从实施到实施

Question

我想将“A”的类型转换为“B”，将“A”的 collections 转换为“B”的 collections（反之亦然）。

我对该机制的工作原理有一些误解。

我假设在基本类型上实现From将类似地转移到 collections 而无需显式实现。

例如：

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=aaffccd542d750a4e7061fc0045b712c

struct A {
    text: String
}

struct B {
    text: String
}

impl From<A> for B {
    fn from(a: A) -> Self {
        B { text: a.text } 
    }
}
fn main() {
    let a = A { text: "hello".to_string() };
    let b = B::from(a); // works
    let a2 = A { text: "hello".to_string() };
    let b2 = a.into(); // works
    let v1 = vec![A { text: "hello".to_string()}];
    let v2 = Vec::<B>::from(v1); // doesn't work
    let v2 : Vec<B> = v1.into(); // doesn't work
}

我在转换 collections 时遇到的错误：

Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `Vec<B>: From<Vec<A>>` is not satisfied
  --> src/main.rs:20:14
   |
20 |     let v2 = Vec::<B>::from(v1); // doesn't work
   |              ^^^^^^^^^^^^^^ the trait `From<Vec<A>>` is not implemented for `Vec<B>`
   |
   = help: the following implementations were found:
             <Vec<T, A> as From<Box<[T], A>>>
             <Vec<T> as From<&[T]>>
             <Vec<T> as From<&mut [T]>>
             <Vec<T> as From<BinaryHeap<T>>>
           and 6 others
   = note: required by `from`

error[E0277]: the trait bound `Vec<B>: From<Vec<A>>` is not satisfied
  --> src/main.rs:21:26
   |
21 |     let v2 : Vec<B> = v1.into(); // doesn't work
   |                          ^^^^ the trait `From<Vec<A>>` is not implemented for `Vec<B>`
   |
   = help: the following implementations were found:
             <Vec<T, A> as From<Box<[T], A>>>
             <Vec<T> as From<&[T]>>
             <Vec<T> as From<&mut [T]>>
             <Vec<T> as From<BinaryHeap<T>>>
           and 6 others
   = note: required because of the requirements on the impl of `Into<Vec<B>>` for `Vec<A>`

这些嵌套转换是否有“一揽子”实现？ 如果不是，实现这种灵活性的最佳方式是什么？

Answer 1

在将项目收集到新向量之前，使用into_iter和 map Into::into使用 vec：

let v2: Vec<B> = v1.into_iter().map(Into::into).collect();

操场

Answer 2

如果您正在寻找与From::from和Into::into提供的语法类似的语法，您可能需要考虑编写自己的通用特征FromVec ，例如：

trait FromVec<T> {
   fn from_vec(val: Vec<T>) -> Self;
}

impl<T, S: From<T>> FromVec<T> for Vec<S> {
   fn from_vec(val: Vec<T>) -> Self {
      val.into_iter().map(Into::into).collect()  
   }
}

连同其相应的伙伴特征IntoVec ：

trait IntoVec<T> {
   fn into_vec(self) -> Vec<T>;
}

并为其提供一揽子实施：

impl<T, S> IntoVec<T> for Vec<S>
   where Vec<T>: FromVec<S>
{
   fn into_vec(self) -> Vec<T> {
      Vec::from_vec(self)
   }
}

这样，您将以与调用from() () 和 into() 类似的方式调用from_vec()和into_vec() into() ：

fn main() {
    let vec_a = vec![A { text: "hello".to_string() } ];
    let vec_b = Vec::<B>::from_vec(vec_a);

    let vec_a = vec![A { text: "hello".to_string() } ];
    let vec_b: Vec<B> = vec_a.into_vec();
}