从特质和专业化实施

问题描述

根据专业化RFC，我应该能够在一个 struct 上有多个相同 trait 的 impl ，通过指定一个作为默认值.

According to the specialization RFC, I should be able to have multiple impls of the same trait on a struct by specifying one as default.

我有代码:

#![feature(specialization)]
struct A(u32);

trait Dummy {}

impl<T> From<T> for A
where
    T: Into<u32>,
{
    default fn from(item: T) -> Self {
        A(item.into())
    }
}

impl<T> From<T> for A
where
    T: Dummy,
{
    fn from(item: T) -> Self {
        A(2)
    }
}

即使其中一个实现是默认的，编译器仍然告诉我这两个实现是冲突的.

Even though one of the implementations is default, the compiler still tells me that the two are conflicting implementations.

推荐答案

您的第二个实现不是第一个的专业化.这是一个与第一个冲突的替代实现.

Your second implementation is not a specialization of the first. It's an alternative implementation that conflicts with the first.

专业化要求与您的第二个 impl 匹配的所有类型也与您的第一个 impl 匹配.换句话说，您的专业化边界需要是默认实现边界的严格子集.来自 RFC:

Specialization requires that all types matching your second impl also match your first impl. In other words the bounds of your specialization need to be a strict subset of the bounds of the default implementation. From the RFC:

该 RFC 提出了一种专门化设计，它允许多个 impl 块应用于同一类型/特征，只要其中一个块明显比另一个更具体".

将您的特征定义更改为

trait Dummy: Into<u32> {}

使您的代码编译.

查看 https://github.com/rust-lang/rfcs/blob/master/text/1210-impl-specialization.md 了解更多详情.

Check out https://github.com/rust-lang/rfcs/blob/master/text/1210-impl-specialization.md for more details.

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