pub type FN_FaceDetector = Option<Box<dyn FnMut(*const c_void, *const c_void) -> bool + Send + Sync + 'static>>;
enum FN_FaceDetector { None, Some(Box<dyn FnMut(*const c_void, *const c_void) -> bool + Send + Sync>), }
No value.
Some value of type T.
T