Here's the thing though: Rust could have every function be an async state machine, automatically. And then the compiler optimizes away that code when it isn't needed. It would be a big pain to implement, but it's doable, and it would deliver a developer experience much closer to Go's. There isn't a technical reason for why Rust couldn't do this.
FYI you can't poll an async result in a sync context in Rust, either.
> There isn't a technical reason for why Rust couldn't do this.
First, it would be a huge undertaking. That in itself is a huge time/resource burden.
Second, it would add overhead to any non-async function call. Because async introduces branching on every function invocation, it would make the resulting assembly even harder to understand. This strongly goes against the zero overhead/ zero cost abstraction idea of Rust.
By the same measure, Go could technically remove (almost) all GC, add some kind of borrowing mechanism and steal Rust's thunder.
FYI you can't poll an async result in a sync context in Rust, either.