"semantically compatible but not syntactically compatible" would have been a more impressive way of expressing it.

If Rust is highly semantically compatible with C++, then automated porting of C++ to Rust isn't such a far-out idea.

Definitely it'd be interesting. But I feel like such a thing is going to have the same issues I worry about with the ISO Core C++ lifetime checker: existing C++ code just isn't designed for lifetimes, so you will have to rework the logic a lot to get the static analysis to pass. When you have that much manual intervention, I wonder whether you're porting or really just rewriting—and if you're rewriting, syntactic compatibility doesn't matter so much.