There are a number of crates with Rust bindings to the Guile C API: https://lib.rs/search?q=guile
Most of them are just the result of running the bindgen tool, which autogenerates Rust bindings from C header files. A few of them have slightly more convenient wrapper but none is really well-developed AFAICS. Honestly, I think the easiest at this time is to do the interfacing with Guile from C. Create a Rust module which exports a function through C FFI, and convert between C and Guile types in C. Alternatively, use the Guile FFI facilities for the conversion. It is probably simpler than doing these conversions in Rust. It is, in any case, safer, because I'm quite sure that the Rust compiler will be happy to do optimizations that defeat conservative GC, by hiding pointers in local variables (which Rust can more easily do, thanks to its aliasing guarantees). Be careful with calling Guile code from Rust code. This is fraught, because Guile code can raise exceptions, which are basically propagated through longjmp, and it is UB in Rust to unwind through Rust code with longjmp. Basically, if you call Guile from Rust then it must be wrapped in exception handling. An exception must not propagate through Rust stack frames. call/cc would be a problem too, so you likely want to use with-continuation-barrier.
