...as long as you're fine shelling out to cargo, right? What if that's not appropriate or ergonomic?

There's a world where Rust is as easy to use as C, C++, Fortran, etc. outside of cargo, but it's not an interesting use case for the Rust community to support right now for whatever reason. That's basically my point.

And it's common enough to have linked programs in which python calls C that calls C++ that calls Fortran that calls C. Replacing arbitrary layers with Rust is technically possible, though Rust doesn't tend to be deployed in situations like that, at least not yet. I'll consider the Rust ecosystem more mature when you see Rust used mixed ELF situations like that.