error[E0277]: `impl Future` cannot be sent between threads safely --> $DIR/send-on-async-fn-in-trait.rs:13:16 | LL | needs_send(T::test()); | ---------- ^^^^^^^^^ `impl Future` cannot be sent between threads safely | | | required by a bound introduced by this call | = help: the trait `Send` is not implemented for `impl Future` note: required by a bound in `needs_send` --> $DIR/send-on-async-fn-in-trait.rs:12:27 | LL | fn needs_send(_: impl Send) {} | ^^^^ required by this bound in `needs_send` help: `Send` can be made part of the associated future's guarantees for all implementations of `Foo::test` | LL - async fn test() -> () {} LL + fn test() -> impl std::future::Future + Send { async {} } | error[E0277]: `impl Future` cannot be sent between threads safely --> $DIR/send-on-async-fn-in-trait.rs:15:16 | LL | needs_send(T::test2()); | ---------- ^^^^^^^^^^ `impl Future` cannot be sent between threads safely | | | required by a bound introduced by this call | = help: the trait `Send` is not implemented for `impl Future` note: required by a bound in `needs_send` --> $DIR/send-on-async-fn-in-trait.rs:12:27 | LL | fn needs_send(_: impl Send) {} | ^^^^ required by this bound in `needs_send` help: `Send` can be made part of the associated future's guarantees for all implementations of `Foo::test2` | LL - async fn test2() -> i32 { 1 + 2 } LL + fn test2() -> impl std::future::Future + Send {async { 1 + 2 } } | error: aborting due to 2 previous errors For more information about this error, try `rustc --explain E0277`.