How do you make a trait function return a future with a lifetime?

[josephg]

Hi! I'm trying to make a trait method return a future. The future might need to access member variables, and I can't get it to work.

#![feature(type_alias_impl_trait)]
use std::future::*;

struct Foo(i32);

// Outside of a trait this all works fine
type T<'a> = impl Future<Output=i32> + 'a;
impl Foo {
    async fn ref_async(self: &Self) -> i32 {
        self.0
    }
    // All these variants are ok.
    // fn blah<'a>(self: &'a Self) -> impl Future<Output=i32> + 'a {
    // fn blah<'a>(self: &'a Self) -> T<'a> {
    fn blah(self: &Self) -> T<'_> {
        self.ref_async()
    }
}

But everything goes a bit hairbrained in a trait:

trait SomeTrait {
    type TT;
    fn blah(self: &Self) -> Self::TT;
}

impl SomeTrait for Foo {
    type TT<'a> = impl Future<Output=i32> + 'a;
    fn blah<'a>(self: &'a Self) -> Self::TT<'a> { // <- Errors..
        self.ref_async()
    }
}

(Playground)

I can't seem to figure out how to associate the lifetime of my self object with the return value. Does this need GAT or something?

[sfackler]

Yes, you can't express that associated type currently.

The alternative is fn blah<'a>(&'a self) -> Pin<Box<dyn Future<Output = i32> + 'a>> { ... }

[josephg]

Aww! I was hoping to avoid allocations, though I suppose it shouldn't make much difference in practice for what I'm working on. It itches doing it this way knowing allocation-free futures are right around the corner though

Thanks for your help!

[Yandros]

I don't know if it really applies to your use case, but for a related issue there is the ::async_trait crate, that will automagically Pin<Box<dyn Future>> the return type of the async fns methods of a trait.

[josephg]

I saw that and it looks nice, but seems that it just adds sugar on top of doing what @sfackler said above. It also makes errors in my functions get misattributed to the macro in my IDE, which is pretty bad while I'm learning (when I make mistakes I need all the help I can get).