Function builtin_formals 

fn builtin_formals(
    args: &[RValue],
    _: &[(String, RValue)],
) -> Result<RValue, RError>

formals(fn) — return the formal parameter list of a function as a named list.

For closures, returns param names with defaults. For trait-based builtins with @param docs, returns the parameter names. Otherwise returns NULL.