C++'s Core Guidelines have been worked on for the last two years, and weren't even trying to get full compatibility with Rust's guarantees, and still aren't really done yet. It's great work, but it's far from "trivial". If it were trivial to turn C++ into Rust, we wouldn't have bothered making Rust.

That's... not true at all. Even aside from the memory safety stuff steveklabnik mentioned in a sibling comment, C++ is struggling to get things like concepts, pattern matching, or modules, and had to do a lot of non-trivial work in the past to get things like move semantics.

And even then none of that solves the problems I mentioned in the comment you replied to.

Java is only fossilized on Google's Android fork.

Who knows, maybe it even gets value types before the C++ community can agree what modules are supposed to be.