Function do_call_on_exit 

fn do_call_on_exit(
    args_val: &RValue,
    named: &[(String, RValue)],
    target_env: &Environment,
) -> Result<RValue, RError>

Handle do.call(on.exit, list(expr, add), envir=env).

on.exit is a pre-eval builtin that stores unevaluated expressions. When called via do.call, the expression is already an RValue (typically a Language/call object). We convert it back to an Expr for storage.