Rust provides best effort transformation for its Editions (which only touch syntax) in cargo fix --edition, but even that's only best effort. If a proc macro does something ludicrous (see Mara's whichever-compiles! macro for example) how can the transformation hope to keep up?

C++ versions are in much deeper, they not only change the language syntax, but also semantics of existing features. Sometimes the intended effect is null (but intentions may not match reality) and sometimes it is not.

C++ also substantially changes the standard library. Rust's standard library grows but doesn't get redefined in new Editions, so if you call a deprecated Rust function it's not going anywhere, whereas a deprecated C++ method might actually go away in a future version.

It's a shame that the best effort tools aren't provided with C++ compilers, but even if they were provided on a large codebase it's only the beginning for large projects.