Reminds me of the Tomorrow Corporation (developer of World of Goo) tech demo: https://www.youtube.com/watch?v=72y2EC5fkcE
Their entire stack is custom (language, compiler, IDE, build system, engine). Has neat features like time-travelling debugging and instant hot-reloading for code and assets.
What's time traveling debugging?
https://en.wikipedia.org/wiki/Time_travel_debugging
Basically it’s rewinding a process so you can observe changes or twiddle the knobs.
Neat~ it kind of reminds me of Bret Victor who did a lot of research and design into experimental programming environments where changing code instantly updates the visual output.
https://www.youtube.com/watch?v=a-OyoVcbwWE