There are various PHP7-only features that we should IMHO recommend in our coding conventions and during code review, but that we cannot currently use with HHVM. Note, this task is not meant to be a tracking task. It should collect what we think are the most useful features, and can be considered resolved as soon as all of these features are either recommended on mw.org, or enforced via PHPCS depending on the single case. The ones I can think of now: (see T231710#5520397)
- Scalar typehints - https://gerrit.wikimedia.org/r/538726
- Explicit variargs in method signatures - https://gerrit.wikimedia.org/r/539177
- Constant arrays
- Nullable typehints (i.e. ?MyClass $x instead of MyClass $x = null) - PHP71+ - https://gerrit.wikimedia.org/r/539308
- Visibility on class constants
Also, note that this task shouldn't become a lame list of new PHP7 features. Everything included here should either improve (static) analysis, code's self-documentation, or overcome annoying HHVM limitations that force(d) us to write worse code (e.g. constant arrays and variargs).