On Thu, Dec 04, 2025 at 12:57:31PM +0100, Miguel Ojeda wrote: > On Thu, Dec 4, 2025 at 12:11 PM Peter Zijlstra <[email protected]> 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? > 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 ;-) > There is also the question about GCC. A deeper integration into > `rustc` would ideally need to have a way (perhaps depending on the > backend picked?) to support GCC builds properly (to read the header > and flags as expected, as you mention). Right, so the backend that spits out C could obviously just pass through any C headers. But otherwise, inlining C headers (and inline functions) would be something that is independent of the C files. At the end of the day all that really matters is the architecture C ABI. That is, if rustc inlines a C function from a header, it doesn't matter it used libclang to do so, even if the C files are then compiled with GCC. > And finally there is the question of what GCC Rust would do in such a > case. Things have substantially changed on the GCC Rust in the last > years, and they are now closer to build the kernel, thus I think their > side of things is getting important to consider too. > > Cc'ing Emilio (`bindgen`), Antoni (GCC backend) and Arthur (GCC Rust) > so that they are in the loop -- context at: Right, so clearly GCC has the capability to parse C headers :-) So I would imagine their Rust front-end would be able to hand off C headers and get back IR much like LLVM based projects can using libclang.
