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playground.rs

#![feature(specialization)]

use std::fmt::*;

trait Maybe<T: ?Sized> {
    fn implements_this(&self) -> bool;
    fn fmt(&self, &mut Formatter) -> Result;
}

macro_rules! impl_maybe {
    ($t:path) => {
        impl<T: ?Sized> Maybe<$t> for T {
            default fn implements_this(&self) -> bool { false }
            default fn fmt(&self, _: &mut Formatter) -> Result { 
                println!("nah");
                Ok(())
            }
        }
        
        impl<T: ?Sized + $t> Maybe<$t> for T {
            fn implements_this(&self) -> bool { true }
            fn fmt(&self, f: &mut Formatter) -> Result { 
                println!("ok");
                <Self as $t>::fmt(self, f)
            }
        } 
    }
}

impl_maybe!(Debug);
impl_maybe!(Display);
impl_maybe!(LowerHex);

trait MaybeAll: Maybe<Debug> + Maybe<Display> + Maybe<LowerHex> {}
impl<T: ?Sized> MaybeAll for T
    where T: Maybe<Debug> + Maybe<Display> + Maybe<LowerHex> {}

fn rtformat(s: &str, args: &[&MaybeAll]) {
    for (c, t) in s.chars().zip(args) {
        match c {
            'd' => println!("{:?}", t)
        }
    }
}


fn main() {
    rtformat("dDhh", &[&123u8, &"test", &0xffu8, &0xffu8]);
}