Macro define_function_enum 

macro_rules! define_function_enum {
    (
        $(#[$meta:meta])*
        $vis:vis enum $name:ident {
            $(
                $(#[$variant_meta:meta])*
                $variant:ident => {
                    name: $func_name:literal,
                    arity: $arity:expr,
                    variadic: $variadic:expr,
                    description: $desc:literal,
                    handler: $handler:expr
                }
            ),* $(,)?
        }
    ) => { ... };
}

Macro for defining an enumeration of built-in functions

Auto-generate name(), arity(), is_variadic(), description(), execute()