Threads and async have different restrictions and costs. No shared abstraction is possible for a language like Rust without losing what makes it Rust. Minimal-

Threads and async have different restrictions and costs. No shared abstraction is possible for a language like Rust without losing what makes it Rust. Minimal-cost threading support requires taking Send + Sync into account, essentially thread-safety: and the thread-safe primitives are more expensive than the thread-local primitives. Send + Sync matters become pervasive through the design of threaded code, even if they don’t need to be spelled out most of the time. Minimal-cost async requires compiling the code into a state machine, which means you’ve got to restrict borrowing across suspend points, or mess around with Pin. Threading and async are quite different in a language like Rust. They can even play quite nicely together. But you really can’t unify them. Sure, you could do simple cases like single-threaded blocking-or-async, e.g. `async? fn foo() { bar().await; }` compiling to something like strawman `fn sync#foo() { sync#bar(); } async fn async#foo() { async#bar().await; }`, but this falls apart completely as soon as you want to do anything beyond immediate .await.