rust - 如何创建一个特征来指定超特征的关联类型？

Question

我想创建一个抽象特征来指定索引类型和值类型，其中实现特征的任何结构都必须实现 Index<IndexType>和 IndexMut<IndexType>并定义了 Output在实现特征的每个结构中保持相同的类型。

我尝试创建一个特征，但似乎无法指定输出类型:

use std::ops::{Index, IndexMut};

struct Coord;
struct LightValue;

trait LightMap: Index<Coord> + IndexMut<Coord> {}

impl LightMap {
    type Output = LightValue;
}

warning: trait objects without an explicit `dyn` are deprecated
 --> src/lib.rs:8:6
  |
8 | impl LightMap {
  |      ^^^^^^^^ help: use `dyn`: `dyn LightMap`
  |
  = note: `#[warn(bare_trait_objects)]` on by default

error[E0191]: the value of the associated type `Output` (from the trait `std::ops::Index`) must be specified
 --> src/lib.rs:8:6
  |
8 | impl LightMap {
  |      ^^^^^^^^ associated type `Output` must be specified

error[E0202]: associated types are not yet supported in inherent impls (see #8995)
 --> src/lib.rs:9:5
  |
9 |     type Output = LightValue;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^

如果我不指定输出类型，那么 associated type Output must be specified在我尝试使用该特征的任何地方都会发生。

Answer 1

您需要对超特征进行关联类型的限制:

trait LightMap: Index<Coord, Output = LightValue> + IndexMut<Coord> {}

另见:

• Traits with stricter associated type bounds than supertrait
• How do I bound a trait with a supertrait that uses the trait's associated type as a parameter?
• Constraints on associated trait types
• Matching a generic parameter to an associated type in an impl
• Is there any way to create a type alias for multiple traits?
• What does "dyn" mean in a type?
• Why would I implement methods on a trait instead of as part of the trait?