问题

我有两个相关的特性 `List` 和 `ListItem` ([playground 链接][1])。
```rust
trait List {
    type Item: ListItem;
    fn items(&self) -> Vec<Self::Item>;
    /* 更多内容 */
}

其中 Item 是 List 上的关联类型，而 ListItem 对于 PartialEq 和 Debug 有自定义实现。

在一个看起来像这样的 List 特性方法内：

fn foo(&self, other: &dyn List<Item=Self::Item>)

我想要比较项目的子切片，使用为 ListItem 定义的 PartialEq。
例如：

let lhs: &[&dyn ListItem] = ...;
let rhs: &[&dyn ListItem] = ...;
assert_eq!(lhs, rhs);

但我无法将切片强制转换为正确的类型。

fn foo(&self, other: &dyn List<Item = Self::Item>) {
   let items = self.items();
   let items_slice: &[&dyn ListItem] = items.as_slice(); // <-- 这里有错误

error[E0308]: 不匹配的类型
   |
40 |         let these: &[&dyn ListItem] = these_items.as_slice();
   |                    ----------------   ^^^^^^^^^^^^^^^^^^^^^^ 期望类型为 `&[&dyn ListItem]`，找到类型为 `&[<Self as List>::Item]`
   |                    |
   |                    期望类型
   |
   = 注意：期望引用 `&[&dyn ListItem]`
              找到引用 `&[<Self as List>::Item]`
   = 帮助：考虑将关联类型 `<Self as List>::Item` 约束为 `&dyn ListItem`
   = 注意：更多信息请访问 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html

(playground 链接)

我该如何强制转换切片为正确的类型？

<details>
<summary>英文:</summary>

I have two related traits `List` and `ListItem` ([playground link][1]).
```rust
trait List {
    type Item: ListItem;
    fn items(&amp;self) -&gt; Vec&lt;Self::Item&gt;;
    /* more stuff */
}

where Item is an associated type on List and ListItem has a custom implementation for PartialEq and Debug

Inside of a List trait method that looks like

fn foo(&self, other: &dyn List<Item=Self::Item>)

I would like to compare sub-slices of the items, using the PartialEq defined for ListItem.
e.g.

let lhs: &[&dyn ListItem] = ...;
let rhs: &[&dyn ListItem] = ...;
assert_eq!(lhs, rhs);

but I cannot coerce the slices to the correct type.

fn foo(&self, other: &dyn List<Item = Self::Item>) {
   let items = self.items();
   let items_slice: &[&dyn ListItem] = items.as_slice(); // <-- Error here

error[E0308]: mismatched types
   |
40 |         let these: &[&dyn ListItem] = these_items.as_slice();
   |                    ----------------   ^^^^^^^^^^^^^^^^^^^^^^ expected `&[&dyn ListItem]`, found `&[<Self as List>::Item]`
   |                    |
   |                    expected due to this
   |
   = note: expected reference `&[&dyn ListItem]`
              found reference `&[<Self as List>::Item]`
   = help: consider constraining the associated type `<Self as List>::Item` to `&dyn ListItem`
   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html

(playground link)

How can I coerce the slice to the correct type?

答案1

得分: 1

你不能。首先，你需要引用，而 items 包含拥有的项目。其次，你需要胖指针，而 items 具有具体的类型。

你需要将它们收集到一个 Vec 中：

let items = self.items();
let items_dyn = items
    .iter()
    .map(|item| item as &dyn ListItem)
    .collect::<Vec<_>>();

let items = self.items();
let items_dyn = items
    .iter()
    .map(|item| item as &dyn ListItem)
    .collect::<Vec<_>>();