As of today in systemd Git it's now possible for the systemd programs to be written in the Rust programming language as an alternative to C.

Core systemd code like the systemd library doesn't yet support Rust code, but for the various systemd programs / leaf components is now support for using Rust. That's Rust without Cargo. For Rust in core systemd code, there are some Rust blockers that would first need to be resolved.