error: future cannot be sent between threads safely
  --> $DIR/issue-70935-complex-spans.rs:15:23
   |
LL | fn foo(x: NotSync) -> impl Future + Send {
   |                       ^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
   |
   = help: within `NotSync`, the trait `Sync` is not implemented for `*mut ()`
note: future is not `Send` as this value is used across an await
  --> $DIR/issue-70935-complex-spans.rs:20:12
   |
LL |           baz(|| async {
   |  _____________-
LL | |             foo(x.clone());
LL | |         }).await;
   | |         -  ^^^^^- the value is later dropped here
   | |         |  |
   | |_________|  await occurs here, with the value maybe used later
   |           has type `{closure@$DIR/issue-70935-complex-spans.rs:18:13: 18:15}` which is not `Send`

error: aborting due to previous error