I'm not sure this is really possible. Given that async programming is cooperative by nature, how do you tell the difference between a blocking IO task, and a really long running loop in a piece of code that is itself blocking others from executing because it's doing too much work?

The blocking IO might be something in Rust that a type could be created for to denote that they are not async, and therefor warn you in some way, but I think that one is easy to detect in testing.