How to accept an async function as an argument?

The async |...| expr closure syntax is available on the nightly channel enabling the feature async_closure.

#![feature(async_closure)]

use futures::future;
use futures::Future;
use tokio;

pub struct Bar;

impl Bar {
    pub fn map<F, T>(&self, f: F)
    where
        F: Fn(i32) -> T,
        T: Future<Output = Result<i32, i32>> + Send + 'static,
    {
        tokio::spawn(f(1));
    }
}

async fn foo(x: i32) -> Result<i32, i32> {
    println!("running foo");
    future::ok::<i32, i32>(x).await
}

#[tokio::main]
async fn main() {
    let bar = Bar;
    let x = 1;

    bar.map(foo);

    bar.map(async move |x| {
        println!("hello from async closure.");
        future::ok::<i32, i32>(x).await
    });
}

See the 2394-async_await RFC for more detalis

async functions are effectively desugared as returning impl Future. Once you know that, it's a matter of combining existing Rust techniques to accept a function / closure, resulting in a function with two generic types:

use std::future::Future;

async fn example<F, Fut>(f: F)
where
    F: FnOnce(i32, i32) -> Fut,
    Fut: Future<Output = bool>,
{
    f(1, 2).await;
}

This can also be written as

use std::future::Future;

async fn example<Fut>(f: impl FnOnce(i32, i32) -> Fut)
where
    Fut: Future<Output = bool>,
{
    f(1, 2).await;
}

• How do you pass a Rust function as a parameter?
• What is the concrete type of a future returned from `async fn`?
• What is the purpose of async/await in Rust?
• How can I store an async function in a struct and call it from a struct instance?
• What is the difference between `|_| async move {}` and `async move |_| {}`