[aw1621107]

> Have a "full" standard library where std::thread is M:N green threads and I/O is implemented behind the scenes that way, and a "thin" version where std::thread is OS threads and I/O is just passed through to the OS. (Then of course no_std for embedded.)

That sounds vaguely like what Rust used to have with its I/O functions before its green thread implementation was removed [0]? Not sure it's quite the same thing, though:

> In today's [2014] Rust, there is a single I/O API -- std::io -- that provides blocking operations only and works with both threading models. Rust is somewhat unusual in allowing programs to mix native and green threading, and furthermore allowing some degree of interoperation between the two.

> [snip]

> In this setup, libstd works directly against the runtime interface. When invoking an I/O or scheduling operation, it first finds the current Task, and then extracts the Runtime trait object to actually perform the operation.

> On native tasks, blocking operations simply block. On green tasks, blocking operations are routed through the green scheduler and/or underlying event loop and nonblocking I/O.

[0]: https://github.com/rust-lang/rfcs/blob/master/text/0230-remo...