The Rust approach is, Rust's safety guarantees work for concurrency, but the details of that concurrency are left up to libraries, not the language. Since the safety is in the language, things are always safe, but you get the flexibility to do what you need.
You have to remember, Rust is a systems language. Which means that you need access to what the system gives you, and that means at least OS threads and both synchronous and asynchronous IO. We can't just decree "the world must use only aio and green threads", or we would be compromising Rust's fundamental design goals.
