I thought Rust async is a colored stackless coroutine model and thus it would be unsafe to continue execution of previously executing async functions.

To explain, generally speaking, stackless coroutine async only need coloring because they are actually “independent stack”less coroutines. What they actually do is that they share the stack for their local state. This forces async function execution to proceed in LIFO order so you do not blow away the stack of the async function executing immediately after which demands state machine transforms to be safe. This is why you need coloring unlike stackful coroutine models which can execute, yield, and complete in arbitrary order since their local state is preserved in a safe location.