Rust From error

main.rs

// in the following snippet,  cargo complains:
//
// error[E0277]: the trait bound `B: std::convert::From<&A>` is not satisfied
//   --> src/main.rs:54:21
//    |
// 54 |         let dn: B = B::from(n);
//    |                     ^^^^^^^ the trait `std::convert::From<&A>` is not implemented for `B`
//    |
//    = help: the following implementations were found:
//              <B as std::convert::From<A>>
//    = note: required by `std::convert::From::from`
// 
//
// While what is missing in the `Clone` trait on A.
// 
// Why is there no error on self.clone() ?

extern crate serde;

use serde::Serialize;
use serde::ser::Serializer;

#[macro_use]
extern crate serde_derive;

// #[derive(Clone)]
struct A {
    vec: i32,
}

impl From<A> for B {
    fn from(n: A) -> Self {
        Self { vec: n.vec.into() }
    }
}

#[derive(Serialize)]
struct B {
    vec: i32,
}

impl Serialize for A
where
    B: From<A>+Serialize,
{
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        let n = self.clone();
        let dn: B = B::from(n);
        dn.serialize(serializer)
    }
}

fn main() {}