Comparing trait objects

[John_Nagle]

I want to compare two trait objects. If they're the same trait implementation, apply PartialEq to the items, otherwise false. Rust won't derive PartialEq for a trait, which would be nice. So

trait Viewable {
   fn is_same(&self, other: &dyn Viewable) -> bool;
}

#[derive (PartialEq)]
struct SculptData {
}

impl Viewable For SculptData {
    fn is_same(&self, other: &dyn Viewable) -> bool {
       self == other // can't do that, of course
    }
}

So, how to do this? Do I have to go through the whole Any/downcast thing, or is there some automation for this?

[H2CO3]

@John_Nagle

If they're the same trait implementation

Do you mean that they are the same type? Then you should probably add an Any bound (and an explicit conversion method to &dyn Any in the absence of supertrait coercions), and use <dyn Any>::downcast_ref::<Self>() to check whether other has the same type as self. Playground

Note that this won't work transitively, since a trait object (as a concrete type) itself can still be wrapped into another level of trait objects, so this might not always work as expected.

[quinedot]

I wouldn't be surprised if there's a crate for this, but it came up in this thread and I threw together a PoC which may be useful.

[John_Nagle]

I know how to do it the hard way. I was hoping for an easy way. Like being able to derive PartialEq for traits and getting all that machinery generated.

[quinedot]

Built in? No. This crate exists.

[John_Nagle]

That worked great. Thanks.