关联类型

通过将内部类型局部地移动到 trait 作为输出类型，使用“关联类型”可以提高代码的整体可读性。 trait 定义的语法如下：

#![allow(unused)]
fn main() {
// `A` and `B` are defined in the trait via the `type` keyword.
// (Note: `type` in this context is different from `type` when used for
// aliases).
trait Contains {
    type A;
    type B;

    // Updated syntax to refer to these new types generically.
    fn contains(&self, _: &Self::A, _: &Self::B) -> bool;
}
}

请注意，使用 trait Contains 的函数不再需要表达 A 或 B

// Without using associated types
fn difference<A, B, C>(container: &C) -> i32 where
    C: Contains<A, B> { ... }

// Using associated types
fn difference<C: Contains>(container: &C) -> i32 { ... }

让我们使用关联类型重写上一节中的示例

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