Coccinelle for C, used by Linux kernel devs for decades, here's an article from 2009:

https://lwn.net/Articles/315686

Also IDE tooling for C#, Java, and many other languages; JetBrains' IDEs can do massive refactorings and code fixes across millions of lines of code (I use them all the time), including automatically upgrading your code to new language features. The sibling comment is slightly "wrong" — they've been available for decades, not mere years.

Here's a random example:

https://www.jetbrains.com/help/rider/ConvertToPrimaryConstru...

These can be applied across the whole project with one command, rewriting however many problems there are.

Also JetBrains has "structural search and replace" which takes language syntax into account, it works on a higher level than just text like what you'd see in text editors and pseudo-IDEs (like vscode):

https://www.jetbrains.com/help/idea/structural-search-and-re...

https://www.jetbrains.com/help/idea/tutorial-work-with-struc...

For modern .NET you have Roslyn analyzers built in to the C# compiler which often have associated code fixes, but they can only be driven from the IDE AFAIK. Here's a tutorial on writing one:

https://learn.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/t...