1use crate::future::Future;
2
3/// Conversion into a `Future`.
4///
5/// By implementing `IntoFuture` for a type, you define how it will be
6/// converted to a future.
7///
8/// # `.await` desugaring
9///
10/// The `.await` keyword desugars into a call to `IntoFuture::into_future`
11/// first before polling the future to completion. `IntoFuture` is implemented
12/// for all `T: Future` which means the `into_future` method will be available
13/// on all futures.
14///
15/// ```no_run
16/// use std::future::IntoFuture;
17///
18/// # async fn foo() {
19/// let v = async { "meow" };
20/// let mut fut = v.into_future();
21/// assert_eq!("meow", fut.await);
22/// # }
23/// ```
24///
25/// # Async builders
26///
27/// When implementing futures manually there will often be a choice between
28/// implementing `Future` or `IntoFuture` for a type. Implementing `Future` is a
29/// good choice in most cases. But implementing `IntoFuture` is most useful when
30/// implementing "async builder" types, which allow their values to be modified
31/// multiple times before being `.await`ed.
32///
33/// ```rust
34/// use std::future::{ready, Ready, IntoFuture};
35///
36/// /// Eventually multiply two numbers
37/// pub struct Multiply {
38/// num: u16,
39/// factor: u16,
40/// }
41///
42/// impl Multiply {
43/// /// Construct a new instance of `Multiply`.
44/// pub fn new(num: u16, factor: u16) -> Self {
45/// Self { num, factor }
46/// }
47///
48/// /// Set the number to multiply by the factor.
49/// pub fn number(mut self, num: u16) -> Self {
50/// self.num = num;
51/// self
52/// }
53///
54/// /// Set the factor to multiply the number with.
55/// pub fn factor(mut self, factor: u16) -> Self {
56/// self.factor = factor;
57/// self
58/// }
59/// }
60///
61/// impl IntoFuture for Multiply {
62/// type Output = u16;
63/// type IntoFuture = Ready<Self::Output>;
64///
65/// fn into_future(self) -> Self::IntoFuture {
66/// ready(self.num * self.factor)
67/// }
68/// }
69///
70/// // NOTE: Rust does not yet have an `async fn main` function, that functionality
71/// // currently only exists in the ecosystem.
72/// async fn run() {
73/// let num = Multiply::new(0, 0) // initialize the builder to number: 0, factor: 0
74/// .number(2) // change the number to 2
75/// .factor(2) // change the factor to 2
76/// .await; // convert to future and .await
77///
78/// assert_eq!(num, 4);
79/// }
80/// ```
81///
82/// # Usage in trait bounds
83///
84/// Using `IntoFuture` in trait bounds allows a function to be generic over both
85/// `Future` and `IntoFuture`. This is convenient for users of the function, so
86/// when they are using it they don't have to make an extra call to
87/// `IntoFuture::into_future` to obtain an instance of `Future`:
88///
89/// ```rust
90/// use std::future::IntoFuture;
91///
92/// /// Convert the output of a future to a string.
93/// async fn fut_to_string<Fut>(fut: Fut) -> String
94/// where
95/// Fut: IntoFuture,
96/// Fut::Output: std::fmt::Debug,
97/// {
98/// format!("{:?}", fut.await)
99/// }
100/// ```
101#[stable(feature = "into_future", since = "1.64.0")]
102#[rustc_diagnostic_item = "IntoFuture"]
103pub trait IntoFuture {
104 /// The output that the future will produce on completion.
105 #[stable(feature = "into_future", since = "1.64.0")]
106 type Output;
107
108 /// Which kind of future are we turning this into?
109 #[stable(feature = "into_future", since = "1.64.0")]
110 type IntoFuture: Future<Output = Self::Output>;
111
112 /// Creates a future from a value.
113 ///
114 /// # Examples
115 ///
116 /// Basic usage:
117 ///
118 /// ```no_run
119 /// use std::future::IntoFuture;
120 ///
121 /// # async fn foo() {
122 /// let v = async { "meow" };
123 /// let mut fut = v.into_future();
124 /// assert_eq!("meow", fut.await);
125 /// # }
126 /// ```
127 #[stable(feature = "into_future", since = "1.64.0")]
128 #[lang = "into_future"]
129 fn into_future(self) -> Self::IntoFuture;
130}
131
132#[stable(feature = "into_future", since = "1.64.0")]
133impl<F: Future> IntoFuture for F {
134 type Output = F::Output;
135 type IntoFuture = F;
136
137 fn into_future(self) -> Self::IntoFuture {
138 self
139 }
140}
141