I have something that looks like ```rust // crateA/lib.rs #[cfg(not(test))] extern "C" { fn ext_fn(); } #[cfg(test)] fn ext_fn() { unimplemented!(); } pub fn do_ext() { unsafe { ext_fn() }; } ``` ```rust // crateB/lib.rs crateA::do_ext() ``` but despite using `cargo test`, I still get a link error saying that `ext_fn` isn't found. Is `#[cfg(not(test))]` a valid attribute?