How it works...

Let's start by introducing the Future trait: 

• Implementing the Future trait requires only three constraints: an Item  type, an Error type, and a poll() function. The actual trait looks as follows:

pub trait Future {
    type Item;
    type Error;
    fn poll(
        &mut self, 
        cx: &mut Context
    ) -> Result<Async<Self::Item>, Self::Error>;
}

• The Poll<Self::Item, Self::Error> is a type that translates into Result<Async<T>, E> , where T = Item and E = Error. This is what our example is using on line 50.
• poll() is called upon whenever a futures::task::Waker (can also be known as a Task) is executed with one of our executors located at futures::executor, or manually woken up by building a futures::task::Context and running with a future wrapper such as futures::future::poll_fn.

Now, onto our local_until() function:

• LocalPool offers us...