Rust is not "runtime free". Rust needs a "minimal runtime", as stated on the project home page. You need at least a memory allocator. Even C programs are usually linked to a runtime library (like glibc).
Rust is usable with an extremely minimal runtime that approximates having no runtime at all (with #[no_std]). There are a few intrinsics you have to define, but none of them involve memory allocation. It also has a core crate that does not expose anything that requires allocation.
