> Edit: oof, though the stray character in question is inside a perfectly legitimate C string, so to catch this, any such diffs would need to Matroyshka down and that seems unsolvable / intractable.

Not sure, you could also just forbid code that's too complex to analyse without going down a rabbit hole. Instead of trying to go down the rabbit hole.

In general, it's hard to analyse arbitrary code. But we don't have to allow arbitrarily complex code when doing code review.