If I can autoport my C++ to Rust, and the port is confirmed identical, and the Rust is confirmed safe, can't I use that to reason about the safety of my C++? Is safe C++ just a matter of proving it has a safe Rust equivalent?
It depends on what “confirmed identical” means. Most compilers (or translators as you like) only guarantee that the output program represents a subset of the behaviors of the input program so it could be that subset is the “safe as in Rust” subset.
GP is referring to the article's title, as in "safe Rust", as in "Rust with no unsafe keyword", as in whether it has undefined behavior or not.
Granted the premise is rather brittle, since all Cpp2Rust does is to convert UB to runtime crashes, which can be undecidable, and cannot be used to prove anything about the original C++ code.
