The difference between Rust 2015 and 2018 is less than most people think. Originally, yes, Rust 2018 was used to push the new borrow checker without affecting code using Rust 2015. But unlike a "version" that gets deprecated and left behind, a Rust edition perpetually continues to be supported and benefit from improvements with every compiler release. But this means that in order to support different borrow checkers in different editions the compiler had to continue to ship with both borrow checkers, which, considering how much code that is and the aforementioned bugs, was an intractable proposition in the long term.