> The key part is that editions are configured per-library - libraries A and B might use editions 2015 and 2021

In what way is that key? It still reads as being the same as the Go version directive. Obviously there are some differences in the implementation. For example, Go puts it in go.mod, while Rust puts it in Cargo.toml, but at a conceptual level I fail to see any fundamental difference. As you describe it, and how the documentation describes it, they attempt to accomplish the same thing for the same reason.

But, as phire puts it, they are "very different". But I don't see how. The carrying on of the tradition of the Rust community being horrible at communication carries on, I'm afraid. As before, you are going to have to speak to those who aren't deep in the depths of programming languages. Dumb it down for the reader who uses PHP and who has never touched Go or Rust in their life.

> they don't enable Go to make breaking changes to the language.

What, exactly, do you mean? The change to loop variable semantics comes to mind that was clearly a breaking change to the language, but gracefully handled with the version directive. What purpose are you under the impression the directive serves if not for dealing with breaking changes?