rust - Impl Into 所有类型的 impl Into 特性

Question

我想实现一个转换特性，涵盖支持现有转换的所有类型。我认为这可以通过以下方式完成:

impl<T> Into<B> for T
where
    T: Into<A>,
{
  fn into(self) -> B {
    let a: A = self.into();
    B::CaseA(a)
  }
}

但是编译器会抛出以下错误:

error[E0119]: conflicting implementations of trait `std::convert::Into<block::ItemContent>`
   --> yrs\src\block.rs:945:1
    |
945 | / impl<T> Into<B> for T
946 | | where
947 | |     T: Into<A>,
948 | | {
...   |
952 | |     }
953 | | }
    | |_^
    |
    = note: conflicting implementation in crate `core`:
            - impl<T, U> Into<U> for T
              where U: From<T>;

error[E0210]: type parameter `T` must be covered by another type when it appears before the first local type (`B`)
   --> yrs\src\block.rs:945:6
    |
945 | impl<T> Into<B> for T
    |      ^ type parameter `T` must be covered by another type when it appears before the first local type (`B`)
    |
    = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local, and no uncovered type parameters appear before that first local type
    = note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`, where `T0` is the first and `Tn` is the last

这有可能实现吗？我正在寻找一种方法来为所有实例自动扩展新类型的转换，支持转换为另一种现有类型。

Answer 1

通过用本地类型覆盖 T 可以实现您正在尝试做的事情(如错误所述)。没有它你不能这样做的原因是因为否则其他人也可以做同样的事情并且 Rust 将无法判断要使用哪个 impl 。因此，您需要一个本地包装器 类型来保持分离。

错误解释中引用了这背后的完整推理:

https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md

和https://rust-lang.github.io/rfcs/2451-re-rebalancing-coherence.html

希望这有帮助；)

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