rustp-14.自定义迭代器

struct Counter {
    count: u32,
  }
impl Counter {
    fn new() -> Counter {
        Counter {
            count: 0,
          }
      }
  }
impl Iterator for Counter {
    type Item = u32;
    fn next(&mut self) -> Option<Self::Item> {
        self.count += 1;
        if self.count < 6 {
            Some(self.count) 
          } else {
            None 
          }
      }
  }
fn main() {
  let mut counter = Counter::new();
  for i in (0..6) {
      if let Some(v) = counter.next() {
          println!("i = {},v = {}",i,v);
        } else {
          println!("i = {},at end",i);
          break;
        }
    }
  }