How to write a helper function to retry a future in a loop?

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2

I've found a similar question in Issue with FnOnce and async function with a &reference type argument, and managed to apply it to this simplified example, using the nightly type_alias_impl_trait feature.

#![feature(type_alias_impl_trait)]

use std::future::Future;

struct Foo {}

type FutBool<'a> = impl 'a + Future<Output = bool>;

impl Foo {
    // Operation that may fail and should be retried.
    async fn operation(&mut self) -> bool {
        unimplemented!()
    }

    async fn indirect(&mut self) {
        self.retry_loop(|this| this.operation()).await
    }

    async fn retry_loop<F>(&mut self, mut f: F)
    where
        for<'any> F: FnMut(&'any mut Foo) -> FutBool<'any>,
    {
        loop {
            if f(self).await {
                return;
            }
        }
    }
}

However, one thing to note (which is however non-blocking) is that I couldn't make a type alias generic over the future's Output.

type Fut<'a, T> = impl 'a + Future<Output = T>;
type FutBool<'a> = Fut<'a, bool>;

error[E0271]: type mismatch resolving `<impl futures::Future as futures::Future>::Output == T`
 --> src/foo.rs:5:19
  |
5 | type Fut<'a, T> = impl 'a + Future<Output = T>;
  |              -    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `bool`, found type parameter `T`
  |              |
  |              this type parameter
  |
  = note:        expected type `bool`
          found type parameter `T`

Now, this was a simplified example, in reality the type Foo is rather a Bar<'a> which contains a reference. I'm now blocked at the following stage.

struct Bar<'a> {
    data: &'a str,
}

type FutBool<'x> = impl 'x + Future<Output = bool>;

impl<'a> Bar<'a> {
    async fn indirect_4(&mut self) {
        self.retry_loop_4(|this| this.operation()).await
    }

    async fn retry_loop_4<F>(&mut self, mut f: F)
    where
        for<'any> F: FnMut(&'any mut Bar) -> FutBool<'any>,
    {
        loop {
            if f(self).await {
                return;
            }
        }
    }

    async fn operation(&mut self) -> bool {
        unimplemented!()
    }
}

error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
  --> src/bar.rs:11:34
   |
11 |         self.retry_loop_4(|this| this.operation()).await
   |                                  ^
   |
note: hidden type `impl futures::Future` captures the anonymous lifetime #2 defined on the body at 11:27
  --> src/bar.rs:11:27
   |
11 |         self.retry_loop_4(|this| this.operation()).await
   |                           ^^^^^^^^^^^^^^^^^^^^^^^

error[E0477]: the type `impl futures::Future` does not fulfill the required lifetime
 --> src/bar.rs:7:20
  |
7 | type FutBool<'x> = impl 'x + Future<Output = bool>;
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: type must outlive the lifetime `'x` as defined on the item at 7:14
 --> src/bar.rs:7:14
  |
7 | type FutBool<'x> = impl 'x + Future<Output = bool>;