error[E0505]: cannot move out of `s` because it is borrowed
  --> $DIR/btreemap_dropck.rs:15:10
   |
LL |     let s = String::from("Hello World!");
   |         - binding `s` declared here
LL |     let _map = BTreeMap::from_iter([((), PrintOnDrop(&s))]);
   |                                                      -- borrow of `s` occurs here
LL |     drop(s);
   |          ^ move out of `s` occurs here
LL | }
   | - borrow might be used here, when `_map` is dropped and runs the `Drop` code for type `BTreeMap`
   |
help: consider cloning the value if the performance cost is acceptable
   |
LL |     let _map = BTreeMap::from_iter([((), PrintOnDrop(&s.clone()))]);
   |                                                        ++++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0505`.