Messages in this thread |  | | From | Miguel Ojeda <> | | Date | Wed, 21 May 2025 15:45:18 +0200 | | Subject | Re: [PATCH] rust: workaround `bindgen` issue with forward references to `enum` types |
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On Tue, Mar 25, 2025 at 7:43 PM Miguel Ojeda <ojeda@kernel.org> wrote: > > `bindgen` currently generates the wrong type for an `enum` when there > is a forward reference to it. For instance: > > enum E; > enum E { A }; > > generates: > > pub const E_A: E = 0; > pub type E = i32; > > instead of the expected: > > pub const E_A: E = 0; > pub type E = ffi::c_uint; > > The issue was reported to upstream `bindgen` [1]. > > Now, both GCC and Clang support silently these forward references to > `enum` types, unless `-Wpedantic` is passed, and it turns out that some > headers in the kernel depend on them. > > Thus, depending on how the headers are included, which in turn may depend > on the kernel configuration or the architecture, we may get a different > type on the Rust side for a given C `enum`. > > That can be quite confusing, to say the least, especially since > developers may only notice issues when building for other architectures > like in [2]. In particular, they may end up forcing a cast and adding > an `#[allow(clippy::unnecessary_cast)]` like it was done in commit > 94e05a66ea3e ("rust: hrtimer: allow timer restart from timer handler"), > which isn't great. > > Instead, let's have a section at the top of our `bindings_helper.h` that > `#include`s the headers with the affected types -- hopefully there are > not many cases and there is a single ordering that covers all cases. > > This allows us to remove the cast and the `#[allow]`, thus keeping the > correct code in the source files. When the issue gets resolved in upstream > `bindgen` (and we update our minimum `bindgen` version), we can easily > remove this section at the top. > > Link: https://github.com/rust-lang/rust-bindgen/issues/3179 [1] > Link: https://lore.kernel.org/rust-for-linux/87tt7md1s6.fsf@kernel.org/ [2] > Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Andreas: are you OK with something along the lines of this change?
If you don't have anything against it, then I think it would be nice to put it in, mainly so that the issue is documented, and so that we don't forget about this possible workaround. Plus it removes an `allow` and a cast, which is also nice.
The only downside, as far as I know, is that the workaround cannot be always applied (see my sibling reply), but at least for this case, it can be.
Thanks!
Cheers, Miguel
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