Autoformatters are deterministic tools that are tested regularly, with extensive test suites and went through a very long process of production usage and review before the reformat:

    > We also built a tool to diff ripper trees across formatted files, accounting for things like rubyfmt converting single quotes to double quotes. Combined with our extensive test suite, we built confidence slowly and deliberately.

They also didn't do the entire thing in one weekend—that was just the article title clickbait. they did it file by file, incrementally, over the course of months:

    > Rolling out a novel autoformatter to 25 million lines of code has two big risks: merge conflicts and correctness. A bug affecting just 0.01% of lines would still touch tens of thousands of files. To manage both, we built in a per-file opt-in so rubyfmt would only format files that explicitly asked for it. Following the Developer Productivity org’s typical pattern, we started with systems we owned and could observe closely, then expanded coverage gradually as our confidence grew.