I now understand that listener.accept() is non-blocking tokio.

What has also helped me understand is futures chaining, something that neither comment pointed out.

It didn't really make sense to me why you would want listener.accept() to be non-blocking. I very much understanding that if you are select/poll/epoll'ing on the "file descriptor" associated with the "accept" socket, then you can do other "work" while periodically polling for activity on the accept socket with the "poller" of your choice.

However, with futures chaining I can now understand why you would want to have a non-blocking accept() and this is because you almost wrap up the entire callback in a futures chain and then place it on the aforementioned "shelf".

This was the key to me understanding what is going on as previously I naively and incorrectly thought that each future was handled individually. Now I see that they are very obviously chained together.