guard.rs

macro_rules! guard {
    (let $($tt:tt)*) => {
        let guard!(@lets (_)  let $($tt)*)
          = guard!(@step (()) let $($tt)*)
    };
    (match $($tt:tt)*) => {
        let guard!(@lets (_)  match $($tt)*)
          = guard!(@step (()) match $($tt)*)
    };

    // the @lets rules discard all but the very last statement's `let` pattern.
    // the @step rules do all the actual matching.
    // the @else rules skip statements until the next `else` statement.

    // plain match;
    (@lets ($last:pat)  match $e:expr { $pat:pat }; $($tt:tt)*) => {
        guard!(@lets (_) $($tt)*)
    };
    (@step ($last:expr) match $e:expr { $pat:pat }; $($tt:tt)*) => {
        match $e { $pat => guard!(@step (()) $($tt)*), _ => guard!(@else $($tt)*) }
    };
    (@else match $e:expr { $pat:pat }; $($tt:tt)*) => (guard!(@else $($tt)*));

    // match if condition;
    (@lets ($last:pat)  match $e:expr { $pat:pat if $cond:expr }; $($tt:tt)*) => {
        guard!(@lets (_) $($tt)*)
    };
    (@step ($last:expr) match $e:expr { $pat:pat if $cond:expr }; $($tt:tt)*) => {
        match $e { $pat if $cond => guard!(@step (()) $($tt)*), _ => guard!(@else $($tt)*) }
    };
    (@else match $e:expr { $pat:pat if $cond:expr }; $($tt:tt)*) => (guard!(@else $($tt)*));

    // let tuple = match;
    (@lets ($last:pat)  let ($($a:ident),+) = match $e:expr { $pat:pat }; $($tt:tt)*) => {
        guard!(@lets (($($a),+)) $($tt)*)
    };
    (@step ($last:expr) let ($($a:ident),+) = match $e:expr { $pat:pat }; $($tt:tt)*) => {
        match $e { $pat => guard!(@step (($($a),+)) $($tt)*), _ => guard!(@else $($tt)*) };
    };
    (@else let ($($a:ident),+) = match $e:expr { $pat:pat }; $($tt:tt)*) => (guard!(@else $($tt)*));

    // let tuple = match if condition;
    (@lets ($last:pat)  let ($($a:ident),+) = match $e:expr { $pat:pat if $cond:expr }; $($tt:tt)*) => {
        guard!(@lets (($($a),+)) $($tt)*)
    };
    (@step ($last:expr) let ($($a:ident),+) = match $e:expr { $pat:pat if $cond:expr }; $($tt:tt)*) => {
        match $e { $pat if $cond => guard!(@step (($($a),+)) $($tt)*), _ => guard!(@else $($tt)*) };
    };
    (@else let ($($a:ident),+) = match $e:expr { $pat:pat if $cond:expr }; $($tt:tt)*) => (guard!(@else $($tt)*));

    // let var = match;
    (@lets ($last:pat)  let $a:ident = match $e:expr { $pat:pat }; $($tt:tt)*) => {
        guard!(@lets ($a) $($tt)*)
    };
    (@step ($last:expr) let $a:ident = match $e:expr { $pat:pat }; $($tt:tt)*) => {
        match $e { $pat => guard!(@step ($a) $($tt)*), _ => guard!(@else $($tt)*) }
    };
    (@else let $a:ident = match $e:expr { $pat:pat }; $($tt:tt)*) => (guard!(@else $($tt)*));

    // let var = match if condition;
    (@lets ($last:pat)  let $a:ident = match $e:expr { $pat:pat if $cond:expr }; $($tt:tt)*) => {
        guard!(@lets ($a) $($tt)*)
    };
    (@step ($last:expr) let $a:ident = match $e:expr { $pat:pat if $cond:expr }; $($tt:tt)*) => {
        match $e { $pat if $cond => guard!(@step ($a) $($tt)*), _ => guard!(@else $($tt)*) }
    };
    (@else let $a:ident = match $e:expr { $pat:pat if $cond:expr }; $($tt:tt)*) => (guard!(@else $(tt)*));

    // else err;
    (@lets ($last:pat)  else $err:expr; $($tt:tt)+) => (guard!(@lets ($last) $($tt)+));
    (@step ($last:expr) else $err:expr; $($tt:tt)+) => (guard!(@step ($last) $($tt)+));
    (@else              else $err:expr; $($tt:tt)+) => ($err);

    // end condition
    (@lets ($last:pat)  else $err:expr) => ($last);
    (@step ($last:expr) else $err:expr) => ($last);
    (@else              else $err:expr) => ($err);
}