soundchange::subst_rules!

macro_rules! subst_rules {
    ($e:expr => =>) => ($e.into_owned()); // XXX oops!
    ($e:expr => => [$($pre:tt)*] $from:tt [$($post:tt)*] => $to:expr; $($t:tt)*) =>
        (subst_rules!(subst(&$e,
                            &[$($pre.into_pre_cond()),*], $from, &[$($post.into_post_cond()),*],
                            $to.into_transform(),
                            concat!("[", stringify!($($pre)*), "] ", stringify!($from),
                                   " [", stringify!($($post)*), "] => ", stringify!($to)))
                      => => $($t)*));
    ($e:expr => => [$($pre:tt)*] $from:tt => $to:expr; $($t:tt)*) =>
        (subst_rules!(subst(&$e,
                            &[$($pre.into_pre_cond()),*], $from, &[],
                            $to.into_transform(),
                            concat!("[", stringify!($($pre)*), "] ", stringify!($from),
                                   " => ", stringify!($to)))
                      => => $($t)*));
    ($e:expr => => $from:tt [$($post:tt)*] => $to:expr; $($t:tt)*) =>
        (subst_rules!(subst(&$e,
                            &[], $from, &[$($post.into_post_cond()),*],
                            $to.into_transform(),
                            concat!(stringify!($from), " [", stringify!($($post)*), "] => ",
                                    stringify!($to)))
                      => => $($t)*));
    ($e:expr => => $from:tt => $to:expr; $($t:tt)*) =>
        (subst_rules!(subst(&$e,
                            &[], $from, &[],
                            $to.into_transform(),
                            concat!(stringify!($from), " => ", stringify!($to)))
                      => => $($t)*));

    // has to be come later
    ($e:expr => $($t:tt)*) => ({
        use std::borrow::Cow;
        use $crate::{Search, Transform, IntoTransform, Cond, IntoCond};

        #[inline(always)]
        fn subst<'a, From: Search>(s: &'a str, preconds: &[Cond], from: From, postconds: &[Cond],
                                   to: Transform, rulestring: &str) -> Cow<'a, str> {
            let ret = $crate::subst(s, preconds, from, postconds, to);
            if s != ret { debug!("{} --> {} ({})", s, ret, rulestring); }
            ret
        }

        subst_rules!($e => => $($t)*)
    });
}