[puzzle]

Google also has this:

https://source.bazel.build

[wocram]

That's not an open source product and not available otherwise, though it is backed by kythe.

[hanwenn]

If you want to have an open source index search, you can try out github.com/google/zoekt/ . See here for a demo site: https://cs.bazel.build/

For example: https://cs.bazel.build/search?q=r%3Atorvalds+meltdown&num=50 searches the Linux kernel for "meltdown"