Subject: Re: [PATCH 3/3] rust: generate bindings via cbindgen

From: Patrick Steinhardt <[email-masked]>
Date: Thu, 30 Oct 2025 10:50:55 +0100

On Mon, Oct 27, 2025 at 09:14:51PM +0000, brian m. carlson wrote:
> On 2025-10-27 at 20:35:59, Ezekiel Newren wrote:
> > On Fri, Oct 24, 2025 at 12:37 AM Patrick Steinhardt <[email-masked]> wrote:
> > > > cbindgen is a Rust crate and it should be specified in the Cargo.toml
> > > > under [build-dependencies] block.
> > >
> > > What is the benefit for us? The generated code is not a dependency of
> > > the Rust code, and neither do we use it via "build.rs". And if we use
> > > cbindgen via "Cargo.toml" we'd be forced to build it first, which slows
> > > down our CI jobs.
> > >
> > > Please let me know in case I miss any reasons to have it in our build
> > > dependencies instead.
> > 
> > You're targeting a very old version of Rust (1.49). I'm not even sure
> > that cbindgen will work with a version that old, but if it does then
> > we should use it in build.rs to make sure we're not using any features
> > of cbindgen that aren't available until later versions. If we use
> > cbindgen that is packaged with the platform then we can't precisely
> > control which version of cbindgen is being used. This is a matter of
> > reproducibility. There may be platforms that can compile Rust, but
> > can't generate C header files via cbindgen because cbindgen hard codes
> > that a certain minimum Rust version is required in its own Cargo.toml
> > file.
> 
> Yes, I agree with this.  Not all systems have cbindgen and it's not
> guaranteed that the system's cbindgen will work with the version of Rust
> that you want to target or that's being used to compile.

Okay. In that case the question to me is how to drive cbindgen from our
Makefile and from Meson if it's going to be invoked via "build.rs". It
doesn't make much sense from my PoV to make generation of the C headers
depend on building the complete Rust library. Doubly so because we'd now
have a chicken-and-egg problem:

  1. To build libgit.a we need to have the C interop header.

  2. To build the C interop header we need to build the Rust library.

  3. The Rust library depends on libgit.a.

So am I missing anything obvious here for how to declare cbindgen in our
"Cargo.toml" file and invoke it directly from our other build systems?

Thanks!

Patrick