Re: [PATCH 4/4] build: rust: provide an option to inline C helpers into Rust

["Emilio Cobos Álvarez" <emilio@crisal.io>]

On 12/4/25 12:57 PM, Miguel Ojeda wrote:
> On Thu, Dec 4, 2025 at 12:11 PM Peter Zijlstra <peterz@infradead.org> wrote:
>>
>> Right. Earlier I also proposed using libclang to parse the C header and
>> inject that. This might be a little simpler, in that..
> 
> Yeah, that would be closer to the `bindgen` route in that `libclang`
> gets already involved.

Yeah, so... there are existing tools (c2rust [0] being the actively 
maintained one IIUC) that in theory could do something like that 
(translate the bodies of the functions from C to Rust so that rustc 
could consume them directly rather than via LLVM LTO).

I think the intended use case is more "translate a whole C project into 
rust", but it could be interesting to test how well / poorly it performs 
with the kernel helpers / with a single header translated to Rust.

I personally haven't tried it because for work I need to deal with C++, 
which means that automatic translation to Rust is a lot harder / 
probably impossible in general. So for Firefox we end up relying on 
bindgen + cross-language LTO for this kind of thing, and it works well 
for us.

If I'm understanding correctly, it seems the kernel needs this extra bit 
of help (__always_inline) to push LLVM to inline C functions into rust, 
which is a bit unfortunate... But this approach seems sensible to me, 
for now at least.

FWIW Bindgen recently gained an option to generate inline functions [1], 
which could help avoid at least the bindgen ifdef in the patch series?

Anyways, it might be interesting to give c2rust a go on the kernel 
helpers if nobody has done so, and see how well / poorly it works in 
practice? Of course probably introducing a new dependency would be kind 
of a pain, but could be a good data point for pushing into adding 
something like it built into rustc...

Thanks,
  -- Emilio

[0]: https://github.com/immunant/c2rust
[1]: 
https://docs.rs/bindgen/latest/bindgen/struct.Builder.html#method.generate_inline_functions