> Certainly any language that differentiates between async code and non-async code has the async code be a pain.

Function colouring is not the only problem with async code. The difference is that concurrency in other high-level languages usually don't break down polymorphism and other language features. Also, they don't push you to deal with lifetimes, which is a serious issue in Rusty async.

Writing async code in C# is a lot easier to me than in Rust. Unfortunately, I didn't have a chance to write async in functional languages, such as Haskell or F#, because they are well-known for elegant concurrency.