Julia isn't primarily implemented in C++. Most of the implementation is in Julia (68%). About 25% is C/C++, but that could be changed if someone wrote a good enough compiler in a fast enough memory safe language. The rest is in a custom scheme dialect (femptolisp) and is in the process of being replaced by Julia (see JuliaSyntax.jl).
I'm hoping we get there. LLVM has really good codegen, but I think we can probably do a better job on the optimizer side than they do. Don't expect anything soon though. Replacing LLVM is (kinda obviously) a ridiculously huge project.
Apparently AUTOSAR and MISRA are relevant enough for Rust, that efforts are being made to allow a Rust certification subset as well.