Having a global switch to switch the program over to async only works if all your concurrency primitives are aware of it: e.g. mutexes. Otherwise you get deadlo

Having a global switch to switch the program over to async only works if all your concurrency primitives are aware of it: e.g. mutexes. Otherwise you get deadlocks. But Rust is designed to work with raw OS concurrency primitives. So that solution isn't available.