Self-reference is allowed, but it prevents moving.

  fn main() {
      // ok
      let mut inp = Input { words: vec![], phrase: String::from("foo bar") };
      inp.words.push(&inp.phrase[0..3]);
      inp.words.push(&inp.phrase[4..7]);
      println!("{:?}", inp);

      // this would give "cannot move out of `inp` because it is borrowed"
      //let x = inp;
  }