(side note: why does this forum not have a category: "discussion"? "help" seems overly broad.)
As advised I'm starting a new topic.
The question is about tokio's LocalSet which guarantees a sequentially consistent single-thread like environment across .await point by ensuring that they execute on a single thread (the current thread). This feature allows the building of more traditional event-loop based environments (such as a traditional JS runtime). However, fundamentally, tokio appears designed to provide an M:N threading implementation so internally it is a multi-threaded program.
The question is to what extent LocalSet provides an efficient single-threaded execution engine given that it provides the full feature set of the tokio API. For instance, if a task calls tokio::sleep it seems that there's no separate code path for a call from a LocalSet task (the code seems to be using an Arc here unless I'm misreading it). So an interesting question is to what extent such a use would pay for unnecessary synchronization overhead (use of Arc where Rc would suffice, etc.), and potentially whether there's a need or use for a purely single-threaded async engine (if it doesn't already exist.) Of course, certain system interactions (like with a I/O threadpool to offload blocking I/O, etc.) will require synchronization in any event if this environment were to provide support for them.