library/core/src/future/into_future.rs

   1 use 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 pub trait IntoFuture {
 103     /// The output that the future will produce on completion.
 104     #[stable(feature = "into_future", since = "1.64.0")]
 105     type Output;
 106
 107     /// Which kind of future are we turning this into?
 108     #[stable(feature = "into_future", since = "1.64.0")]
 109     type IntoFuture: Future<Output = Self::Output>;
 110
 111     /// Creates a future from a value.
 112     ///
 113     /// # Examples
 114     ///
 115     /// Basic usage:
 116     ///
 117     /// ```no_run
 118     /// use std::future::IntoFuture;
 119     ///
 120     /// # async fn foo() {
 121     /// let v = async { "meow" };
 122     /// let mut fut = v.into_future();
 123     /// assert_eq!("meow", fut.await);
 124     /// # }
 125     /// ```
 126     #[stable(feature = "into_future", since = "1.64.0")]
 127     #[lang = "into_future"]
 128     fn into_future(self) -> Self::IntoFuture;
 129 }
 130
 131 #[stable(feature = "into_future", since = "1.64.0")]
 132 impl<F: Future> IntoFuture for F {
 133     type Output = F::Output;
 134     type IntoFuture = F;
 135
 136     fn into_future(self) -> Self::IntoFuture {
 137         self
 138     }
 139 }