We're still pretty confident that Rust will be able to support a high quality green threading library and other M:N concurrency libraries without needing to be integrated into the language itself. All the type magic that we do to pass data between threads should work quite well for a green threading library. We already have a couple projects experimenting with this [1], [2], [3]. Servo also has a pretty nifty work-stealing queue [4] that is yet another abstraction for easily sharing data between threads.

1: https://github.com/rustcc/coroutine-rs 2: https://github.com/zonyitoo/simplesched 3: https://github.com/dpc/mioco 4: https://github.com/servo/servo/blob/c76720d67c0d04f6d77c138c...