It doesn't actually expand to that any more. You can see what it does expand to by running rustc --pretty expanded with a recent compiler:

  for pattern in iterator {
      body
  }

becomes

  match &mut iterator {
      i => loop {
          match i.next() {
              None => break ,
              Some(mut _value) => { let pattern = _value; { body } }
          }
      }
  }