enum_len!

enum_len.rs

/// Defines an enum and implements a static function for it
/// that returns the number of variants it can take.
macro_rules! enum_len {
    // 'Public API': 0 variants
    ($name:ident, []) => {
        enum_len!($name, [], [], 0);
    };

    // 'Public API': 1 variant
    ($name:ident, [$first:ident]) => {
        enum_len!($name, [$first,]);
    };

    // 'Public API': the normal case
    // Trailing comma
    ($name:ident, [$first:ident, $($remainder:ident),*,]) => {
        enum_len!($name, [$($remainder,)*], [$first], 1);
    };
    // No trailing comma
    ($name:ident, [$first:ident, $($remainder:ident),*]) => {
        enum_len!($name, [$($remainder,)*], [$first], 1);
    };

    ($name:ident, [$first:ident, $($remainder:ident,)*], [$($vars:ident),*], $cnt:expr) => {
        enum_len!($name, [$($remainder,)*], [$($vars),*, $first], $cnt + 1);
    };

    ($name:ident, [], [$($vars:ident),*], $cnt: expr) => {
        enum $name {
            $($vars),*
        }

        impl $name {
            fn num_variants() -> usize {
                $cnt
            }
        }
    }
}

enum_len!(Direction, [North, East, South, West]);

fn main() {
    println!("{}", Direction::num_variants());
}