Providing additional constraint on type

I have the following code (playground):

trait Animal: std::fmt::Debug {}

trait Zoo {
    fn animals(&self) -> Vec<Box<dyn Animal>>;

impl Zoo for String {
    fn animals(&self) -> Vec<Box<dyn Animal>> {

fn test() {
    assert_eq!("".to_string().animals(), vec![]);

The last line assert_eq!("".to_string().animals(), vec![]); fails to compile (binary operation ==cannot be applied to typeVec<Box>``). I believe this is because there is no constraint of PartialEquality on the elements returned from animals(). But I am not sure how to specify one. How do I express a constraint something along the lines of dyn Animal + PartialEq?

Note that in my real program the equivalent of the Animal type comes from a different crate and thus cannot be modified.

Comparing trait objects is pretty tricky and requires some dynamic dispatch tricks and downcasting. Here's one way to do it (playground).


Thank you so much. It fits my needs perfectly!