Macro parser_state 

macro_rules! parser_state {
    (
        $(#[$attrs:meta])*
        $vis:vis struct $name:ident <
            $I:ident $(: $Itrait:path)?
            $(, $Ps:ident: $trait:ident)*
            $(; $($Ts:ident $(:$def:ident)?),*)?
            $(| $(const $Cs:ident : $C_ty:ty),*)? $(,)?
        > {
            $($(#[$opt:ident$(($($k:ident = $v:ident),*$(,)?))?])? $field:ident : $ty:ty),*$(,)?
        }
    ) => { ... };
}

Automatically generates a Parser::State or IterableParser::State.