If the stack could be sufficiently small, it's not that different from heap allocated async state. But you probably needs segmented stacks, or at least separate stack async preemptible or non-async-preemptible code (has anyone tried making a system like this?)

It's isn't a given that M:N threading is slower than 1:1 threading even in Rust. A particular implementation you tried exhibited that behavior.

> One has a stack, and the other doesn't. That's a significant difference.

They both have some memory area to which they write state. Calling it "a stack" refers to the abstraction in the programmer's mind, not to how the memory is actually written/read. It is true that in order to support recursion, a thread might need to dynamically allocate memory, but so would async/await, except it'll make it more explicit.

> It's isn't a given that M:N threading is slower than 1:1 threading even in Rust. A particular implementation you tried exhibited that behavior.

I don't see any way around the problems of segmented stacks and FFI. There is no way to implement stack growth by reallocating stacks and rewriting pointers in Rust, even in theory. It would break too much code: there is a lot of unsafe (and even safe!) code out there that assumes that stack pointer addresses are stable. In fact, async/await in Rust had to introduce a new explicit pinning concept in order to solve this exact problem while remaining backwards compatible. And when calling the FFI, you have to switch to a big stack, which was an insurmountable performance problem. Rust code by its nature is FFI-heavy; it's part of the niche that Rust finds itself in.

You can make what are virtually zero-cost copies from what you call a "big stack" to a resizable stack with virtual memory tricks. You don't even need to copy the entire stack, but cleverly rewrite the return address stored on the stack to do this kind of "code-switching". But it does mean doing backend manipulations in a platform-dependent way. There are several good ways to do this, none of them particularly easy. What is perhaps impossible is allowing FFI code to block the lightweight thread, but async/await doesn't solve this, either.

> You can make what are virtually zero-cost copies from what you call a "big stack" to a resizable stack with virtual memory tricks. You don't even need to copy the entire stack, but cleverly rewrite the return address stored on the stack to do this kind of "code-switching".

We tried it. It was too slow.

OK. It really is hard when you're what you call "FFI-heavy" and don't like a significant runtime. So Rust has several * self-imposed* constraints (whether they're all essential for its target domains is a separate discussion, but some of those constraints certainly are) that makes this task particularly hard, but my point is that there is nothing fundamental to n:m threading that makes it slower than async/await, and async/await does fundamentally come at the significant cost of a particularly viral form of accidental complexity.