async fn unmet lifetime constraints produce confusing diagnostics

[s97712]

use std::future::Future;

struct Foo<'a> {
  pub value: &'a str
}

fn mode_1(_b: Foo<'_>) -> impl Future<Output=()> {
  async {}
}

async fn mode_2(_b: Foo<'_>) {
}


fn test<F, Fut>(_f: F ) 
  where
    Fut: Future<Output=()>,
    F: FnOnce(Foo<'_>) -> Fut,
{
    
}


fn main() {

    // here can compiled
    test(mode_1);
    
    // here can't compiled
    test(mode_2);
}

   Compiling playground v0.0.1 (/playground)
error[E0271]: type mismatch resolving `for<'r> <for<'_> fn(Foo<'_>) -> impl std::future::Future {mode_2} as std::ops::FnOnce<(Foo<'r>,)>>::Output == _`
  --> src/main.rs:29:5
   |
14 | fn test<F, Fut>(_f: F ) 
   |    ----
...
17 |     F: FnOnce(Foo<'_>) -> Fut,
   |                           --- required by this bound in `test`
...
29 |     test(mode_2);
   |     ^^^^ expected bound lifetime parameter, found concrete lifetime

error: aborting due to previous error

For more information about this error, try `rustc --explain E0271`.

rust playground

[estebank]

Note that you can write code that will compile for this, it is just not obvious (note the added explicit lifetime 'r):

fn test<'r, F, Fut>(_f: F) 
  where
    Fut: Future<Output=()> + 'r,
    F: FnOnce(Foo<'r>) -> Fut,

The reason one works and the other one doesn't is because they are not equivalent. mode_2 gets desugared to

fn mode_2(_b: Foo<'_>) -> impl Future<Output=()> + '_ { // note the relaxing of the lifetime constraint to the return type
  async {}
}

which also doesn't compile unless you add a lifetime to test.

I believe there are already quite a few A-async-await tickets about the default behavior of lifetimes needing some work.

[s97712]

I try to use async fn as callback:

fn test<F, Fut>(_f: F) 
  where
    Fut: Future<Output=()>,
    F: for<'r> FnOnce(Foo<'r>) -> Fut +'r,

but get a error:

17 |     F: for<'r> FnOnce(Foo<'r>) -> Fut + 'r,
   |                                         ^^ undeclared lifetime

Is there a solution to this problem?

[Dylan-DPC]

Current output:

Error[E0308]: mismatched types
  --> src/main.rs:29:5
   |
29 |     test(mode_2);
   |     ^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected opaque type `impl for<'a> Future<Output = ()>`
              found opaque type `impl Future<Output = ()>`
   = note: distinct uses of `impl Trait` result in different opaque types
note: the lifetime requirement is introduced here
  --> src/main.rs:17:27
   |
17 |     F: FnOnce(Foo<'_>) -> Fut,
   |                           ^^^

For more information about this error, try `rustc --explain E0308`.