error[E0271]: type mismatch resolving `<[async block@$DIR/async.rs:12:17: 12:25] as Future>::Output == i32` --> $DIR/async.rs:12:17 | LL | needs_async(async {}); | ----------- ^^^^^^^^ types differ | | | required by a bound introduced by this call | note: required by a bound in `needs_async` --> $DIR/async.rs:8:31 | LL | fn needs_async(_: impl Future) {} | ^^^^^^^^^^^^ required by this bound in `needs_async` error: aborting due to previous error For more information about this error, try `rustc --explain E0271`.