>if half of your files in a million line codebase are unsafe that doesn't tell you much any more.

If half of your files in the first pass of a million line rewrite are unsafe then that's completely fine. Do you understand what the tag actually is? It doesn't even mean that the code is actually unsafe, just that the compiler can't guarantee its safety, which can happen for a number of reasons, some benign.

Who rewrites a 700K codebase trying to be idiomatic from the get go ? That's setting yourself up for failure, whether you're a human or a machine.