This is not true. Imagine code like this:

    const n = try reader.interface.readVec(&data);

Can you guess if it's going to do blocking or non-blocking I/O read?

The io parameter is not really "coloring", as defined by the async/await debate, because you can have code that is completely unaware of any async I/O, pass it std.Io.Reader and it will just work, blocking or non-blocking, it makes no difference. Heck, you even even wrap this into C callbacks and use something like hiredis with async I/O.

Stackful coroutines need more memory, because you need to pre-allocate large enough stack for the entire lifetime. With stackless coroutines, you only need the current state, but with the disadvantage that you need frequent allocations.