One of the common comments in this thread is "can't we just make standard interfaces in std?" "well, no, sync+send is hard"

I can't help but wonder if there are two sets of interfaces necessary? a set of standard single-threaded traits and a set of multi-threaded traits? would that be sufficient?

As an aside, what workloads require true multithreaded reactors as opposed to a runtime which uses multiple singlethreaded reactors?