Code shared from the Rust Playground

trait_and_trait_object.rs

trait A{
    fn a(&self);
}

trait B{
    fn b(&self);
    
}

impl B for A{
    fn b(&self){
        println!("B1");
        self.a();
    }
}

impl B for &A{
    fn b(&self){
        println!("B2");
        (*self).a();
    }
}

struct S;
impl A for S{
    fn a(&self){
        println!("S");
    }
}





fn t<T: B+?Sized>(a: &T) {
    a.b();
}

fn main() {
    let a:&dyn A = &S;
    let b:&dyn B = &a;
    b.b();
    
    t(a);

}