Rust doesn’t have a way in the type system to tell if a function that isn’t declared async is going to block or not. Their point is that any of those functions

Rust doesn’t have a way in the type system to tell if a function that isn’t declared async is going to block or not. Their point is that any of those functions could block, and having them block wouldn’t be an obvious breaking change, and so you need to know about their internals to know if they’re okay to call.