If you want to have annotations on IO code you could get it using attributes - and you could even go further by using more precise annotations like `network`, `disk`, etc. The problem with using the type system is that now you need to account for the distinction everywhere (ex. interfaces/traits must support IO-based implementations) only to carry this metadata which should not affect the behavior. In Haskell, IO exists not to track a behavior but to enforce it in a lazy language. In Rust is done due to the lack of a runtime.

I recommend reading https://degoes.net/articles/no-effect-tracking . In summary, most languages could do with the Go/Java virtual thread async model dropping async/await entirely.