At a guess, it will increase until the necessary-but-not-currently-handled constructs are dealt with (arena-based memory management, I'm looking at you) and then decrease asymptotically. Already in Rust, if you adopt C++ STL idioms (and don't want to squeeze more performance out) and don't need to visit currently-unwrapped interfaces, you won't need unsafe at all.
Rust is a very good C++ replacement.