Maybe it's not as necessary with a codebase as well-organized as Oxide's, but I found gemini 3 useful for a refactor of some completely test-free ML research code, recently. I got it to generate a test case which would exercise all the code subject to refactoring, got it to do the refactoring and verify that it leads to exactly the same state, then finally got it to randomize the test inputs and keep repeating the comparison.