Many comments are right this is prototype and there isn't any code guarantee! It is purely test case driven.
But this prototype sort of proves that to have Rust-equivalent memory safety, you don't really need to completely ditch C++, and all those "rewrite in Rust" clones of C++ repos. The time I spent on this project is very limited. I did maybe half of the dev on my phone through Happy. If Microsoft or Google who has lots of C++ code is willing to put some serious resources on this idea, I am sure they can have something a lot more solid. And they don't have to give up C++ (they shouldn't, it is very unclever engineering wise).
To me personally this prototype is a "usable" alternative to Circle C++. It saved me a lot of hard debugging time.