warning: unused coroutine that must be used
  --> $DIR/yield-in-args-rev.rs:13:18
   |
LL |       #[coroutine] || {
   |  __________________^
LL | |         let b = true;
LL | |         foo(yield, &b);
LL | |     };
   | |_____^
   |
   = note: coroutines are lazy and do nothing unless resumed
   = note: `#[warn(unused_must_use)]` on by default

warning: 1 warning emitted