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.

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 allow their values to be modified multiple times before being .awaited.

use std::future::{ready, Ready, IntoFuture};

/// Eventually multiply two numbers
pub struct Multiply {
    num: u16,
    factor: u16,
}

impl Multiply {
    /// Constructs 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:

use std::future::IntoFuture;

/// Converts 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)
}