Last time i checked c++ debugger still used 16 bit line numbers. My attempts to make a plugin for vs also left an impression that it contains large amount of modules glued together using various technologies. Things like 5 layers of c# API wrappers named api10, api20, api30 on top of some c code communicating using COM objects. I wouldn't be surprised if they still had some third party libraries for which they only have a binaries and vendor doesn't exist anymore.