Important: The headline was modified since I submitted this. Apple didn't actually open-source the code. They give you a 90-day licence to read the code.

> Apple grants you, for a period of ninety days from the date you downloaded the Apple Software, a limited, non-exclusive, non-licensable license under Apple's copyrights in the Apple Software to make a reasonable number of copies of, compile and run the Apple Software internally within your organization only on devices and computers you own or control for the sole purpose of verifying the security characteristics and correct functioning of the Apple Software;...