In my understanding, this alone isn't good enough. You need to: 1. Say what you expect an implementation to do (the spec) 2. Check that the tool does what you

In my understanding, this alone isn't good enough. You need to: 1. Say what you expect an implementation to do (the spec) 2. Check that the tool does what you say in step 1 (the implementation) Part 2 on its own is not enough. The reference implementation is an implementation, but you still need an explicitly different reference to check said implementation against. That it's a reference implementation means it's an appropriate thing to derive 1 from, which is of course what's happening here. And rustc's massive test suite is very, very helpful in demonstrating 2.