Trait Upcasting

Trait upcasting, stabilized in Rust 1.86 (April 2025), lets you coerce &dyn Sub to &dyn Super — a long-requested feature that previously required manual workarounds.

trait Animal: std::fmt::Debug + std::any::Any {
    fn name(&self) -> &str;
}

fn print_and_downcast(animal: &dyn Animal) {
    let debuggable: &dyn std::fmt::Debug = animal;  // upcast to supertrait
    println!("{debuggable:?}");

    let any: &dyn std::any::Any = animal;            // upcast to Any
    if let Some(dog) = any.downcast_ref::<Dog>() {
        println!("It's a dog named {}!", dog.0);
    }
}

// Works with Box, Arc, Rc too
fn upcast_box(animal: Box<dyn Animal>) -> Box<dyn std::fmt::Debug> {
    animal
}

This completes a gap in Rust's trait object model that previously required either downcasting hacks or the as-any pattern. Combined with gats and RPITIT, the Rust type system is now nearly feature-complete for practical use. See modern-rust-features for the timeline.