Linux

I think you're making some poorly-researched assumptions.

In the embedded world, there often aren't "system libraries," depending on just what you're targeting. But if, for some reason, you really do want to use libc but not the Rust standard library, you can certainly do that; for instance, here's a crate that reimplements the Rust standard library's output and formatting capabilities using libc: https://github.com/mmastrac/rust-libc-print

Rust provides essentially the same memory control as C does. You can also have inline assembly in Rust, just as in C.