error: future cannot be sent between threads safely --> $DIR/issue-70935-complex-spans.rs:13:45 | LL | fn foo(tx: std::sync::mpsc::Sender) -> impl Future + Send { | ^^^^^^^^^^^^^^^^^^ future created by async block is not `Send` | = help: the trait `Sync` is not implemented for `Sender` note: future is not `Send` as this value is used across an await --> $DIR/issue-70935-complex-spans.rs:19:11 | LL | baz(|| async{ | _____________- LL | | foo(tx.clone()); LL | | }).await; | | - ^^^^^^- the value is later dropped here | | | | | |_________| await occurs here, with the value maybe used later | has type `[closure@$DIR/issue-70935-complex-spans.rs:17:13: 17:15]` which is not `Send` error: aborting due to previous error