See my sibling comment. Using Rust implies many more changes than just moving code to an incompatible-but-close version of C and C++ that adds lifetime annotations and unsafe blocks. Compare it with the Python 2/3 split vs. rewriting all your Python 2 code to, say, Ruby.
Then there is the OOB issues too, like using another build system, a single compiler based on LLVM, etc.
Even moving large enterprise codebases from python 2 to python 3 is taking so long they had to push that deadline back years. Given the distance you'd have to cross to push an older C++ codebase to something with similar guarantees as Rust, I don't know if any company would voluntarily cross that gap.
Worth noting that the vast majority of lifetime annotations in Rust are “elided”, i.e. inferred by the compiler.
I’d say that a successful “safe C” compiler would accept any existing C code that never invokes undefined behaviour, without a runtime performance penalty.
That's unfortunately what's not possible. The C source code doesn't contain enough information to prove safety (a lot of real world C code depends on invariants that happen to be true at runtime, but aren't provably true at compile time)