Comment by bskap

Well, indexing and searching is kind of Google's thing. It also helps that they're using a unified build system. They can instrument the compiler to get cross references from the build instead of trying to figure out out by parsing the text themselves.

They open sourced the tool to do it- https://kythe.io - but I think it would be a pain to make it work for anything like GitHub that supports arbitrary languages and build systems with untrusted code