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async / await + Box<Future> + Send does not compile #60658

carllerche opened this issue May 9, 2019 · 1 comment


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commented May 9, 2019


#![feature(async_await, await_macro)]

use std::future::Future;

pub trait Foo<'a> {
    type Future: Future<Output = ()>;
    fn foo() -> Self::Future;

struct MyType<T>;

impl<'a, T> Foo<'a> for MyType<T>
    T: Foo<'a>,
    T::Future: Send,
    type Future = Box<Future<Output = ()> + Send>;
    fn foo() -> Self::Future {
        Box::new(async move {

fn main() {


   Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
  --> src/
21 | /         Box::new(async move {
22 | |             await!(T::foo())
23 | |         })
   | |__________^ one type is more general than the other
   = note: expected type `std::marker::Send`
              found type `std::marker::Send`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: Could not compile `playground`.

To learn more, run the command again with --verbose.

Compiler version: rustc 1.36.0-nightly (2019-05-07)


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commented May 16, 2019

Expanding the constraint to

for<'b> <T as Foo<'b>>::Future: Send

gets past this error. I'm not sure why it needs this more general constraint though.

reproduction directly with generators

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