Question

当创建存储DST的结构（例如，原始切片）时，我可以使用普通的#[derive(Eq, PartialEq, Ord, PartialOrd)]工具来获取我的类型上该特征的实现：

#[derive(PartialEq, Eq, PartialOrd, Ord)]
struct A([u8]);

但是，如果我手动实现它们，那么编译器会抱怨我的类型没有实现Sized：

struct A([u8]);

impl AsRef<[u8]> for A {
    fn as_ref(&self) -> &[u8] {
        &self.0
    }
}

impl<S: AsRef<[u8]>> PartialEq<S> for A {
    fn eq(&self, other: &S) -> bool {
        self.0.eq(other.as_ref())
    }
}

impl Eq for A { }

impl<S: AsRef<[u8]>> PartialOrd<S> for A {
    fn partial_cmp(&self, other: &S) -> Option<Ordering> {
        let  slice: &[u8] = &self.0;
        slice.partial_cmp(other.as_ref())
    }
}

impl Ord for A {
    fn cmp(&self, other: &Self) -> Ordering {
        self.partial_cmp(&other).unwrap()
    }
}

编译结果：

rustc 1.12.0 (3191fbae9 2016-09-23)
error[E0277]: the trait bound `[u8]: std::marker::Sized` is not satisfied
  --> <anon>:20:6
   |
20 | impl Eq for A { }
   |      ^^
   |
   = note: `[u8]` does not have a constant size known at compile-time
   = note: required because it appears within the type `A`
   = note: required because of the requirements on the impl of `std::cmp::PartialEq` for `A`
   = note: required by `std::cmp::Eq`

error[E0277]: the trait bound `[u8]: std::marker::Sized` is not satisfied
  --> <anon>:29:6
   |
29 | impl Ord for A {
   |      ^^^
   |
   = note: `[u8]` does not have a constant size known at compile-time
   = note: required because it appears within the type `A`
   = note: required because of the requirements on the impl of `std::cmp::PartialOrd` for `A`
   = note: required by `std::cmp::Ord`

如果我创建具有固定大小的类型的变体（例如通过将其转换为固定大小的数组），那么我可以手动实现这些特征没问题。

struct B([u8; 5]);

impl AsRef<[u8]> for B {
    fn as_ref(&self) -> &[u8] {
        &self.0
    }
}

impl<S: AsRef<[u8]>> PartialEq<S> for B {
    fn eq(&self, other: &S) -> bool {
        self.0.eq(other.as_ref())
    }
}

impl Eq for B { }

impl<S: AsRef<[u8]>> PartialOrd<S> for B {
    fn partial_cmp(&self, other: &S) -> Option<Ordering> {
        let  slice: &[u8] = &self.0;
        slice.partial_cmp(other.as_ref())
    }
}

impl Ord for B {
    fn cmp(&self, other: &Self) -> Ordering {
        self.partial_cmp(&other).unwrap()
    }
}

Here is a playground link showing the issue

我的问题是 - 如何在我的自定义DST上实施Ord和Eq，以便我可以利用任何partial_cmp/eq AsRef<[u8]>的事实}，还可以使用它来强制执行Ord / Eq特征实现的边界，就像我在我的示例中使用struct B一样？

Answer 1

问题是S: AsRef<[u8]>默认情况下还限制S到Sized种类型。您需要使用?Sized选择退出。

所有泛型类型参数都隐式绑定Sized，因此?Sized可用于选择退出隐式绑定。

From the Rust Book

以下编辑对我来说很好：

use std::cmp::Ordering;

struct A([u8]);

impl AsRef<[u8]> for A {
    fn as_ref(&self) -> &[u8] {
        &self.0
    }
}

impl<S: AsRef<[u8]> + ?Sized> PartialEq<S> for A {
    fn eq(&self, other: &S) -> bool {
        self.0.eq(other.as_ref())
    }
}

impl Eq for A {}

impl<S: AsRef<[u8]> + ?Sized> PartialOrd<S> for A {
    fn partial_cmp(&self, other: &S) -> Option<Ordering> {
        let slice: &[u8] = &self.0;
        slice.partial_cmp(other.as_ref())
    }
}

impl Ord for A {
    fn cmp(&self, other: &Self) -> Ordering {
        self.partial_cmp(&other).unwrap()
    }
}

Demo

为什么我得到一个`trait bound` [T]：当我尝试在unsized类型上手动实现Ord和Eq时，不满足std :: marker :: Sized`？

1 个答案: