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

[Alice Ryhl]

On Thu, Dec 04, 2025 at 01:39:06PM +0100, Peter Zijlstra wrote:
> On Thu, Dec 04, 2025 at 12:57:31PM +0100, Miguel Ojeda wrote:
> > On Thu, Dec 4, 2025 at 12:11 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.
> > 
> > > ... if you build rustc against libclang they are necessarily from the
> > > same LLVM build.
> > 
> > So currently there are 3 "LLVMs" that get involved:
> > 
> >   - The one Clang uses (in LLVM=1 builds).
> 
> Well, being on Debian, I'm more likely to be using LLVM=-22 (or whatever
> actual version is required, 22 just being the latest shipped by Debian
> at this point in time).
> 
> >   - The one `rustc` uses (the LLVM backend).
> >   - The one `bindgen` uses (via libclang).
> 
> These are not necessarily the same? That is, is not bindgen part of the
> rustc project and so would be built against the same LLVM?

Rustc and bindgen are both part of the Rust project, but they're two
different repos:
https://github.com/rust-lang/rust
https://github.com/rust-lang/rust-bindgen

Generally you need to ensure that bindgen matches clang. This ensures
that bindgen and clang agree on the interpretation of C headers.

> > If that is all done within `rustc` (so no `bindgen`), then there may
> > still be `rustc` vs. Clang mismatches, which are harder to resolve in
> > the Rust side at least (it is easier to pick another Clang version to
> > match).
> > 
> > For those using builds from distros, that shouldn't be a problem.
> > Others using external `rustc` builds, e.g. from `rustup` (e.g. for
> > testing different Rust versions) it would be harder.
> 
> Make rust part of LLVM and get them all built and distributed
> together... such that LLVM=-23 will get me a coherent set of tools.
> 
> /me runs like crazeh ;-)

Maybe clang itself should have an option to emit Rust headers, taking
over the role of bindgen?

/me runs like crazeh ;-)

Alice