The code for initializing TcpStreams looks like this:

fn init_stream(serv_addr: &str, serv_port: u16) -> TcpStream {
    let stream = TcpBuilder::new_v4().unwrap().to_tcp_stream().unwrap();
    stream.set_nonblocking(true).unwrap();
    // This will fail with EINPROGRESS
    let _ = stream.connect((serv_addr, serv_port));
    stream
}

Here connect() first calls to_socket_addrs() which does address name resolving by calling getaddrinfo(), which in turn blocks the thread.

One alternative is to use getaddrinfo_a(), but I'm not sure how portable this would be.

Another alternative is to use futures to run getaddrinfo() and poll the results in tick handlers.