The previous parser is mostly a declarative grammar file, which is extremely readable. It codegens a C++ parser, which is hard to read. It depends which of those you count as the previous parser's source code!

In the future, we'd make changes by modifying the ANTLR parser first, then using the same approach as in the blog post to get the new parser to parity. We have no plans to get rid of the C++ parser as an oracle!

Yeah, one of the interesting parts to me while working on this is that the breakpoint for when it's worth writing your own parser vs accepting ANTLR's slowness has shifted massively. Previously it would have been someone's full-time job to maintain. Now with this approach you can get the best of both worlds.