Implement Traits on External Types in Rust

Say you are using an type from an external crate and you want to implement a specific functionality/method on that type. As an example say you are using the linked list of my recently developed rust_dsa crate and you want to implement a method on it to print it's values in the following format.



let mut h = Node::new(Some(0));
// Snippet
println!("Linked list: {}", head);

If it's from a local trait, you can simply implement the Display trait on it as follows



impl<T: Debug + Clone + Copy> fmt::Display for Node<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match &self.next {
            Some(next) => {
                print!(
                    "{} <- ",
                    Node {
                        value: self.value,
                        next: Some(next.cl)
                    }
                );
                write!(f, "{:?}", self.value.as_ref().unwrap())
            }
            None => {
                write!(f, "{:?}", self.value.as_ref().unwrap())
            }
        }
    }
}

But since you are using Node which is from an external trait you will face the following error when you compile your binary

So what can you do about it? You can simply implement a wrapper for that specific type and implement the Trait of your choice as follows by accessing the Type you want as a child of the Wrapper as a tuple element.



struct  Wrapper<'a, T>(&'a Box<Node::<T>>);



impl<'a, T: Debug> fmt::Display for Wrapper<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match &self.0.next {
            Some(next) => {
                print!("{} <- ", Wrapper(&next));
                write!(f, "{:?}", self.0.value.as_ref().unwrap())
            },
            None => {
                write!(f, "{:?}", self.0.value.as_ref().unwrap())
            }
        }
    }
}

That's it for new types and feel free to explore more on new types in Rust book's guide on Advanced traits.

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Implement Traits on External Types in Rust

Shakya Peiris

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