trait Ext {
    fn test(&self);
}


// A


struct A(String);


impl A {
    fn new(v: impl AsRef<str>) -> A { A(v.as_ref().to_owned()) }
}


impl Ext for A {
    fn test(&self) {
        println!("A [{}]: Hello, world!", self.0);
    }
}


// B with A inside


struct B<T>(String, T);


impl<T: Ext> B<T> {
    fn new(v: impl AsRef<str>, inner: T) -> B<T> { B(v.as_ref().to_owned(), inner) }
}


impl<T: Ext> Ext for B<T> {
    fn test(&self) {
        self.1.test();
        println!("B [{}]: Hello, world!", self.0);
    }
}


// APP


struct App {
    inner: Box<dyn Ext>,
}


impl App {
    fn test(&self) { self.inner.test() }
}


fn main() {
    let app = App {
        inner: Box::new(A::new("test-1")),
    };

    // TODO:...

    app.test();
}