Comment by const_cast a day ago
I mean from an outsiders perspective on Rust this is how I saw it.
Rust is in a strange place because they're a systems language directly competing with C++. Async, in general, doesn't vibe with that but green threads definitely don't.
If you're gonna do green threads you might as well throw in a GC too and get a whole runtime. And now you're writing Go.
There are other languages with green threads and folks are free to use those. Zig is trying to do interesting things with stackful coroutines.
I don't think I nor most systems programmers would have chosen rust if it required green threads instead of stackless coroutines for async. If you work on embedded or low level environments like kernels and whatnot, you need something that falls back to callbacks for async. I'm sure folks who work on servers would have been fine with green threads but they were not the target audience for rust. Being upset because you fall outside the target demographic of a particular language doesn't mean they made the wrong choice. It just means you should look for something else.
Hardware does not catches up with language requirements. If anything, it is languages/compilers that catch up with hardware, like SSE instructions and loop parallel ism.
For me the mistake that Rust made was that it tried too hard to behave like C/C++ with its single execution stack.
Ada uses two stacks allowing a callee to return a stack-allocated arrays to the caller. Not only it allows to avoid dynamic allocations in many cases where C++ allocates memory, but it also reduces the need for pointers making the code safer even without the borrow checker.
If instead of async Rust spent efforts on implementing something like that or even allow for explicit stack control from safe code so green threads or co-routines could be implemented as a library it could be more compatible with io_uring world.
> Ada uses two stacks allowing a callee to return a stack-allocated arrays to the caller.
You could do this manually by threading a pointer to a separately-allocated stack (could be on the heap or perhaps just a static allocation) as an extra function parameter. It's just a very simple case of arena allocation, with similar advantages and disadvantages. (For example, the caller must ensure that enough space is available on the dynamic-data stack for anything that the callee might want to push onto it.) In general it's just not really worth it, because it turns out that dynamically-sized data that one would not want to simply place on the heap is rare anyway.
I don't think doing green threads equates to 'well might as well have a GC now!'. I think they made the wrong tradeoff too, because hardware will inevitably catch up to the language requirements, especially if its desirable to use. Not to mention over time things can be made more efficient from the Rust side as well, with compiler improvements, better programming techniques etc.
I think they made the wrong bet, personally. Having worked in enough languages that have function coloring problems I would avoid it as a language design as a line in the sand item, regardless of tradeoffs