You can build Rust code as a static library and make the C build system consume that instead. This is the approach that librsvg took. Moving C build to build.r

You can build Rust code as a static library and make the C build system consume that instead. This is the approach that librsvg took. Moving C build to build.rs is not necessary. It's usually done only because people used to Cargo don't like bringing CMake along. If you were to publish this as a Rust crate, it'd be slightly easier for downstream Rust users to have one less external tool to install.