compare October 27, 2025 18:25

bors 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 Oct 27, 2025

Zalathar mentioned this pull request Oct 28, 2025

Rollup of 13 pull requests #148195

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Zalathar mentioned this pull request Oct 28, 2025

Rollup of 14 pull requests #148202

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bors merged commit ef8003b into rust-lang:master Oct 28, 2025
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RalfJung deleted the repr-c-not-zst branch October 29, 2025 07:32

RalfJung mentioned this pull request Oct 29, 2025

perf experiment for #147185 #148243

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Auto merge of #148243 - RalfJung:perf-transparent-checks, r=<try>

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perf experiment for #147185

rust-bors bot added a commit that referenced this pull request Oct 29, 2025

Auto merge of #148243 - RalfJung:perf-transparent-checks, r=<try>

9fcd3c8

perf experiment for #147185

rust-bors bot added a commit that referenced this pull request Oct 29, 2025

Auto merge of #148243 - RalfJung:perf-transparent-checks, r=<try>

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perf experiment for #147185

repr(transparent): do not consider repr(C) types to be 1-ZST #147185

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