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/// /// Constructs 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/// /// Converts 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"]
103#[diagnostic::on_unimplemented(
104 label = "`{Self}` is not a future",
105 message = "`{Self}` is not a future",
106 note = "{Self} must be a future or must implement `IntoFuture` to be awaited"
107)]
108pub trait IntoFuture {
109 /// The output that the future will produce on completion.
110 #[stable(feature = "into_future", since = "1.64.0")]
111 type Output;
112
113 /// Which kind of future are we turning this into?
114 #[stable(feature = "into_future", since = "1.64.0")]
115 type IntoFuture: Future<Output = Self::Output>;
116
117 /// Creates a future from a value.
118 ///
119 /// # Examples
120 ///
121 /// Basic usage:
122 ///
123 /// ```no_run
124 /// use std::future::IntoFuture;
125 ///
126 /// # async fn foo() {
127 /// let v = async { "meow" };
128 /// let mut fut = v.into_future();
129 /// assert_eq!("meow", fut.await);
130 /// # }
131 /// ```
132 #[stable(feature = "into_future", since = "1.64.0")]
133 #[lang = "into_future"]
134 fn into_future(self) -> Self::IntoFuture;
135}
136
137#[stable(feature = "into_future", since = "1.64.0")]
138impl<F: Future> IntoFuture for F {
139 type Output = F::Output;
140 type IntoFuture = F;
141
142 fn into_future(self) -> Self::IntoFuture {
143 self
144 }
145}
146