It would be cool to converge on some kind of textual format for AST early on, so that testing harness can be basically a glorified
diff. That is, it would be useful to be able to compare some parser against current implementation without needing to build rustc, and using some test corpus instead.
JSON could be used for that, but I think that a home-grown text format could be much more readable. I kinda like the one use in IntelliJ/libsyntax2: just an indentation based tree where nodes are marked with textual offsets and kinds. Here’s an example: