pub type dm_log_fn = Option<unsafe extern "C" fn(level: c_int, file: *const c_char, line: c_int, f: *const c_char, ...)>;
enum dm_log_fn { None, Some(unsafe extern "C" fn(_: i32, _: *const i8, _: i32, _: *const i8, ...)), }
No value.
Some value of type T.
T