I needed to parse some MIG (Mach Interface Generator) files to feed functions with out/inout variable directions into a static analyzer step over XNU. This is the hacky method I took to do so in case it saves anyone else some time. First, MIG files have C-style preprocessor directives, I resolved all of them by running the following command in the XNU source tree:
find . -name \*.defs | xargs cat | clang -E -DIOKIT -DKERNEL -Iosfmk - > master.mig
You might want to change the command line #defines to suit your particular environment.
Next, I found a slightly dated MIG ANTLR definition here and updated it to ANTLR3. I added some tree rewriting rules (e.g. look for ROUTINE_DEF in the grammar) to easily pull out routines, arguments, and basic type information. I generated the lexer/parser in generated/ by running antlr4ruby -fo generated/ Mig.g.
Finally ./parse.rb master.mig parses out the required information. Happy parsing!