async/await: cannot move out of captured variable in an `Fn` closure

[leinlawun]

Hello, recently faced with strange behavior, please comment, is this a bug, or not? Compiler version: 1.35.0-nightly 2019-04-12 99da733f7f38ce8fe684

#![feature(futures_api, async_await, await_macro)]

use std::future::Future;

struct Task<T> {
    task: T,
}

impl<T> Task<T>
where
    T: Fn(),
{
    fn new(task: T) -> Self {
        Self { task }
    }

    fn execute(&self) {
        (self.task)();
    }
}

struct AsyncTask<T> {
    task: T,
}

impl<T, F> AsyncTask<T>
where
    T: Fn() -> F,
    F: Future<Output = ()>,
{
    fn new(task: T) -> Self {
        Self { task }
    }

    async fn execute(&self) {
        await!((self.task)());
    }
}

fn main() {
    let string = "Hello, World!".to_string();
    let _task = Task::new(move || {
        println!("{}", string);
    });

    let string = "Hello, World!".to_string();
    let _async_task = AsyncTask::new(async move || {
        println!("{}", string);
    });
}

(Playground) (Updated Playground

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0507]: cannot move out of captured variable in an `Fn` closure
  --> src/main.rs:47:52
   |
46 |       let string = "Hello, World!".to_string();
   |           ------ captured outer variable
47 |       let _async_task = AsyncTask::new(async move || {
   |  ____________________________________________________^
48 | |         println!("{}", string);
49 | |     });
   | |_____^ cannot move out of captured variable in an `Fn` closure

error: aborting due to previous errors

For more information about this error, try `rustc --explain E0507`.
error: Could not compile `playground`.

To learn more, run the command again with --verbose.

[nikomatsakis]

Marking as deferred -- async-await closures are not part of the core async fn functionality we expect to stabilize.

@leinlawun -- I'm not sure precisely what the problem is here, but in general async closures don't really work correctly.

[compiler-errors]

This is expressible now with reworked async closures (#120361) and async Fn() bounds:

#![feature(async_closure)]
#![allow(unused)]

struct Task<T> {
    task: T,
}

impl<T> Task<T>
where
    T: Fn(),
{
    fn new(task: T) -> Self {
        Self { task }
    }

    fn execute(&self) {
        (self.task)();
    }
}

struct AsyncTask<T> {
    task: T,
}

impl<T> AsyncTask<T>
where
    T: async Fn(),
{
    fn new(task: T) -> Self {
        Self { task }
    }

    async fn execute(&self) {
        (self.task)().await;
    }
}

fn main() {
    let string = "Hello, World!".to_string();
    let _task = Task::new(move || {
        println!("{}", string);
    });

    let string = "Hello, World!".to_string();
    let _async_task = AsyncTask::new(async move || {
        println!("{}", string);
    });
}

[dev-ardi]

Shouldn't this be closed then?

[compiler-errors]

@dev-ardi: No. Async closures are not stable.

[dev-ardi]

Is that how it works? If an issue in stable 1.78 is fixed in release 1.80 when is the issue closed? When the fixing PR is closed, at the beta release or when the fix becomes stable?