Green threads have no place in a low level systems language like Rust whose design goals are zero cost abstraction and trivial C interop.

D made a similar mistake by requiring GC/runtime from start and now even though they added ways to avoid it the ecosystem and the language design are "poisoned" by it an itmakeas it a very hard sell in some places where it could be sold as a C++ successor.

Because rust made the right choice in time it's now a contender in that space, if it chose to go down the runtime required/custom threading model route it would have much less practical appeal. If you can swallow runtime/threading abstraction overhead why not just bolt on a GC and use Go