Yeah, the crucial trick is that Rust is a single language but the Editions system lets people keep older syntax in source code, knowing it will be transformed into the single abstract syntax of that language.

Rust editions are not versions of the language, they're just syntax.

New features of the language in a new library or compiler version work fine in all editions unless they require syntax which isn't in your chosen edition.

Rust 2015 edition is only restricted to the syntax from 2015, all the features of today are available unless they're gated off somehow by syntax. For example the "async" keyword didn't exist in Rust 2015, so, you won't be using that from 2015 edition even today. But even though in 2015 you certainly couldn't go around evaluating u32::checked_div() in a constant, it works just fine in Rust 2015 edition today, because it's not gated by syntax.