lint on `core::ffi::c_void` as a return type by euclio · Pull Request #156379 · rust-lang/rust

euclio · 2026-05-10T02:24:27Z

This PR introduces a new ~~deny-by-default~~ warn-by-default lint c_void_returns that fires on usage of core::ffi::c_void as a return type. This is never correct, and is a potential stumbling block for users coming from C.

rustbot · 2026-05-10T02:24:33Z

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Kivooeo · 2026-05-10T05:59:12Z

why is deny by default? what about things like bindgen that generates bindgings from c code?

asquared31415 · 2026-05-10T15:25:18Z

You're never meant to have a value of type c_void, it's only meant to be used in raw pointers, as an equivalent to void*. () is the equivalent to returning void.

in fact I expect that UB arises from the internal implementation details if you have a void meow() and call it as fn meow() -> c_void because the rust c_void type actually is not zero sized for Internal Reasons.

Kivooeo · 2026-05-10T23:45:27Z

I'd suggest warn rather than deny by default for a few reasons:

Using c_void as a return type in extern blocks is extremely common pattern in existing FFI code and bindgen output
Even outside extern blocks, there are plenty cases show real-world code that would break, and while the pattern is incorrect, making it a hard error seems like a high bar for ecosystem disruption
warn would still surface the issue without being a breaking change. In a future edition it could be escalated to deny, but only for functions outside extern blocks, the cascading wrapper case shows that blindly denying it everywhere creates unnecessary churn in FFI-heavy codebases

asquared31415 · 2026-05-11T14:07:13Z

I believe that every single one of those cases that are defining a -> c_void { ... } function are relying on internal details and so it's not currently UB, but could be in the future.

All the cases that are declaring a -> c_void; function are currently UB (and will almost certainly always remain UB) to link to a -> () (or C -> void), and on top of that, this UB cannot be detected in most of those cases, because miri won't support the typical use cases. I believe that a deny by default lint here is more than warranted.

Additionally, the c_void docs explicitly say "this is not the same as C’s void return type, which is Rust’s () type."

here is an example of a case where miri can detect the UB, but this is rarely possible.

Kivooeo · 2026-05-11T20:28:41Z

As I noted earlier, I agree this pattern is incorrect. My concern is primarily about ecosystem impact - a GitHub search returns over 1,000 instances of this pattern, including in production codebases from organizations like Microsoft and Azure.

Making this deny by default would break these codebases on the next compiler update with no warning period. warn by default would still surface the issue while giving the ecosystem time to adapt.

rustbot · 2026-05-13T15:08:21Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

joshtriplett · 2026-05-13T15:14:07Z

Looks reasonable. We'd like to start with warn at first, and ramp up to deny after a couple of releases.

@rfcbot merge lang

rust-rfcbot · 2026-05-13T15:14:09Z

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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traviscross · 2026-05-13T15:14:59Z

Thanks @euclio.

@rfcbot reviewed

tmandry · 2026-05-13T15:16:23Z

@rfcbot reviewed

rust-rfcbot · 2026-05-13T15:16:49Z

🔔 This is now entering its final comment period, as per the review above. 🔔

rust-rfcbot · 2026-05-23T15:19:05Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

theemathas · 2026-06-28T15:17:52Z

@rustbot reroll

mati865

Thanks, looks good but needs a rebase.

View changes since this review

rustbot · 2026-06-30T02:29:37Z

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.

euclio · 2026-06-30T12:41:50Z

@mati865 Rebased.

mati865 · 2026-06-30T12:58:07Z

@bors r+

rust-bors · 2026-06-30T12:58:10Z

📌 Commit b268dee has been approved by mati865

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 1. This pull request will be tested once the tree is reopened.

lint on `core::ffi::c_void` as a return type Fixes rust-lang#100972. This PR introduces a new ~deny-by-default~ warn-by-default lint `c_void_returns` that fires on usage of `core::ffi::c_void` as a return type. This is never correct, and is a potential stumbling block for users coming from C.

…uwer Rollup of 3 pull requests Successful merges: - #156379 (lint on `core::ffi::c_void` as a return type) - #157347 (Implement `Box::as_non_null()`.) - #158614 (Fix error message when rejecting implicit stage != 2 in CI)

…uwer Rollup of 7 pull requests Successful merges: - #156379 (lint on `core::ffi::c_void` as a return type) - #157347 (Implement `Box::as_non_null()`.) - #157650 (rustc_target: Add OpenEmbedded/Yocto Linux base targets) - #158569 ([rustdoc] Fix handling of inlining of `no_inline` of foreign items) - #158573 (stabilize `feature(atomic_from_mut)`) - #158614 (Fix error message when rejecting implicit stage != 2 in CI) - #158616 (Remove dependency from `rustc_metadata` on `rustc_incremental`)

Rollup merge of #156379 - euclio:c-void-returns, r=mati865 lint on `core::ffi::c_void` as a return type Fixes #100972. This PR introduces a new ~deny-by-default~ warn-by-default lint `c_void_returns` that fires on usage of `core::ffi::c_void` as a return type. This is never correct, and is a potential stumbling block for users coming from C.

