Code search is indeed hard. Stop words, stemming and such do rule out most off the shelf indexing solutions but you can usually turn them off. You can even get around the splitting issues of things like

    a.toString()

With some pre-processing of the content. However were you really get into a world of pain is allowing someone to search for ring in the example. You can use partial term search, prefix, infix, or suffix but this massively bloats the index and is slow to run.

The next thing you try is trigrams, and suddenly you have to deal with false positive matches. So you add a positional portion to your index, and all of a sudden the underlying index is larger than the content you are indexing.

Its good fun though. For those curious about it I would also suggest reading posts by Michael Stapelberg https://michael.stapelberg.ch/posts/ who writes about Debian Code Search (which I believe he started) in addition to the other posts mentioned here. Shameless plug, I also write about this https://boyter.org/posts/how-i-built-my-own-index-for-search... where I go into some of the issues when building a custom index for searchcode.com

Oddly enough I think you can go a long way brute forcing the search if you don't do anything obviously wrong. For situations where you are only allowed to search a small portion of the content, say just your own (which looks applicable in this situation) that's what I would do. Adding an index is really only useful when you start searching at scale or you are getting semantic search out of it. For keywords which is what the article appears to be talking about, that's what I would be inclined to do.