Macro parser_fn

macro_rules! parser_fn {
    [
            $( #[$meta:meta] )*
            $vis:vis $name:ident
                $( <$a:lifetime> )?
                -> $ret_typ:ty
                = $imp:expr
        ] => { ... };
    [
            $( #[$meta:meta] )*
            $vis:vis $name:ident ($input:ident)
                -> $ret_typ:ty
            $imp:block
        ] => { ... };
    [
            $( #[$meta:meta] )*
            $vis:vis $name:ident ($a:lifetime $input:ident)
                -> $ret_typ:ty
            $imp:block
        ] => { ... };
}

Defines a byte slice parser function, avoiding repetitive type parameters