Lint idea: find known-blocking constructs in async contexts

[skade]

With async/await coming up, we will probably see an uptake of such code:

async hello() {
    println!("hello").
}

Users will most likely not be aware that the call to println are blocking. The same goes to any use of the stdlib io apis, most famously to_socket_addr() calls.

I think there is a chance for clippy to at least find obvious cases of this, for a list of known functions.

[hwchen]

thread::sleep seems to be a pretty common mistake:

https://old.reddit.com/r/rust/comments/e1gxf8/not_understanding_asyncawait_properly/
https://old.reddit.com/r/rust/comments/dtp6z7/what_can_i_actually_do_with_the_new_async_fn/
https://old.reddit.com/r/rust/comments/dt0ruy/how_to_await_futures_concurrently/