Code shared from the Rust Playground

playground.rs

trait Foo {
    fn method(&self);
}

trait Cast<T> {
    fn cast(t: &T) -> &Self;
    
    fn cast_mut(t: &mut T) -> &mut Self;
}

impl<T> Cast<T> for Foo
where
    T: Foo + 'static,
{
    fn cast(t: &T) -> &(Foo + 'static) {
        t
    }
    
    fn cast_mut(t: &mut T) -> &mut (Foo + 'static) {
        t
    }
}

impl Foo for i32 {
    fn method(&self) {
        println!("{}", self);
    }
}

fn generic<A: Cast<B> + ?Sized, B>(b: &B, f: fn(&A)) {
    let obj: &A = A::cast(b);
    f(obj);
}

fn main() {
    generic::<Foo, i32>(&99, Foo::method);
}