error[E0597]: `ref_` does not live long enough
  --> $DIR/dropck.rs:23:18
   |
21 |     gen = || {
   |           -- capture occurs here
22 |         // but the generator can use it to drop a `Ref<'a, i32>`.
23 |         let _d = ref_.take(); //~ ERROR `ref_` does not live long enough
   |                  ^^^^ borrowed value does not live long enough
...
28 | }
   | - borrowed value dropped before borrower
   |
   = note: values in a scope are dropped in the opposite order they are created

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0597"