I have a large existing C++ codebase but haven't worked in Rust yet. But from what I've seen, Rust looks like a very well designed language that has the low-level and performance aspects of C++, with similar flexibility. IMO the killer feature is the borrowing mechanism which provides provably-safe memory access, eliminating whole classes of memory/security bugs, with minimal performance impact. I don't think C++ will ever expand to encompass that. I haven't got much experience with the Rust toolchain but I think that's a strong reason to seriously consider Rust for any new projects.