Return future with lifetime that's different from &selfs

[cpick]

I have a Stream that needs to run a Future from a trait method on each iteration. I've managed to get it to work using a LocaslBoxFuture:

pub trait Make {
    // working
    fn make<'a, T: Default + 'a>(&self) -> LocalBoxFuture<'a, T> {
        Box::pin(future::ready(T::default()))
    }
}

struct Foo {}
impl Make for Foo {}

fn main() {
    let f = Foo {};
    block_on_stream(stream::empty().map(move |_: ()| f.make::<bool>()));
}

However, I haven't been able to figure out how to accomplish the same with async_trait. For example, attempting the following fails:

#[async_trait]
pub trait Make {
    // broken
    async fn make<'a, T: Default + 'a>(&self) -> T {
        T::default()
    }
}

With error:

error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
  --> src/main.rs:20:56
   |
20 |     block_on_stream(stream::empty().map(move |_: ()| f.make::<bool>()));
   |                                                        ^^^^
   |
note: first, the lifetime cannot outlive the lifetime '_ as defined on the body at 20:41...
  --> src/main.rs:20:41
   |
20 |     block_on_stream(stream::empty().map(move |_: ()| f.make::<bool>()));
   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...so that closure can access `f`
  --> src/main.rs:20:54
   |
20 |     block_on_stream(stream::empty().map(move |_: ()| f.make::<bool>()));
   |                                                      ^
note: but, the lifetime must be valid for the method call at 20:21...
  --> src/main.rs:20:21
   |
20 |     block_on_stream(stream::empty().map(move |_: ()| f.make::<bool>()));
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...so that a type/lifetime parameter is in scope here
  --> src/main.rs:20:21
   |
20 |     block_on_stream(stream::empty().map(move |_: ()| f.make::<bool>()));
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

error: could not compile `stream-async-trait`.

I've tried various permutations with 'async_trait and 'a leading to similar results.

I'm trying to figure out if what I'm attempting is supported and I'm just missing something?
Perhaps this is related to #8 (I don't have quite a strong enough handle on Futures/lifetimes to tell for sure)?

[dtolnay]

fn make<'a, T: Default + 'a>(&self) -> LocalBoxFuture<'a, T> {

This signature is not expressible using async fn in Rust (whether or not async_trait is involved). An async fn in Rust does not begin executing until the first time it is awaited, which means every input lifetime is necessarily captured by the future.

[cpick]

Ahhh, that's interesting and makes sense.

So it sounds like while it would be technically possible for async_trait to support this (because of its current implementation), such support wouldn't really be in keeping with the intent of this crate (which I assume is to act just as as closely to how async trait members will work if they're ever implemented upstream)?

If that's true, I think this issue should be closed, thank you.

I realize this is now just a general async rust question, so of course feel free to just redirect me to StackOverflow :), but I was wondering if there was a common/recommended pattern to accomplish what I'm after. I'm thinking that I'll do the following and just accept that LocalBoxFuture will be with me for the duration:

fn make<'a, T: Default + 'a>(&self) -> LocalBoxFuture<'a, T> {
    // do all work with `self` here

    Box::pin(async {
        // use nice `?` error-handling here
        T::default()
    })
}

[dtolnay]

Your approach seems fine to me. Alternatively:

fn make<T: Default>(&self) -> impl Future<Output = T> {
    // using `self`

    async {
        T::default()
    }
}