Box<dyn Foo> vs &Foo, or: combining static and dynamic dispatch

[hjfreyer]

I'm struggling with an error when I combine Trait objects with Trait bounds. Here's an example:

Rust Playground

When I try to use a Box<dyn Foo> as a type parameter with bound Foo, I get:

the trait `Foo` is not implemented for `std::boxed::Box<dyn Foo>`

I feel caught in a Catch-22 here: I can't simply pass around a dyn Foo and apparently I can't use a Box<dyn Foo> in its place. Switching to references would make the lifetime management very complex. I've proposed a workaround, but it really doesn't seem like the right option.

Is there a more idiomatic way to deal with this? Or is the answer to simply not combine static and dynamic dispatch like this?

[alice]

Add the impl to boxes too.

impl<T: Foo + ?Sized> Foo for Box<T> {
    fn bar(&self) -> &'static str {
        (&**self).bar()
    }
}

Note that you probably don't want to use &'static str, because that type can only be used for compile-time constants hard-coded in your codebase.

[hjfreyer]

Aha, so this is basically a less verbose version of my workaround. I didn't realize you could add Traits to shared types like that, good to know.

But is this actually idiomatic?

[alice]

Yeah, if you want boxed trait objects to be usable like that, this is the way to do it.