``` 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
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
```
```use {Future, Poll, Async};

/// Future for the `select` combinator, waiting for one of two futures to
/// complete.
///
/// This is created by the `Future::select` method.
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
pub struct Select<A, B> where A: Future, B: Future<Item=A::Item, Error=A::Error> {
inner: Option<(A, B)>,
}

/// Future yielded as the second result in a `Select` future.
///
/// This sentinel future represents the completion of the second future to a
/// `select` which finished second.
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
pub struct SelectNext<A, B> where A: Future, B: Future<Item=A::Item, Error=A::Error> {
inner: OneOf<A, B>,
}

#[derive(Debug)]
enum OneOf<A, B> where A: Future, B: Future {
A(A),
B(B),
}

pub fn new<A, B>(a: A, b: B) -> Select<A, B>
where A: Future,
B: Future<Item=A::Item, Error=A::Error>
{
Select {
inner: Some((a, b)),
}
}

impl<A, B> Future for Select<A, B>
where A: Future,
B: Future<Item=A::Item, Error=A::Error>,
{
type Item = (A::Item, SelectNext<A, B>);
type Error = (A::Error, SelectNext<A, B>);

fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
let (ret, is_a) = match self.inner {
Some((ref mut a, ref mut b)) => {
match a.poll() {
Err(a) => (Err(a), true),
match b.poll() {
Err(a) => (Err(a), false),
}
}
}
}
None => panic!("cannot poll select twice"),
};

let (a, b) = self.inner.take().unwrap();
let next = if is_a {OneOf::B(b)} else {OneOf::A(a)};
let next = SelectNext { inner: next };
match ret {
Err(e) => Err((e, next)),
}
}
}

impl<A, B> Future for SelectNext<A, B>
where A: Future,
B: Future<Item=A::Item, Error=A::Error>,
{
type Item = A::Item;
type Error = A::Error;

fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
match self.inner {
OneOf::A(ref mut a) => a.poll(),
OneOf::B(ref mut b) => b.poll(),
}
}
}
```