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playground.rs

#![feature(specialization)]

macro_rules! debug_or {
    ($e:expr, $s:expr, $($a:expr),*) => {
        $e.debug_or_else(|f| write!(f, $s, $($a),*))
    };
    ($e:expr, $s:expr) => {
        debug_or!($e, $s,)
    }
}

fn main() {
    struct X;
    #[derive(Debug)] struct Y;

    println!("{:?}", X.debug_or("-"));
    println!("{:?}", X.debug_or(&1234));
    println!("{:?}", X.debug_or_else(|f| write!(f, "{}", "-")));
    println!("{:?}", X.debug_or_else(|f| write!(f, "{}", 1234)));
    println!("{:?}", debug_or!(X, "-"));
    println!("{:?}", debug_or!(X, "{:?}", "-"));
    println!("{:?}", debug_or!(X, "{}", 1234));

    println!("{:?}", Y.debug_or("-"));
    println!("{:?}", Y.debug_or(&1234));
    println!("{:?}", Y.debug_or_else(|f| write!(f, "{}", "-")));
    println!("{:?}", Y.debug_or_else(|f| write!(f, "{}", 1234)));
    println!("{:?}", debug_or!(Y, "-"));
    println!("{:?}", debug_or!(Y, "{:?}", "-"));
    println!("{:?}", debug_or!(Y, "{}", 1234));
}

use std::fmt::Debug;
use std::fmt;

enum DebugOr<'a, A: 'a + ?Sized, Fallback: 'a + ?Sized + Debug> {
    A(&'a A),
    Fallback(&'a Fallback),
}

enum DebugOrElse<'a, A: 'a + ?Sized, Fallback> {
    A(&'a A),
    Fallback(Fallback),
}

trait MaybeDebug {
    fn debug_or<'a, U: ?Sized + Debug>(&'a self, u: &'a U) -> DebugOr<'a, Self, U>;
    fn debug_or_else<'a, F: Fn(&mut fmt::Formatter) -> fmt::Result>(&'a self, f: F) -> DebugOrElse<'a, Self, F>;
}

impl<T: ?Sized> MaybeDebug for T {
    default fn debug_or<'a, U: ?Sized + Debug>(&'a self, u: &'a U) -> DebugOr<'a, Self, U> {
        DebugOr::Fallback(u)
    }
    default fn debug_or_else<'a, F: Fn(&mut fmt::Formatter) -> fmt::Result>(&'a self, f: F) -> DebugOrElse<'a, Self, F> {
        DebugOrElse::Fallback(f)
    }
}

impl<T: ?Sized + Debug> MaybeDebug for T {
    fn debug_or<'a, U: ?Sized + Debug>(&'a self, _: &'a U) -> DebugOr<'a, Self, U> {
        DebugOr::A(self)
    }
    fn debug_or_else<'a, F: Fn(&mut fmt::Formatter) -> fmt::Result>(&'a self, _: F) -> DebugOrElse<'a, Self, F> {
        DebugOrElse::A(self)
    }
}

impl<'a, T: ?Sized, U: ?Sized + Debug> Debug for DebugOr<'a, T, U> {
    default fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            DebugOr::A(_)            => unreachable!(),
            DebugOr::Fallback(ref a) => a.fmt(f),
        }
    }
}

impl<'a, T: ?Sized + Debug, U: ?Sized + Debug> Debug for DebugOr<'a, T, U> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            DebugOr::A(ref a)    => a.fmt(f),
            DebugOr::Fallback(_) => unreachable!(),
        }
    }
}

impl<'a, T: ?Sized, F: Fn(&mut fmt::Formatter) -> fmt::Result> Debug for DebugOrElse<'a, T, F> {
    default fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            DebugOrElse::A(_)            => unreachable!(),
            DebugOrElse::Fallback(ref a) => a(f),
        }
    }
}

impl<'a, T: ?Sized + Debug, F: Fn(&mut fmt::Formatter) -> fmt::Result> Debug for DebugOrElse<'a, T, F> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            DebugOrElse::A(ref a)    => a.fmt(f),
            DebugOrElse::Fallback(_) => unreachable!(),
        }
    }
}