pub type net_input_func = Option<unsafe extern "C" fn(nif: *mut netif_t, pkt: *const u8, len: c_int) -> c_int>;
enum net_input_func { None, Some(unsafe extern "C" fn(_: *mut netif_t, _: *const u8, _: i32) -> i32), }
No value.
Some value of type T.
T