Enable PartialEq on an external type

Question

In Rust, I want to add the PartialEq trait to struct that's defined in an external crate. The PartialEq trait cannot simply be added to the external struct since I want to compare it to a custom type. How can I do that?

I know you can define your own trait (e.g., DoSomething in the example below), implement that for the external struct, then import that trait to use it. However, I can't figure out how to do something similar with any of the standard traits like PartialEq.

// a.rs

use external::Struct;

pub trait DoSomething {
    fn can_do_something(&self) -> bool;
}

impl DoSomething for Struct {
    pub fn can_do_something(&self) -> bool {
        true
    }
}

// b.rs

use external::Struct;
use a::DoSomething;

fn main() {
    let struct = Struct::new();
    println!("can it do something? {}", struct.can_do_something());
}

When I try to do something similar for PartialEq, I get the following error:

use external::Struct;

impl PartialEq for Struct {
     ^^^^^^^^^ only traits defined in the current crate can be implemented
               for types defined outside of the crate define and implement
               a trait or new type instead
    // ...
}