Vim can be configured to use the YouCompleteMe plugin to provide code-completion.
Once you've installed the YouCompleteMe plugin you should whitelist the
configuration file that is part of the SerenityOS repository. You can
do this by adding the following option to your .vimrc
file:
let g:ycm_extra_conf_globlist = ['~/serenity/.ycm_extra_conf.py']