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`？

Question

在创建存储DST的结构(例如,原始切片)时,我可以使用常规#[derive(Eq, PartialEq, Ord, PartialOrd)]工具在我的类型上获得此特征的实现:

#[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`

如果我创建该类型的变体确实具有固定大小(例如,通过把它变成一个固定大小的阵列),那么我可以手动实现性状没有问题.

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()
    }
}

这是一个显示问题的游乐场链接.

我的问题是 - 我如何实现Ord和Eq我的自定义DST,以便我可以利用partial_cmp/eq任何事实AsRef<[u8]>,但也可以使用它来强制执行Ord/ Eqtrait实现的边界,就像我struct B在我的示例中所做的那样？

Answer 1

问题是默认情况下S: AsRef<[u8]>也限制S了Sized类型.您需要使用?Sized选择退出.

所有泛型类型参数都隐式具有Sized绑定,因此?Sized可以用于选择退出隐式绑定.

来自Rust Book.

以下编译对我来说很好:

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()
    }
}

演示