Today, the HHVM developers made an announcement[1] that they have plans of ceasing to maintain 100% PHP7 compatibility and concentrating on Hack instead.
While this does not mean that we need to take an action immediately, eventually we will have to decide something. As I see it, our options are:
1) Continue requiring that MediaWiki uses a common set of HHVM and Zend PHP. This, however, is a dead end and will make things progressively harder as the implementations will diverge and various Composer libraries we use will start requiring some Zend-specific features.
2) Declare our loyalty to HHVM. This will result in most of our current users being unable to upgrade, eventually producing what amounts to a WMF-only product and lots of installations with outdated MediaWiki having security holes. At least we will be able to convert to Hack eventually. This is a very clean-cut case of vendor lock-in though, and if Facebook decides to switch their code base to something shinier, we'll be deep in trouble.
3) Revert WMF to Zend and forget about HHVM. This will result in performance degradation, however it will not be that dramatic: when we upgraded, we switched to HHVM from PHP 5.3 which was really outdated, while 5.6 and 7 provided nice performance improvements.
I personally think that 3) is the only viable option in the long run. What do you think?
---- [1] http://hhvm.com/blog/2017/09/18/the-future-of-hhvm.html