Macro builtin

pub macro builtin(
    $(#[$($meta:tt)*])*
    fn $name:ident ($($arg:pat),*$(,)?) $(@$env:tt)? $body:block
) {
    ...
}

Turns a function definition into a Builtin.

let my_builtin = builtin! {
    /// Use the `@env` suffix to bind the environment!
    /// (needed for recursive calls)
    fn my_builtin(ConValue::Bool(b), rest @ ..) @env {
        // This is all Rust code!
        eprintln!("my_builtin({b}, ..)");
        match rest {
            [] => Ok(ConValue::Empty),
            _ => my_builtin(env, rest), // Can be called as a normal function!
        }
    }
};