repr(transparent): do not consider repr(C) types to be 1-ZST by RalfJung · Pull Request #147185 · rust-lang/rust

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

r? @petrochenkov

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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a few cases of abi_stable-0.9.3, an outdated crate version where the lint probably has been fixed in more recent versions (and I highly doubt there's any repr(C) here, this is going to be about the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 ↩

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.

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

This MR contains the following updates: | Package | Update | Change | |---|---|---| | [rust](https://github.com/rust-lang/rust) | minor | `1.92.0` → `1.93.0` | MR created with the help of [el-capitano/tools/renovate-bot](https://gitlab.com/el-capitano/tools/renovate-bot). **Proposed changes to behavior should be submitted there as MRs.** --- ### Release Notes <details> <summary>rust-lang/rust (rust)</summary> ### [`v1.93.0`](https://github.com/rust-lang/rust/blob/HEAD/RELEASES.md#Version-1930-2026-01-22) [Compare Source](rust-lang/rust@1.92.0...1.93.0) \========================== <a id="1.93.0-Language"></a> ## Language - [Stabilize several s390x `vector`-related target features and the `is_s390x_feature_detected!` macro](rust-lang/rust#145656) - [Stabilize declaration of C-style variadic functions for the `system` ABI](rust-lang/rust#145954) - [Emit error when using some keyword as a `cfg` predicate](rust-lang/rust#146978) - [Stabilize `asm_cfg`](rust-lang/rust#147736) - [During const-evaluation, support copying pointers byte-by-byte](rust-lang/rust#148259) - [LUB coercions now correctly handle function item types, and functions with differing safeties](rust-lang/rust#148602) - [Allow `const` items that contain mutable references to `static` (which is *very* unsafe, but not *always* UB)](rust-lang/rust#148746) - [Add warn-by-default `const_item_interior_mutations` lint to warn against calls which mutate interior mutable `const` items](rust-lang/rust#148407) - [Add warn-by-default `function_casts_as_integer` lint](rust-lang/rust#141470) <a id="1.93.0-Compiler"></a> ## Compiler - [Stabilize `-Cjump-tables=bool`](rust-lang/rust#145974). 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See [the linked issue](rust-lang/rust#150646 (comment)) for details. - [User written types in the `offset_of!` macro are now checked to be well formed.](rust-lang/rust#150465) - `cargo publish` no longer emits `.crate` files as a final artifact for user access when the `build.build-dir` config is unset - [Upgrade the `deref_nullptr` lint from warn-by-default to deny-by-default](rust-lang/rust#148122) - [Add future-incompatibility warning for `...` function parameters without a pattern outside of `extern` blocks](rust-lang/rust#143619) - [Introduce future-compatibility warning for `repr(C)` enums whose discriminant values do not fit into a `c_int` or `c_uint`](rust-lang/rust#147017) - [Introduce future-compatibility warning against ignoring `repr(C)` types as part of `repr(transparent)`](rust-lang/rust#147185) </details> --- ### Configuration 📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined). 🚦 **Automerge**: Disabled by config. 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Pkgsrc changes: * Update version & checksums. * Adapt openssl-src patches to minor version update. Noteable failures at the time of commit: * The cross-build for sparc64 fails, not yet reported. Upstream changes relative to 1.92.0: Version 1.93 (2026-01-22) ========================== Language -------- - [Add warn-by-default `function_casts_as_integer` lint] (rust-lang/rust#141470) - [Add future-incompatibility warning for `...` function parameters without a pattern outside of `extern` blocks] (rust-lang/rust#143619) - [Stabilize several s390x `vector`-related target features and the `is_s390x_feature_detected!` macro] (rust-lang/rust#145656) - [Stabilize declaration of C-style variadic functions for the `system` ABI] (rust-lang/rust#145954) - [Emit error when using some keyword as a `cfg` predicate] (rust-lang/rust#146978) - [Introduce future-compatibility warning for `repr(C)` enums whose discriminant values do not fit into a `c_int` or `c_uint`] (rust-lang/rust#147017) - [Introduce future-compatibility warning against ignoring `repr(C)` types as part of `repr(transparent)`] (rust-lang/rust#147185) - [Stabilize `asm_cfg`] (rust-lang/rust#147736) - [Upgrade the `deref_nullptr` lint from warn-by-default to deny-by-default] (rust-lang/rust#148122) - [During const-evaluation, support copying pointers byte-by-byte] (rust-lang/rust#148259) - [Add warn-by-default `const_item_interior_mutations` lint to warn against calls which mutate interior mutable `const` items] (rust-lang/rust#148407) - [LUB coercions now correctly handle function item types, and functions with differing safeties] (rust-lang/rust#148602) - [Allow `const` items that contain mutable references to `static` (which is *very* unsafe, but not *always* UB)] (rust-lang/rust#148746) Compiler -------- - [Stabilize `-Cjump-tables=bool`] (rust-lang/rust#145974). The flag was previously called `-Zno-jump-tables`. - [Promote `riscv64a23-unknown-linux-gnu` to Tier 2 (without host tools)] (rust-lang/rust#148435) Platform Support ---------------- Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. [platform-support-doc]: https://doc.rust-lang.org/rustc/platform-support.html Libraries --------- - [Stop internally using `specialization` on the `Copy` trait as it is unsound in the presence of lifetime dependent `Copy` implementations. This may result in some performance regressions as some standard library APIs may now call `Clone::clone` instead of performing bitwise copies] (rust-lang/rust#135634) - [Allow the global allocator to use thread-local storage and `std::thread::current()`] (rust-lang/rust#144465) - [Make `BTree::append` not update existing keys when appending an entry which already exists] (rust-lang/rust#145628) - [Don't require `T: RefUnwindSafe` for `vec::IntoIter<T>: UnwindSafe`] (rust-lang/rust#145665) Stabilized APIs --------------- - [`<MaybeUninit<T>>::assume_init_drop`] (https://doc.rust-lang.org/stable/core/mem/union.MaybeUninit.html#method.assume_init_drop) - [`<MaybeUninit<T>>::assume_init_ref`] (https://doc.rust-lang.org/stable/core/mem/union.MaybeUninit.html#method.assume_init_ref) - [`<MaybeUninit<T>>::assume_init_mut`] (https://doc.rust-lang.org/stable/core/mem/union.MaybeUninit.html#method.assume_init_mut) - [`<[MaybeUninit<T>]>::write_copy_of_slice`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.write_copy_of_slice) - [`<[MaybeUninit<T>]>::write_clone_of_slice`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.write_clone_of_slice) - [`String::into_raw_parts`] (https://doc.rust-lang.org/stable/std/string/struct.String.html#method.into_raw_parts) - [`Vec::into_raw_parts`] (https://doc.rust-lang.org/stable/std/vec/struct.Vec.html#method.into_raw_parts) - [`<iN>::unchecked_neg`] (https://doc.rust-lang.org/stable/std/primitive.isize.html#method.unchecked_neg) - [`<iN>::unchecked_shl`] (https://doc.rust-lang.org/stable/std/primitive.isize.html#method.unchecked_shl) - [`<iN>::unchecked_shr`] (https://doc.rust-lang.org/stable/std/primitive.isize.html#method.unchecked_shr) - [`<uN>::unchecked_shl`] (https://doc.rust-lang.org/stable/std/primitive.usize.html#method.unchecked_shl) - [`<uN>::unchecked_shr`] (https://doc.rust-lang.org/stable/std/primitive.usize.html#method.unchecked_shr) - [`<[T]>::as_array`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.as_array) - [`<[T]>::as_array_mut`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.as_mut_array) - [`<*const [T]>::as_array`] (https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.as_array) - [`<*mut [T]>::as_array_mut`] (https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.as_mut_array) - [`VecDeque::pop_front_if`] (https://doc.rust-lang.org/stable/std/collections/struct.VecDeque.html#method.pop_front_if) - [`VecDeque::pop_back_if`] (https://doc.rust-lang.org/stable/std/collections/struct.VecDeque.html#method.pop_back_if) - [`Duration::from_nanos_u128`] (https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.from_nanos_u128) - [`char::MAX_LEN_UTF8`] (https://doc.rust-lang.org/stable/std/primitive.char.html#associatedconstant.MAX_LEN_UTF8) - [`char::MAX_LEN_UTF16`] (https://doc.rust-lang.org/stable/std/primitive.char.html#associatedconstant.MAX_LEN_UTF16) - [`std::fmt::from_fn`] (https://doc.rust-lang.org/stable/std/fmt/fn.from_fn.html) - [`std::fmt::FromFn`] (https://doc.rust-lang.org/stable/std/fmt/struct.FromFn.html) Cargo ----- - [Enable CARGO_CFG_DEBUG_ASSERTIONS in build scripts based on profile] (rust-lang/cargo#16160) - [In `cargo tree`, support long forms for `--format` variables] (rust-lang/cargo#16204) - [Add `--workspace` to `cargo clean`] (rust-lang/cargo#16263) Rustdoc ----- - [Remove `#![doc(document_private_items)]`](rust-lang/rust#146495) - [Include attribute and derive macros in search filters for "macros"](rust-lang/rust#148176) - [Include extern crates in search filters for `import`](rust-lang/rust#148301) - [Validate usage of crate-level doc attributes](rust-lang/rust#149197). This means if any of `html_favicon_url`, `html_logo_url`, `html_playground_url`, `issue_tracker_base_url`, or `html_no_source` either has a missing value, an unexpected value, or a value of the wrong type, rustdoc will emit the deny-by-default lint `rustdoc::invalid_doc_attributes`. Compatibility Notes ------------------- - [Introduce `pin_v2` into the builtin attributes namespace] (rust-lang/rust#139751) - [Update bundled musl to 1.2.5] (rust-lang/rust#142682) - [On Emscripten, the unwinding ABI used when compiling with `panic=unwind` was changed from the JS exception handling ABI to the wasm exception handling ABI.] (rust-lang/rust#147224) If linking C/C++ object files with Rust objects, `-fwasm-exceptions` must be passed to the linker now. On nightly Rust, it is possible to get the old behavior with `-Zwasm-emscripten-eh=false -Zbuild-std`, but it will be removed in a future release. - The `#[test]` attribute, used to define tests, was previously ignored in various places where it had no meaning (e.g on trait methods or types). Putting the `#[test]` attribute in these places is no longer ignored, and will now result in an error; this may also result in errors when generating rustdoc. [Error when `test` attribute is applied to structs] (rust-lang/rust#147841) - Cargo now sets the `CARGO_CFG_DEBUG_ASSERTIONS` environment variable in more situations. This will cause crates depending on `static-init` versions 1.0.1 to 1.0.3 to fail compilation with "failed to resolve: use of unresolved module or unlinked crate `parking_lot`". See [the linked issue] (rust-lang/rust#150646 (comment)) for details. - [User written types in the `offset_of!` macro are now checked to be well formed.] (rust-lang/rust#150465) - `cargo publish` no longer emits `.crate` files as a final artifact for user access when the `build.build-dir` config is unset

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

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

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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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perf experiment for #147185 #148243

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perf experiment for #147185

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perf experiment for #147185

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

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perf experiment for #147185

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Draft release notes for 1.93 #150760

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