> I still think sans-io at the language level might be the future, but this isn't a complete solution. Maybe we should be simply compiling all fns to state machines (with the Rust polling implementation detail, a sans-io interface could be used to make such functions trivially sync - just do the syscall and return a completed future).

Can you be more specific what is missing in sans-io with explicit state machine for static and dynamic analysis would not be a complete solution? Serializing the state machine sounds excellent for static and dynamic analysis. I'd guess the debugging infrastructure for optimization passes and run-time debugging are missing or is there more?