If I implement From for B, will From> for Vec also be implemented?

Question

It seems like no, because I have code that implements From<A> for <B>, and I can convert an A to a B with .into(), but the same thing isn't working for a Vec<A> .into() a Vec<B>.

Either I've screwed up something that's preventing the implementation from being derived, or this isn't supposed to happen.

If it's not supposed to work, why not? It seems like code such as this would work:

impl<A: From<B>, B> From<Vec<A>> for Vec<B> {
    // ... map .into onto vec of As to vec of Bs ...
}

I'm guessing there's more complexity to it than that.

Shepmaster · Accepted Answer · 2017-10-14T14:11:31.113

There's no need to guess at which implementations of From exist for Vec; they are all listed in the docs. The list as of Rust 1.21.0:

impl<'a, T> From<&'a mut [T]> for Vec<T> { /**/ }

impl<T> From<BinaryHeap<T>> for Vec<T> { /**/ }

impl<T> From<VecDeque<T>> for Vec<T> { /**/ }

impl<'a, T> From<&'a [T]> for Vec<T>  { /**/ }

impl From<String> for Vec<u8> { /**/ }

impl<'a, T> From<Cow<'a, [T]>> for Vec<T> { /**/ } 

impl<'a> From<&'a str> for Vec<u8> { /**/ }

impl<T> From<Box<[T]>> for Vec<T> { /**/ }

Instead, you will want to do something like:

let b: Vec<Wrapper> = a.into_iter().map(Into::into).collect();

If you tried to implement this, you'd get a failure:

error[E0119]: conflicting implementations of trait `core::convert::From<vec::Vec<_>>` for type `vec::Vec<_>`:
    --> /Users/shep/Projects/rust/src/liballoc/vec.rs:2190:1
     |
2190 | / impl<A, B> From<Vec<A>> for Vec<B>
2191 | |     where A: Into<B>
2192 | | {
2193 | |     fn from(s: Vec<A>) -> Vec<B> {
2194 | |         s.into_iter().map(Into::into).collect()
2195 | |     }
2196 | | }
     | |_^
     |
     = note: conflicting implementation in crate `core`

Nothing prevents A and B from being the same type. In that case, you'd be conflicting with the reflexive implementation of From: impl<T> From<T> for T.

If I implement From for B, will From> for Vec also be implemented?

1 Answers1