iota_const.rs

/// The `iota_const!` macro constructs a sequence of constants of a given type, starting with `1 << 0`.
/// Usage:
///     `iota_const!(u8, A, B, C);
/// Will generate:
///     pub const A: u8 = 1 << 0;     // 1 << 0
///     pub const B: u8 = 1 << 0+1;   // 1 << 1
///     pub const C: u8 = 1 << 0+1+1; // 1 << 2
macro_rules! iota_const {
    // There's no way to iterate over the args, so I need to run private macro recirsively
    // to accumulate a counter expression like `(0+1+...+1)`.    
    ( $const_type:ty, $name:ident, $( $rest:tt )* )=> {
        __iota_const_impl!($const_type, (0), $name, $( $rest )*);
    };
}

macro_rules! __iota_const_impl {
    ( $const_type:ty, ($i:expr), $name:ident, $( $rest:tt )+ ) => {
        pub const $name: $const_type = 1<<($i);
        __iota_const_impl!($const_type, ($i+1), $( $rest )+);
    };
    ( $const_type:ty, ($i:expr), $name:ident, $last:ident ) => {
        pub const $name: $const_type = 1<<($i);
        __iota_const_impl!($const_type, ($i+1), $last);
    };
    ( $const_type:ty, ($i:expr), $name:ident ) => {
        pub const $name: $const_type = 1<<($i);
    };
}