Rollup merge of #97876 - yoshuawuyts:into-future-docs, r=JohnTitor,yaahc

update docs for `std::future::IntoFuture`

Ref #67644.

This updates the docs for `IntoFuture` providing a bit more guidance on how to use it. Thanks!

Author: JohnTitor

library/core/src/future/into_future.rs

@@ -1,6 +1,109 @@
 use crate::future::Future;
 
 /// Conversion into a `Future`.
+///
+/// By implementing `Intofuture` for a type, you define how it will be
+/// converted to a future.
+///
+/// # `.await` desugaring
+///
+/// The `.await` keyword desugars into a call to `IntoFuture::into_future`
+/// first before polling the future to completion. `IntoFuture` is implemented
+/// for all `T: Future` which means the `into_future` method will be available
+/// on all futures.
+///
+/// ```no_run
+/// #![feature(into_future)]
+///
+/// use std::future::IntoFuture;
+///
+/// # async fn foo() {
+/// let v = async { "meow" };
+/// let mut fut = v.into_future();
+/// assert_eq!("meow", fut.await);
+/// # }
+/// ```
+///
+/// # Async builders
+///
+/// When implementing futures manually there will often be a choice between
+/// implementing `Future` or `IntoFuture` for a type. Implementing `Future` is a
+/// good choice in most cases. But implementing `IntoFuture` is most useful when
+/// implementing "async builder" types, which allows the type to be modified
+/// multiple times before being `.await`ed.
+///
+/// ```rust
+/// #![feature(into_future)]
+///
+/// use std::future::{ready, Ready, IntoFuture};
+///
+/// /// Eventually multiply two numbers
+/// pub struct Multiply {
+///     num: u16,
+///     factor: u16,
+/// }
+///
+/// impl Multiply {
+///     /// Construct a new instance of `Multiply`.
+///     pub fn new(num: u16, factor: u16) -> Self {
+///         Self { num, factor }
+///     }
+///
+///     /// Set the number to multiply by the factor.
+///     pub fn number(mut self, num: u16) -> Self {
+///         self.num = num;
+///         self
+///     }
+///
+///     /// Set the factor to multiply the number with.
+///     pub fn factor(mut self, factor: u16) -> Self {
+///         self.factor = factor;
+///         self
+///     }
+/// }
+///
+/// impl IntoFuture for Multiply {
+///     type Output = u16;
+///     type IntoFuture = Ready<Self::Output>;
+///
+///     fn into_future(self) -> Self::IntoFuture {
+///         ready(self.num * self.factor)
+///     }
+/// }
+///
+/// // NOTE: Rust does not yet have an `async fn main` function, that functionality
+/// // currently only exists in the ecosystem.
+/// async fn run() {
+///     let num = Multiply::new(0, 0)  // initialize the builder to number: 0, factor: 0
+///         .number(2)                 // change the number to 2
+///         .factor(2)                 // change the factor to 2
+///         .await;                    // convert to future and .await
+///
+///     assert_eq!(num, 4);
+/// }
+/// ```
+///
+/// # Usage in trait bounds
+///
+/// Using `IntoFuture` in trait bounds allows a function to be generic over both
+/// `Future` and `IntoFuture`. This is convenient for users of the function, so
+/// when they are using it they don't have to make an extra call to
+/// `IntoFuture::into_future` to obtain an instance of `Future`:
+///
+/// ```rust
+/// #![feature(into_future)]
+///
+/// use std::future::IntoFuture;
+///
+/// /// Convert the output of a future to a string.
+/// async fn fut_to_string<Fut>(fut: Fut) -> String
+/// where
+///     Fut: IntoFuture,
+///     Fut::Output: std::fmt::Debug,
+/// {
+///     format!("{:?}", fut.await)
+/// }
+/// ```
 #[unstable(feature = "into_future", issue = "67644")]
 pub trait IntoFuture {
     /// The output that the future will produce on completion.
@@ -12,6 +115,22 @@ pub trait IntoFuture {
     type IntoFuture: Future<Output = Self::Output>;
 
     /// Creates a future from a value.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// #![feature(into_future)]
+    ///
+    /// use std::future::IntoFuture;
+    ///
+    /// # async fn foo() {
+    /// let v = async { "meow" };
+    /// let mut fut = v.into_future();
+    /// assert_eq!("meow", fut.await);
+    /// # }
+    /// ```
     #[unstable(feature = "into_future", issue = "67644")]
     #[lang = "into_future"]
     fn into_future(self) -> Self::IntoFuture;