Green threading used to be default. And used libuv for io. Then native threading was avaliable. Then it became default. Now green threading is being moved out of builtin libs.

Starting with green threads is really easy too:

    extern crate green;
    extern crate rustuv;

    #[start]
    fn start(argc: int, argv: *const *const u8) -> int {
        green::start(argc, argv, rustuv::event_loop, main)
    }

    fn main() {
        // this code is running in a pool of schedulers all     powered by libuv
    }

Edit: You can also start up a new libgreen scheduler pool at any time and add some native threads to the pool. So you can have some threads running with libgreen and some with libnative. (So you could theoretically embed a go program inside a rust program)