error[E0626]: borrow may still be in use when coroutine yields
  --> $DIR/coroutine-with-nll.rs:8:17
   |
LL |     || {
   |     -- within this coroutine
...
LL |         let b = &mut true;
   |                 ^^^^^^^^^
LL |
LL |         yield ();
   |         -------- possible yield occurs here
   |
help: add `static` to mark this coroutine as unmovable
   |
LL |     static || {
   |     ++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0626`.