Green threads/Virtual threads are one solution, but then you need to ship a runtime for them, then you pay for what you don't use.

Or don't standardize on them and have a split ecosystem of half libs supporting use `green-thread.rs` and the other half supporting `rust-gthreads`.

I wonder if you could have hidden green threads from the application entirely. 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.)

Of course the problem then would be that Rust makes it trivially easy to call C code. As soon as someone in a green thread runtime called libc I/O functions directly they'd be in for a weird surprise when the call froze all threads on their worker.

Of course the "full" M:N runtime could be instrumented to print a warning when this occurs unless explicitly told not to, and std::thread in the "full" stdlib could allow explicit creation of full OS threads outside the runtime.

There is an attempt at doing this out there:

https://github.com/Xudong-Huang/may

> 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...

May has soundness issues around TLS. You can cause undefined behavior in safe code. They do not plan on changing that (and I'm not sure it's even possible) but there's no way that it will become super popular due to this issue.

(and yes, as my sibling commentor mentions, Rust tried this before. Doesn't mean nobody can do it, but I suspect that it is not possible.)

> Green threads/Virtual threads are one solution, but then you need to ship a runtime for them, then you pay for what you don't use.

Indeed, and IIRC that's one of the reasons Rust removed its green thread implementation in the first place.

> Or don't standardize on them and have a split ecosystem of half libs supporting use `green-thread.rs` and the other half supporting `rust-gthreads`.

Don't forget the third lib for abstracting over the two!