pub type CK_C_DigestKey = Option<unsafe extern "C" fn(hSession: CK_SESSION_HANDLE, hKey: CK_OBJECT_HANDLE) -> CK_RV>;
enum CK_C_DigestKey { None, Some(unsafe extern "C" fn(_: u64, _: u64) -> u64), }
No value.
Some value of type T.
T