…uwer Rollup of 7 pull requests Successful merges: - rust-lang/rust#156379 (lint on `core::ffi::c_void` as a return type) - rust-lang/rust#157347 (Implement `Box::as_non_null()`.) - rust-lang/rust#157650 (rustc_target: Add OpenEmbedded/Yocto Linux base targets) - rust-lang/rust#158569 ([rustdoc] Fix handling of inlining of `no_inline` of foreign items) - rust-lang/rust#158573 (stabilize `feature(atomic_from_mut)`) - rust-lang/rust#158614 (Fix error message when rejecting implicit stage != 2 in CI) - rust-lang/rust#158616 (Remove dependency from `rustc_metadata` on `rustc_incremental`)

This MR contains the following updates: | Package | Type | Update | Change | |---|---|---|---| | [rust](https://github.com/rust-lang/rust) | tools | minor | `1.97.1` → `1.98.0` | 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> ### [`v1.98.0`](https://github.com/rust-lang/rust/blob/HEAD/RELEASES.md#Version-1980-2026-08-20) [Compare Source](rust-lang/rust@1.97.1...1.98.0) \========================== <a id="1.98.0-Language"></a> ## Language - [Allow shortening lifetime of `&mut` when unsize-coercing, even in an invariant position.](rust-lang/rust#149219) For example, you can now coerce a `Cell<&'long mut i32>` to a `Cell<&'short mut dyn Send>`. Such shortenings were already previously allowed when coercing a `&mut` to a `&`, or coercing a `&` to a `&`. - [Add deny-by-default `invalid_runtime_symbol_definitions` lint and warn-by-default `suspicious_runtime_symbol_definitions` lint](rust-lang/rust#155521) - The lints currently specifically targets `core` runtime symbols like `memcmp`, `memset`, `strlen`, ... and is planned to be expanded in the next few releases. - [Add warn-by-default `c_void_returns` lint to check `core::ffi::c_void` as a return type](rust-lang/rust#156379) <a id="1.98.0-Platform-Support"></a> ## Platform Support - [Add `powerpc64-unknown-linux-gnuelfv2` as Tier 3](rust-lang/rust#144220) - [Add `aarch64-unknown-linux-pauthtest` as Tier 3 target](rust-lang/rust#155722) - [Promote `thumbv7a-none-eabi` to Tier 2](rust-lang/rust#155763) - [Promote `thumbv7a-none-eabihf` to Tier 2](rust-lang/rust#155763) - [Promote `thumbv7r-none-eabi` to Tier 2](rust-lang/rust#155763) - [Promote `thumbv7r-none-eabihf` to Tier 2](rust-lang/rust#155763) - [Promote `thumbv8r-none-eabihf` to Tier 2](rust-lang/rust#155763) 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 <a id="1.98.0-Libraries"></a> ## Libraries - [Change `Location<'_>` lifetime to `'static` in `Panic[Hook]Info`](rust-lang/rust#146561) - [Document panic in `RangeInclusive::from(legacy::RangeInclusive)`](rust-lang/rust#155421) - [Document that `ManuallyDrop`'s `Box` interaction has been fixed](rust-lang/rust#155750) - [Stabilize LoongArch CRC Intrinsics](rust-lang/rust#156908) - [The `derive` macro is available at `{core,std}::derive`.](rust-lang/rust#154645) This was previously [unintentionally stabilized in 1.96](rust-lang/rust#159856), but is now [explicitly accepted](rust-lang/rust#154645) as a stabilized API. - Please note that the MSRV for `{core,std}::derive` will be 1.96, and not 1.98. <a id="1.98.0-Stabilized-APIs"></a> ## Stabilized APIs - [`str::substr_range`](https://doc.rust-lang.org/stable/std/primitive.str.html#method.substr_range) - [`[T]::subslice_range`](https://doc.rust-lang.org/stable/std/primitive.slice.html#method.subslice_range) - [`core::fmt::NumBuffer`](https://doc.rust-lang.org/stable/core/fmt/struct.NumBuffer.html) - [`<{integer}>::format_into`](https://doc.rust-lang.org/stable/core/primitive.usize.html#method.format_into) - [`Send/Sync for std::process::CommandArgs`](https://doc.rust-lang.org/stable/std/process/struct.CommandArgs.html#impl-Send-for-CommandArgs%3C'a%3E) - [`{fN}::algebraic_add`](https://doc.rust-lang.org/stable/core/primitive.f32.html#method.algebraic_add) - [`{fN}::algebraic_sub`](https://doc.rust-lang.org/stable/core/primitive.f32.html#method.algebraic_sub) - [`{fN}::algebraic_mul`](https://doc.rust-lang.org/stable/core/primitive.f32.html#method.algebraic_mul) - [`{fN}::algebraic_div`](https://doc.rust-lang.org/stable/core/primitive.f32.html#method.algebraic_div) - [`{fN}::algebraic_rem`](https://doc.rust-lang.org/stable/core/primitive.f32.html#method.algebraic_rem) - [`NonZero<{integer}>::from_str_radix`](https://doc.rust-lang.org/stable/core/num/struct.NonZero.html#method.from_str_radix-4) - [`String::from_utf16le`](https://doc.rust-lang.org/stable/std/string/struct.String.html#method.from_utf16le) - [`String::from_utf16le_lossy`](https://doc.rust-lang.org/stable/std/string/struct.String.html#method.from