The factparse extension is new in HHVM 3.18. It's built on HHVM's own parser, so should be faster and more compatible than definition-finder.

For this release, it's opt-in: add "parser": "ext-factparse" to your hh_autoload.json to try it. It'll hopefully be the default in future releases when available.

This allows your dependencies to have a separate configuration for hhvm-autoload compared to composer. This is useful if you use 'files' for conditional includes in composer, eg https://github.com/hhvm/definition-finder/blob/master/autoload_files.php