Golang and C++ don't serve the same purpose. Google internally sometimes uses both to the same effect but that's because Google internally has a bunch of stuff (including new kernel syscalls) that makes highly concurrent server development in C++ palatable.

So why "compete" with Rust? Because C++ compatibility was never a design goal of Rust, so gradual, painless migration from C++ to Rust is a non-starter. The best you can hope to do is rewrite entire subsystems where the API is narrow enough. This new language is obviously going to allow something more like Kotlin or even Swift where codebases will be mixed and new code will be written in Carbon.