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Question

I want to transform types of "A" into "B" and collections of "A" to collections of "B" (and vice versa).

I have some misunderstanding of how the mechanism works.

I assumed implementing From on the base type would transfer to collections similarly without explicitly implementing.

For example:

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
}

the error I get for converting the collections:

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>`

Is there a "blanket" implementation for these nested conversions? If not what is the best way to achieve this flexibility?

Answer 1

Consume the vec with into_iter and map Into::into before collecting the items into a new vector:

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

Playground

Answer 2

If you are looking for a similar syntax to the one provided by From::from and Into::into , you may want to consider writing your own generic trait, FromVec , eg:

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()  
   }
}

along with its corresponding buddy trait, IntoVec :

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

and providing it with a blanket implementation:

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

This way, you will be calling from_vec() and into_vec() in a similar way as you would call from() and 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();
}