_utf16le_lossy) - [`String::from_utf16be`](https://doc.rust-lang.org/stable/std/string/struct.String.html#method.from_utf16be) - [`String::from_utf16be_lossy`](https://doc.rust-lang.org/stable/std/string/struct.String.html#method.from_utf16be_lossy) - [`[T]::strip_circumfix`](https://doc.rust-lang.org/stable/core/primitive.slice.html#method.strip_circumfix) - [`str::strip_circumfix`](https://doc.rust-lang.org/stable/core/primitive.str.html#method.strip_circumfix) - [`Atomic<T>::from_mut`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.Atomic.html#method.from_mut) - [`Atomic<T>::get_mut_slice`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.Atomic.html#method.get_mut_slice) - [`Atomic<T>::from_mut_slice`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.Atomic.html#method.from_mut_slice) - [`std::range::legacy`](https://doc.rust-lang.org/stable/std/range/legacy/index.html) <a id="1.98.0-Compatibility-Notes"></a> ## Compatibility Notes - [If fully elided, lifetime bounds of trait object types may now resolve differently or even get rejected in very specific niche scenarios](rust-lang/rust#129543) - [Error in more cases of ambiguous imports](rust-lang/rust#145108) - [Switch the destructors implementation for thread locals on Windows to use Fiber Local Storage (FLS)](rust-lang/rust#148799) - [Convert some cases of the `ambiguous_glob_imports` lint into a hard error](rust-lang/rust#149195) - [Where-bounds of the form `Type = Type` and `Type == Type` are no longer syntactically allowed](rust-lang/rust#153513) - [Ensure Send/Sync is not implemented for std::env::Vars{,Os}](rust-lang/rust#155153) - [Fix that in some attributes, arguments were not properly rejected](rust-lang/rust#155193) - [`repr(transparent)` is now more strict about which fields have "trivial" layout and hence can be ignored: `repr(C)` types, types with private fields, and `#[non_exhaustive]` types are no longer considered "trivial"](rust-lang/rust#155299) - [Correctly check whether types have equal size in `transmute()` when some `repr` attributes are involved.](rust-lang/rust#155418) - [More characters are escaped when printing strings and chars](rust-lang/rust#155527) - [Implement fast path for `derive(PartialOrd)` when deriving `Ord`](rust-lang/rust#155598) This can break crates in practice where a type's PartialOrd and Ord impls were inconsistent with each other. - [Add temporary scope to `assert_eq` and `assert_ne`](rust-lang/rust#155739) - Closed a hole in the pattern matching [structural equality](https://doc.rust-lang.org/reference/patterns.html#constant-patterns) check, preventing cases where a match of a constant would be allowed, despite disagreeing with a manually written `PartialEq` implementation, when a `derive(PartialEq)` implementation for that type also exists. - [On Emscripten the WASM exception handling ABI is now unconditionally used](rust-lang/rust#156928) The `-Zemscripten-wasm-eh=false` flag to switch back to JS exceptions has been removed. - [The UNSAFE\_CODE lint is now consistently emitted for all unsafe attributes](rust-lang/rust#157201) - [Solaris: remove `File::lock` implementation, it has the wrong semantics (return "unsupported" instead)](rust-lang/rust#157509) - [Windows-gnu targets now specify baseline tools versions](rust-lang/rust#158020) - [rustfmt now discovers module files that are defined in `cfg_select!`](rust-lang/rust#158372) This may cause more code to be formatted which was previously ignored. </details> --- ### Configuration 📅 **Schedule**: (UTC) - Branch creation - At any time (no schedule defined) - Automerge - At any time (no schedule defined) 🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied. ♻ **Rebasing**: Whenever MR becomes conflicted, or you tick the rebase/retry checkbox. 🔕 **Ignore**: Close this MR and you won't be reminded about this update again. --- - [ ] If you want to rebase/retry this MR, check this box --- This MR has been generated by [Mend Renovate](https://github.com/renovatebot/renovate).

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Tracking issue for release notes of #156379: lint on core::ffi::c_void as a return type #156853

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lint on core::ffi::c_void as a return type

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Rollup of 3 pull requests #158619

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Rollup of 7 pull requests #158629

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Draft release notes for 1.98.0 #160700

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