使用关联类型时不满足特质界限

问题描述

我正在尝试设计一个道路跟踪应用程序，并提出了以下特征结构（可在playground上使用）：

pub trait Source: Sized {
    type Path: Path<Source = Self>;
}

pub trait Destination<S: Source>{ }

pub trait Path {
    type Source: Source;
    type Destination: Destination<Self::Source>;
}

问题是它无法编译并显示错误

   Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `<<Self as Source>::Path as Path>::Destination: Destination<Self>` is not satisfied
 --> src/lib.rs:2:16
  |
2 |     type Path: Path<Source = Self>;
  |                ^^^^^^^^^^^^^^^^^^^ the trait `Destination<Self>` is not implemented for `<<Self as Source>::Path as Path>::Destination`
...
7 | pub trait Path {
  |           ---- required by a bound in this
8 |     type Source: Source;
9 |     type Destination: Destination<Self::Source>;
  |                       ------------------------- required by this bound in `Path`
  |
help: consider further restricting the associated type
  |
1 | pub trait Source: Sized where <<Self as Source>::Path as Path>::Destination: Destination<Self> {
  |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

不清楚。我将关联类型的绑定指定为type Path: Path<Source = Self>;，这意味着在错误消息中指定的关联类型上的绑定。

但这应该禁止使用错误的类型实现，而不是按原样声明它。有办法解决这个问题吗？

解决方法

我找到了一种通过指定其他关联类型来修复它的方法，但是尚不清楚原始错误的原因。这是工作示例：

pub trait Source: Sized {
    type Path: Path<Source=Self,Destination=Self::Destination>;
    type Destination: Destination<Self>; // Added associated type
}

pub trait Destination<S: Source>{ }

pub trait Path {
    type Source: Source;
    type Destination: Destination<Self::Source>;
}