The code completion in GoWorks is completely implemented using ANTLR 4. The following video shows the level of completion of this code completion engine. The code completion example runs from 5 minutes through the end of the video.
Intro to Tunnel Vision Labs' GoWorks IDE (Preview Release)
I have been working on code completion algorithms for many years, and strongly believe that there is no better solution (automated or manual) for producing a code completion solution for a new language that meets the requirements for what I would call highly-responsive code completion. If you are not interested in that level of performance or accuracy, other solutions may be easier for you to get involved with (I don't work with those personally, because I am too easily disappointed in the results).