无法为实现我拥有的特征的所有类型实现我不拥有的特征 [英] Can't implement a trait I don't own for all types that implement a trait I do own

问题描述

pub trait AllValues {
    fn all_values() -> Vec<Self> where Self: std::marker::Sized;
}

use rand::Rand;
use rand::Rng;
impl<T: AllValues + Sized> Rand for T {
    fn rand<R: Rng, T>(rng: &mut R) -> T {
        let values = T::all_values();

        let len = values.len();

        if len == 0 {
            panic!("Cannot pick a random value because T::all_values() returned an empty vector!")
        } else {
            let i = rng.gen_range(0, len);

            values[i]
        }
    }
}

上述代码产生以下编译时错误:

The preceding code produces the following compile-time error:

error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g. `MyStruct<T>`); only traits defined in the current crate can be implemented for a type parameter
   --> src/lib.rs:137:1
    |
137 | impl<T: AllValues + Sized> Rand for T {
    | ^

根据这里提到的实现traits的限制，我应该能够为 AllValues 实现 Rand，因为 AllValues 是在我的箱子中定义的.一致性/孤儿impl的规则实际上允许这样做吗?如果是这样，为实现 AllValues 的事物实现 Rand 的正确方法是什么?

According to the restrictions on implementing traits mentioned here I should be able to implement Rand for AllValues since AllValues is defined in my crate. Is this actually allowed by the coherence/orphan impls rules? And if so, what is the right way to implement Rand for things that implement AllValues?

推荐答案

我应该能够为 AllValues 实现 Rand，因为 AllValues 是在我的 crate 中定义的.

I should be able to implement Rand for AllValues since AllValues is defined in my crate.

不，您只能为未定义的类型实现自己的特征 AllValues.你不能从逻辑上跳到实现一个你也没有定义的无关特征.

No, you are only allowed to implement your own trait AllValues for types you didn't define. You can't make the logical jump to implementing an unrelated trait that you also didn't define.

有两个注意事项需要记住:

There are two considerations to remember:

• 如果您的 trait 是公开的(它基于您提供的代码)，那么您并不是唯一可以实现该 trait 的人.您的 crate 的使用者可能能够为他们自己的类型实现它，他们也可能决定在其中实现 Rand！
• rand crate 可能会决定在未来某个时候为 T 实现 Rand.

• If your trait is public (which it is based on the code you've provided), you aren't the only one that can implement the trait. Consumers of your crate might be able to implement it for their own types, where they might also decide to implement Rand!
• The rand crate might decide to implement Rand for T some time in the future.

为实现 AllValues 的事物实现 Rand 的正确方法是什么?

What is the right way to implement Rand for things that implement AllValues?

我不相信有.我只是引入了一个包装器类型，它保存一个值或一个值的引用，该值实现了你的 trait 并为此实现了 Rand.

I don't believe there is one. I'd just introduce a wrapper type that holds a value or a reference to a value that implements your trait and implement Rand for that.

另见:

• 如何为我不拥有的类型实现我不拥有的特征?

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