为什么我的 Trait 实现不匹配？

Question

如何编译这段代码？

trait Pair<'a, A, B> {
    fn first_ref(&'a self) -> &'a A;
    fn second_ref(&'a self) -> &'a B;
};

struct PairOwned<A, B> {
    first: A,
    second: B,
}

// Only implemented for the cases we are interested in ...
impl<'a, A, B> Pair<'a, A, B> for &'a PairOwned<&'a A,&'a B> {
    fn first_ref(&'a self) -> &'a A {
        self.first
    }
    fn second_ref(&'a self) -> &'a B {
        self.second
    }
}

impl<'a, A, B> Pair<'a, A, B> for &'a(&'a A, &'a B) {
    fn first_ref(&'a self) -> &'a A {
        self.0
    }
    fn second_ref(&'a self) -> &'a B {
        self.1
    }
}

fn pair_transformer<'a, I, T>(pairs: I) -> String
    where   T: Pair<'a, &'a Str, &'a Str> + 'a,
            I: Iterator<Item=T> {
    let mut s = String::new();
    for pair in pairs {
        s = s
            + pair.first_ref().as_slice()
            + pair.second_ref().as_slice();
    }
    s
}

pair_transformer([PairOwned { first: "a", second: "b" }].iter());
pair_transformer([("a", "b")].iter());

编译器说：

tests/lang.rs:902:5: 902:21 error: the trait `pair_trait_for_iteration::Pair<'_, &core::str::Str, &core::str::Str>` is not implemented for the type `&pair_trait_for_iteration::PairOwned<&str, &str>` [E0277]
 tests/lang.rs:902     pair_transformer([PairOwned { first: "a", second: "b" }].iter());
                      ^~~~~~~~~~~~~~~~
tests/lang.rs:903:5: 903:21 error: the trait `pair_trait_for_iteration::Pair<'_, &core::str::Str, &core::str::Str>` is not implemented for the type `&(&str, &str)` [E0277]
tests/lang.rs:903     pair_transformer([("a", "b")].iter());

备注

我觉得它与指定应实现特征的各种方式有某种关系，我还没有完全理解这一点。

// As stated in the answer at 
// 
impl Add<YourType> for YourType { ... }
impl<'r> Add<YourType> for &'r YourType { ... }
impl<'a> Add<&'a YourType> for YourType { ... }
impl<'r, 'a> Add<&'a YourType> for &'r YourType { ... }

使用rustc 1.0.0-nightly (522d09dfe 2015-02-19) (built 2015-02-19)

Answer 1

您的代码中有几个错误：

您可能想直接为您的类型实现您的特征，因为特征定义的方法采用特征 by reference（[=11 不是这种情况=] 你链接的另一个 post 的特征）
您对 OwnedPair { first: "a", second: "b"} 的使用实际上并未被拥有：您的类型将是 OwnedPair<&'static str, &'static str> 所以我包含了带有 String 的示例（它们是被拥有的），因为我假设这就是您所拥有的通缉
迭代器返回的项实际上是引用，因此您可能希望将 I 绑定到 Iterator<Item=&'a T>

因为我试图尽可能通用（并且为了使用 OwnedPair<&str,&str> 和 OwnedPair<String,String> 进行编译的示例），我使用了特征 std::borrow::Borrow，这基本上意味着它可以从实现此特征的类型借用对类型 T 的引用。

我还需要使用 ?Sized 作为大多数类型参数的边界。这允许使用在编译时大小未知的类型，并将在 "fat pointer" 后面使用。更多信息在this blog post（有点旧）

这是更正后的完整代码（可在 playpen 中运行）

use std::borrow::Borrow;

trait Pair<'a, A: ?Sized, B: ?Sized> {
    fn first_ref(&'a self) -> &'a A;
    fn second_ref(&'a self) -> &'a B;
}

struct PairOwned<A, B> {
    first: A,
    second: B,
}

// Only implemented for the cases we are interested in ...
impl<'a, ARef: ?Sized, BRef: ?Sized, A: Borrow<ARef>, B: Borrow<BRef>> Pair<'a, ARef, BRef> for PairOwned<A,B> {
    fn first_ref(&'a self) -> &'a ARef {
        self.first.borrow()
    }
    fn second_ref(&'a self) -> &'a BRef {
        self.second.borrow()
    }
}

// It should also be possible to be more generic here with Borrow
// But I wanted to leave your original implementation
impl<'a, A: ?Sized, B: ?Sized> Pair<'a, A, B> for (&'a A, &'a B) {
    fn first_ref(&'a self) -> &'a A {
        self.0
    }
    fn second_ref(&'a self) -> &'a B {
        self.1
    }
}

fn pair_transformer<'a, I, T>(pairs: I) -> String
    where   T: Pair<'a, str, str> + 'a,
            I: Iterator<Item=&'a T> {
    let mut s = String::new();
    for pair in pairs {
        s = s
            + pair.first_ref().as_slice()
            + pair.second_ref().as_slice();
    }
    s
}

fn main() {
    pair_transformer([PairOwned { first: "a".to_string(), second: "b".to_string() }].iter());
    pair_transformer([PairOwned { first: "a".to_string(), second: "b" }].iter()); // It is even possible to mix String and &str
    pair_transformer([PairOwned { first: "a", second: "b" }].iter());
    pair_transformer([("a", "b")].iter());
}

为什么我的 Trait 实现不匹配？

Why does my Trait implementation not match?

rust

备注