It’s hard to say. If you want to use green threads, you may want to look at the may crate [1]. The reason Rust prioritized stackless async just like C++ did is that it fits better the systems programming needs (i.e. it’s syntactic sugar for a state machine you could hand implement) while not preventing things like stackful coroutines.
If Rust manages to solve the coloring problem of async (e.g. by adopting effect systems [2] or alternatives), then stackful and stackless coroutines syntactic sugar could conceivably exist within the std language (perhaps leaving out stackless on nostd).
The reason you don’t see both stackless and stackful coroutines in a single language like Rust is the coloring problem is made 50% worse.
You mean the TLS issue called out or something else?
I wasn't trying to recommend may specifically of course. Or are you saying that stackful coroutines must have soundness issues due to missing language features to make it safe?
Yeah, for TLS to work safely I suspect the only way would be language support which knows to media TLS through a "context pointer" so that the green threads could restore the context when resuming. In C++ the story is even worse because someone could take the address of a TLS variable & use it elsewhere. I think in general it's very very tricky to mix TLS and stackful coroutines that can be scheduled on arbitrary threads and languages like Go pick the stackful coroutines and drop the ability to do TLS.
To be fair though, I think people generally just avoid TLS when running with green thread systems.
