trait可以有默认实现，这样可以做到类似template method的设计模式，例如

    1. trait Stores<TModel> {
    2.     fn get_by_id(&self, id: u64) -> TModel;
    3. }
    5. trait Storage {
    6.     fn query<TModel>(&self, id: u64) -> TModel 
    7.         where 
    8.             Self: Stores<TModel> // Self constrain
    9.     {
    10.         self.get_by_id(id)
    11.     }
    12. }
    13. struct UserDatabase {}
    15. impl Storage  for UserDatabase {}

    当UserDatabase实现了Storage这个trait后，就有了query的能力，这里可以看到where约束除了可以约束泛型类型T外，还能约束 Self 这里约束了Self必须实现 Stores<TModel>, 这样在query方法中就能调用Stores 的方法，我们就能为每种类型的实现get_by_id这个方法

    1. impl Stores<User> for UserDatabase {
    2.     fn get_by_id(&self, id: u64) -> User {
    3.         User{id: 123}
    4.     }
    5. }
    6. impl <'a> Stores<Avatar<'a>> for UserDatabase {
    7.     fn get_by_id(&self, id: u64) -> Avatar<'a> {
    8.         Avatar{id: 123, url: url}
    9.     }
    10. }
    13. #[derive(Debug)]
    14. struct User {
    15.     id: u64,
    16. }
    18. #[derive(Debug)]
    19. struct Avatar<'a> {
    20.     id: u64,
    21.     url: &'a str,
    22. }
    1. fn main() {
    2.     let db = UserDatabase{};
    3.     let u = db.query::<User>(1);
    4.     println!("user is {:?}", u);
    5.     let a = db.query::<Avatar>(2);
    6.     println!("Avatar is {:?}", a);
    7. }