In Hack, what are these called/how do they work: <<something>> over methods or classes?

Question

At first glance I thought they were decorators/annotations, but I'm not sure that's exactly what they are.

Answer 1

They are called attributes. Most of them have no inherent meaning, though a handful (__Override, __ConsistentConstruct, __Memoize, __Deprecated) have runtime and type checker meanings.