You’re right about the impossibility of reviewing for style, clarity, and coherence. For correctness though, Windows is famous for being insistent on backwards compatibility over timespans measured in decades and that must surely be automated to the hilt.

As a third-party developer in the late 2000s I remember my boss giving me a CDROM binder (binders?) of every single OS release that Microsoft had ever put out. I assume he’d been given it my his developer-relations rep at Microsoft. My team and I used it to ensure our code worked on every MSDOS/Win* platform we cared to target.

I expect that, internally, the Windows team have crazy amounts of resources to implement the most comprehensive regression testing suite ever created. To that extent, at least, you’d be able to tell if the Rust version did what the old code did even if you didn’t read the code itself.

> For correctness though, Windows is famous for being insistent on backwards compatibility over timespans measured in decades and that must surely be automated to the hilt.

That hasn't been nearly the same goal for decades now.

For instance, Crysis literally won't run on win10 or later anymore.

On top of that, security bugs aren't the kind of thing you can automate away during a rewrite that no one has the bandwidth to actually review.