error[E0502]: cannot borrow `y` as immutable because it is also borrowed as mutable
  --> $DIR/dropck-resume.rs:26:13
   |
LL |     let z = &mut y;
   |             ------ mutable borrow occurs here
...
LL |         r = y.as_ref().unwrap();
   |             ^ immutable borrow occurs here
LL |
LL |     }
   |     - mutable borrow might be used here, when `g` is dropped and runs the destructor for coroutine

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0502`.