We have Send and non-Send primitives in Rust for a reason. You could use Arc/Mutex/AtomicUsize/... everywhere on a single thread, but you should use Rc/RefCell/Cell<usize>/... instead whenever possible since those are just cheaper. The problem is that in the ecosystem we are building the prevailing assumption is that anything async must also be Send, which means we end up using Send primitives even in non-Send contexts, which is always a waste.
> If you want every users to only use single threaded runtime, it's a major loss for the Rust ecosystem.
Running single threaded executors does not prohibit you from using threads, it just depends on how you want to do that. You can:
1. Have a single async executor running a threadpool that requires _everything_ to be Send. 2. Have a single threadpool, each thread running its own async executor, in which case only stuff that crosses thread boundaries needs to be Send.
The argument is that there are many scenarios where 2 is the optimal solution, both for performance and developer experience, but the ecosystem does not support it well.