Comment by chrismorgan

For curious people unfamiliar with the situation with this: Rust cares about backwards-compatibility so that you should still be able to compile code from 2015 in 2042 (some libraries are just stable!), but it also wants to allow certain changes to the language that you might think would be backwards-incompatible. Each crate (library) declares an edition, and the compiler follows the rules of that edition, and then you can link all the different crates together without worrying what edition they were written in. Editions are limited in what they can change, because things like traits are shared between crates, so they can’t be changed in editions; it’s mostly syntactic stuff, like the 2018 edition making async/await keywords and making dyn a full (rather than conditional) keyword.