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Internally at Google we have this functionality. We use Kythe, which is open-sourced: https://kythe.io/

Don't a lot of IDEs offer this functionality?

Yes... and no. See the thing about hundreds of millions of loc. You're not going to load the entire monorepo into the IDE -- it's not practical. But having an OpenGrok or whatever index of the monorepo is reasonable.

it's definitely useful for structural code annotation, which can be used by both search and code navigation. stay tuned.