> 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".
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.
We tried it. It was too slow.