I expect Rust to be inevitable in embedded development, but yeah, that space moves slow, so give it another ten years or so. I will say that embedded is practically jumping on Rust, compared to how glacially it normally moves. You’ve got big vendors committing to offering Rust APIs, because many of their customers just don’t want to code C/C++ anymore.
I’m not sure, I understand the question, because there’s tons of languages that are basically just a superset of another, such as C++ for C, TypeScript for JavaScript, Crystal for Ruby. Heck, the whole family of LISP languages kind of matches your question, because they all use most of the syntax of LISP.