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2cd58817d7
This is a developer QoL improvement, where running codegen will skip writing (and especially formatting) any files that were not changed. **Why?** While code generation in itself was pretty much instant, QL formatting of generated code was starting to take a long time. This made unconditionally running codegen quite annoying, for example before each test run as part of an IDE workflow or as part of the pre-commit hook. **How?** This was not completely straightforward as we could not work with the contents of the file prior to code generation as that was already post-processed by the QL formatting, so we had no chance of comparing the output of template rendering with that. We therefore store the hashes of the files _prior_ to QL formatting in a checked-in file (`swift/ql/.generated.list`). We can therefore load those hashes at the beginning of code generation, use them to compare the template rendering output and update them in this special registry file. **What else?** We also extend this mechanism to detect accidental modification of generated files in a more robust way. Before this patch, we were doing it with a rough regexp based heuristic. Now, we just store the hashes of the files _after_ QL formatting in the same checked file, so we can check that and stop generation if a generated file was modified, or a stub was modified without removing the `// generated` header.