specialization usage to impl a fn with default return values & "override" it for certain types

specialization.rs

#![feature(specialization)]

trait Foo {
    fn foo(&self);
}

impl<T> Foo for T {
    default fn foo(&self) { println!("default foo") }
}

struct SpecialType;
impl Foo for SpecialType {
    fn foo(&self) { println!("foo for SpecialType") }
}

fn main() {
    42_u32.foo();
    SpecialType.foo();
}