I can also give some in-the-trenches perspective of vec's utility, having spent the last month and a half writing Hack. vec is a really useful data structure to be able to use explicitly in code. It makes code that uses it easier to understand.
The main benefit over Vector is that it could be used as a straightforward replacement for array in many cases, with the same copy-on-write semantics helping developers avoid spooky action-at-a-distance. I'm confident that by the time that such a feature would be ready for primetime, there would be tools in place to assist migrations. We could also presumably allow for casts between vec and array. And once opcache is sorted out, I'm also confident that the advice "using vec will make your code a bit faster" would be a successful recruitment tool. Best wishes, Matt
