Yes, this is similar to what IntelliJ does for Java->Kotlin. Do a first pass that's extremely non-idiomatic and mechanical, then do lots of automated refactoring to bring it closer to idiomatic.

But if you're going to do it that way, the right place to start is probably to a safer form of C++ not Rust. That way code can be ported file-at-a-time or even function-at-a-time, and so you'll have a chance to run the assertions in the context of the original code. Which of course may not have good test coverage, as C codebases often don't, so you'll have to be testing your assertions in production.