“The short answer is that we built our own search engine from scratch, in Rust, specifically for the domain of code search.”
GitHub spent most of its history failing to make general text search work on code. Elasticsearch took months to index 8 million repositories. Blackbird now covers 45 million repositories, 115 TB of code, and squeezes it into a 25 TB index that re-indexes in about 18 hours. Brute-force grep on the same data would tie up 2,048 cores for 96 seconds per query.