multirust run nightly rustc --crate-type lib bar.rs && multirust run nightly rustc -L . foo.rs

bar.rs

pub trait MyFrom2<T> {
    fn my_from2(T) -> Self;
}

foo.rs

extern crate bar;

use std::convert::From;

use bar::MyFrom2;

struct MyVec(Vec<u8>);

impl<'a> From<&'a [u8]> for MyVec {
    fn from(s: &[u8]) -> MyVec {
        MyVec(s.to_owned())
    }
}

trait MyFrom<T> {
    fn my_from(T) -> Self;
}

impl<'a> MyFrom<&'a [u8]> for MyVec {
    fn my_from(s: &[u8]) -> MyVec {
        MyVec(s.to_owned())
    }
}

impl<'a> MyFrom2<&'a [u8]> for MyVec {
    fn my_from2(s: &[u8]) -> MyVec {
        MyVec(s.to_owned())
    }
}

fn make(s: &[u8]) -> MyVec {
    MyVec(s.to_owned())
}

fn main() {
    let _v = MyVec::from(&b"123"[..]);
    // let _v = MyVec::from(b"123");
    //
    // Error's with:
    // foo.rs:27:14: 27:25 error: the trait `core::convert::From<&[u8; 3]>` is not implemented for the type `MyVec` [E0277]
    // foo.rs:27     let _v = MyVec::from(b"123");
    //                        ^~~~~~~~~~~

    let _v = MyVec::my_from(&b"123"[..]);
    let _v = MyVec::my_from(b"123");

    let _v = MyVec::my_from2(&b"123"[..]);
    let _v = MyVec::my_from2(b"123");

    let _v = make(b"123");
}