1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37

use {Future, Poll, Async};

/// A future which "fuses" a future once it's been resolved.
///
/// Normally futures can behave unpredictable once they're used after a future
/// has been resolved, but `Fuse` is always defined to return `Async::NotReady`
/// from `poll` after it has resolved successfully or returned an error.
///
/// This is created by the `Future::fuse` method.
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
pub struct Fuse<A: Future> {
    future: Option<A>,
}

pub fn new<A: Future>(f: A) -> Fuse<A> {
    Fuse {
        future: Some(f),
    }
}

impl<A: Future> Future for Fuse<A> {
    type Item = A::Item;
    type Error = A::Error;

    fn poll(&mut self) -> Poll<A::Item, A::Error> {
        let res = self.future.as_mut().map(|f| f.poll());
        match res.unwrap_or(Ok(Async::NotReady)) {
            res @ Ok(Async::Ready(_)) |
            res @ Err(_) => {
                self.future = None;
                res
            }
            Ok(Async::NotReady) => Ok(Async::NotReady)
        }
    }
}