error[E0277]: `Sender<i32>` cannot be shared between threads safely
  --> $DIR/issue-70935-complex-spans.rs:12:45
   |
LL | fn foo(tx: std::sync::mpsc::Sender<i32>) -> impl Future + Send {
   |                                             ^^^^^^^^^^^^^^^^^^ `Sender<i32>` cannot be shared between threads safely
   |
   = help: the trait `Sync` is not implemented for `Sender<i32>`
   = note: required because of the requirements on the impl of `Send` for `&Sender<i32>`
note: required because it's used within this closure
  --> $DIR/issue-70935-complex-spans.rs:16:13
   |
LL |         baz(|| async{
   |             ^^
note: required because it's used within this `async fn` body
  --> $DIR/issue-70935-complex-spans.rs:9:67
   |
LL |   async fn baz<T>(_c: impl FnMut() -> T) where T: Future<Output=()> {
   |  ___________________________________________________________________^
LL | | }
   | |_^
   = note: required because it captures the following types: `ResumeTy`, `impl for<'r, 's, 't0> Future<Output = ()>`, `()`
note: required because it's used within this `async` block
  --> $DIR/issue-70935-complex-spans.rs:15:16
   |
LL |       async move {
   |  ________________^
LL | |         baz(|| async{
LL | |             foo(tx.clone());
LL | |         }).await;
LL | |     }
   | |_____^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.