C++ is somewhat of a low bar, because of the state of the art in package management in C++ land is "download a header-only library". C++ does have sum types in the form of boost::variant, but boost is an abomination.

The danger to Rust is that for many projects it wins by default because the alternative languages are immature, rather than because its performance/security capabilities are actually worth the productivity trade-off compared to other languages.

Once Swift matures a bit, it will take a lot of the lustre from Rust.