Rust <7>：数据结构==>链表

 enum List {
     Cons(u64, Box<List>),
     NULL,
 }
 
 impl List {
     fn new() -> List {
         List::NULL
     }
 
     fn prepend(self, elem: u64) -> List {
         List::Cons(elem, Box::new(self))
     }
 
     fn len(&self) -> u64 {
         let mut t = self;
         let mut res = 0u64;
         while let List::Cons(_, ref next) = *t {
             res += ;
             t = next;
         }
 
         res
     }
 
     fn stringify(&self) -> String {
         let mut t = self;
         let mut res = String::new();
         while let List::Cons(r, ref next) = *t {
             res.push_str(&format!("{}==>", r));
             t = next;
         }
 
         res.push_str(&format!("NULL"));
 
         res
     }
 }
 
 fn main() {
     let mut list = List::new();
 
     for x in .. {
         list = list.prepend(x);
     }
 
     println!("{}", list.stringify());
     println!("linked list has length: {}", list.len());
 }