Non-leaf futures

Non-leaf futures are the kind of futures we as users of a runtime write ourselves using the async keyword to create a task that can be run on the executor.

The bulk of an async program will consist of non-leaf futures, which are a kind of pause-able computation. This is an important distinction since these futures represent a set of operations. Often, such a task will await a leaf future as one of many operations to complete the task.

This is an example of a non-leaf future:

let non_leaf = async {
    let mut stream = TcpStream::connect("127.0.0.1:3000").await.unwrap();
    println!("connected!");
    let result = stream.write(b"hello world\n").await;
    println!("message sent!");
    ...
};

The two highlighted lines indicate points where we pause the execution, yield control to a runtime, and eventually resume. In...