FnOnce closure can be called multiple times

help
6

If I add let ss = s between those two f(), the compiler will complain:

2 |     let s = String::from("test");
  |         - move occurs because `s` has type `String`, which does not implement the `Copy` trait
3 |     
4 |     let f = move || {
  |             ------- value moved into closure here
5 |         println!("{}", s);
  |                        - variable moved due to use in closure
...
9 |     let ss = s;
  |              ^ value used here after move