Lifetime of async closure return type

user_8763273 0
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Consider the following code:

async fn f(x: &i32) -> i32 {
    todo!()
}

fn g<F, Fut>(f: F)
where
    F: Send + Sync + 'static,
    for<'a> F: Fn(&'a i32) -> Fut,
    Fut: Future<Output = i32> + Send + Sync,
{
    todo!()
}

fn main() {
    g(f);
}

playground

The compiler complains with

error[E0308]: mismatched types
  --> src/main.rs:17:5
   |
17 |     g(f);
   |     ^ lifetime mismatch
   |
   = note: expected associated type `<for<'_> fn(&i32) -> impl Future {f} as FnOnce<(&i32,)>>::Output`
              found associated type `<for<'_> fn(&i32) -> impl Future {f} as FnOnce<(&'a i32,)>>::Output`
   = note: the required lifetime does not necessarily outlive the empty lifetime
note: the lifetime requirement is introduced here
  --> src/main.rs:10:31
   |
10 |     for<'a> F: Fn(&'a i32) -> Fut,
   |                               ^^^

I think the issue is that the lifetime of the return value in the async function is tied to the lifetime of x, but I'm not sure how to express this in the where clause for g?

async-await rust lifetime
1 Answers

i think you are looking for this:

use core::future::Future;

async fn f(x: &i32) -> i32 {
    todo!()
}

fn g<'a, F, Fut>(f: F)
where
    F: Send + Sync + 'static,
    F: Fn(&'a i32) -> Fut,
    Fut: Future<Output = i32> + Send + Sync,
{
    todo!()
}

fn main() {
    g(f);
}

Bind the lifetime of x to that of g (don't know for sure if this is what you want .. i don't know of a way to do this in the where clause)

user_161537620
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