有没有办法在多个特征上实现一个特征? [英] Is there some way to implement a trait on multiple traits?

问题描述

为什么这不起作用:

trait Update {
    fn update(&mut self);
}

trait A {}
trait B {}

impl<T: A> Update for T {
    fn update(&mut self) {
        println!("A")
    }
}

impl<U: B> Update for U {
    fn update(&mut self) {
        println!("B")
    }
}

error[E0119]: conflicting implementations of trait `Update`:
  --> src/main.rs:14:1
   |
8  | impl<T: A> Update for T {
   | ----------------------- first implementation here
...
14 | impl<U: B> Update for U {
   | ^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation

如果类型重叠，我会假设稍后会检查它.

I would assume it is checked later if types overlap.

推荐答案

你希望这个程序的输出是什么?

What would you expect the output of this program to be?

struct AAndB {}
impl A for AAndB {}
impl B for AAndB {}

let a_and_b = AAndB {};
a_and_b.update();

有一个不稳定的编译器特性，specialization，您可以启用它在每晚构建中，它允许您有重叠的实例，并且使用最专业"的.

There is an unstable compiler feature, specialization, which you can enable in nightly builds, which lets you have overlapping instances, and the most "specialized" is used.

但是，即使启用了专业化，您的示例也不起作用，因为 A 和 B 是完全等效的，因此您永远无法明确选择一个实例.

But, even with specialization enabled, your example won't work because A and B are completely equivalent so you could never unambiguously pick an instance.

只要有一个明显更专业"的实例，它就会编译并按预期工作 - 前提是您使用的是启用了专业化的 nightly Rust 构建.例如，如果其中一个特征受到另一个特征的约束，那么它就更加专业化，所以这会起作用:

As soon as there is an obviously "more specialized" instance, it will compile and work as expected - provided you are using a nightly build of Rust with specialization enabled. For example, if one of the traits is bounded by the other, then it is more specialized, so this would work:

#![feature(specialization)]

trait Update {
    fn update(&mut self);
}

trait A {}
trait B: A {}

impl<T: A> Update for T {
    default fn update(&mut self) {
        println!("A")
    }
}

impl<U: B> Update for U {
    fn update(&mut self) {
        println!("B")
    }
}

将实现方法指定为 default 允许另一个更具体的实现定义自己的方法版本.

Specifying the implementation method as default allows another more specific implementation to define its own version of the method.

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