pub type SENDASYNCPROC = Option<unsafe extern "C" fn(arg1: HWND, arg2: UINT, arg3: ULONG_PTR, arg4: LRESULT)>;
pub enum SENDASYNCPROC { None, Some(unsafe extern "C" fn(*mut HWND__, u32, u64, i64)), }
No value.
Some value of type T.
T