I'm not totally clear on the status of green threads but I'm not sure they should be encouraged presently. My understanding is they will be an external package and std::io will not support them.
I think that the external package is supposed to work by providing an alternate implementation of std::io and several other parts of the standard library. Green threads will continue to be fully supported, but they are second class.
[1]https://github.com/rust-lang/meeting-minutes/blob/master/wor...