Shared via Rust Playground

playground.rs

trait Hello { fn hello(&self){}}

struct A{_x:f32}
struct B{_y:u32}
impl Hello for A {}
impl Hello for B {}

// #[derive(EnumAsTrait(Hello))]
enum Foo {
    A(A), 
    B(B)
}

pub trait AsTrait {
    type T: ?Sized;
    fn as_trait(&self) -> &Self::T;
}

// The following impl could be autoderived
impl AsTrait for Foo {
    type T=Hello;
    fn as_trait(&self) -> &Self::T {
        match self {
            &Foo::A(ref a) => a as &Self::T,
            &Foo::B(ref b) => b as &Self::T,
        }
    }
}

fn main() {
    let a = Foo::A(A {_x:1.0});
    let b = Foo::B(B {_y:1});
    a.as_trait().hello();
    b.as_trait().hello();

}