This isn't actually a problem most of the time because we can spawn new threads, and so in C++ with co_await I have like three or four different "runtimes" all working at once with their own thread pools and their own I/O loops and I have no issue mixing and matching their behaviors using the obvious syntax. The issue with Rust seems to come down to something much deeper involving either an overzealous attempt to force people to think in terms of "executors" and "runtimes" for entire stacks of coroutines instead of individual tasks or (at best) as a result of decisions made with respect to how the memory management of the mechanism would operate in the context of Rust.