Rust does allow you to implement low-level things in itself, by giving an escape hatch into C/C++-like unsafe code (i.e. risk-of-incorrectness is purely opt-in, rather than always-there).

Examples of things efficiently implemented entirely in the standard library in pure Rust (well, with some calls into the operating system/libc): Vec, the std::vector equivalent. Rc, reference counted pointers (statically restricted to a single thread). Arc, thread-safe reference counted pointers. Mutex. Concurrent queues. Hashmap.