This is, of course, due to the fact that Rust doesn't have a language specification, just a compiler implementation. If an old version of the compiler has a bug, using an old edition will still have that bug. Contrast this with C++, using an old feature you can still benefit from the latest bugfixes, optimisations, target architectures and instruction sets.

With the new GCC Rust work I believe a new solution will need to be found, and it will end up more similar to the situation with C++.