The language itself doesn't need the runtime: it's entirely implemented in libraries, and the libraries are slowly being rearranged to offer as much as possible to clients who wish to avoid the runtime (e.g. "libcore").

High-level description the runtimes: http://doc.rust-lang.org/master/guide-runtime.html

Avoiding the standard library/runtime: http://doc.rust-lang.org/master/guide-unsafe.html#avoiding-t... (don't miss the "Using libcore" subsection)