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.

“Non-blocking” code is basically just code that takes a short enough amount of time that we don’t care that it blocks the thread. It’s inherently a matter of judgement.

Static analysis should help with this. Basically it should identify every call site where I/O happens (and other syscalls), and then you have to check them that they are invoked with the right async/nonblocking dance.

This is basically a code audit problem.

Of course something like taint analysis could also work. Every such callsite should be counted as tainted unless it gets wrapped with something that's whitelisted (or uses the right marker type wrapper).

Even effects as types can't help much, because the basic interfere to the kernels (Linux, WinNT, etc.) are not typesafe, and as long as the language provides FFI/syscall interfaces you have to audit/trust the codebase/ecosystem.