特质名称后的特质是什么意思?

这是在声明 supertrait . 相当于:

trait Enchanter
where
    Self: std::fmt::Debug,
{
}

简而言之,它需要任何想要实现的类型 Enchanter 也要实施 std::fmt::Debug . 否则 an error will be raised :

error[E0277]: `S` doesn't implement `Debug`
 --> src/lib.rs:4:6
  |
4 | impl Enchanter for S {}
  |      ^^^^^^^^^ `S` cannot be formatted using `{:?}`
  |
  = help: the trait `Debug` is not implemented for `S`
  = note: add `#[derive(Debug)]` to `S` or manually `impl Debug for S`
note: required by a bound in `Enchanter`
 --> src/lib.rs:1:18
  |
1 | trait Enchanter: std::fmt::Debug {}
  |                  ^^^^^^^^^^^^^^^ required by this bound in `Enchanter`