composer/composer — pull request #2823 by fredemmott (2014-03-18 21:40:25+00:00)
Title: Add autoload support for .hh files (HHVM)

HHVM is adding support for an alternative extension for files using
HHVM-specific features. Support them in the Class Map and PSR4
autoloaders.

Trivial example: https://github.com/fredemmott/hh_extension_toolset