There are thousands of integration tests. The "correct" output of the parser is well-known for a given input, and those test cases have been accumulating for o

There are thousands of integration tests. The "correct" output of the parser is well-known for a given input, and those test cases have been accumulating for over a decade. But the internal structure of the parser is much more fluid, and so it wasn't (historically) thought worthwhile to try to write tests against that shifting target.