error: future cannot be sent between threads safely
  --> $DIR/issue-70935-complex-spans.rs:10:45
   |
LL | fn foo(tx: std::sync::mpsc::Sender<i32>) -> impl Future + Send {
   |                                             ^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
   |
   = help: the trait `Sync` is not implemented for `Sender<i32>`
note: future is not `Send` as this value is used across an await
  --> $DIR/issue-70935-complex-spans.rs:13:9
   |
LL | /         baz(|| async{
LL | |             foo(tx.clone());
LL | |         }).await;
   | |________________^ first, await occurs here, with the value maybe used later...
note: the value is later dropped here
  --> $DIR/issue-70935-complex-spans.rs:15:17
   |
LL |         }).await;
   |                 ^
note: this has type `[closure@$DIR/issue-70935-complex-spans.rs:13:13: 15:10]` which is not `Send`
  --> $DIR/issue-70935-complex-spans.rs:13:13
   |
LL |           baz(|| async{
   |  _____________^
LL | |             foo(tx.clone());
LL | |         }).await;
   | |_________^

error: aborting due to previous error