[tialaramex]

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.

[doomvox]

> the Editions system lets people keep older syntax in source code

Perl 'features' are lexically scoped, and could be applied in a finer-grained style, if you like. Shutting something off locally is done pretty commonly (e.g. to silence an ignorable warning).

The Perl devs have a deprecation cycle to remove mis-features, even if that breaks backwards compatibility. They don't, as far as I know, make much effort to line that up with major version changes[1], it's more a matter of making sure there's a long period of warning.

[1] The difficulty of course, was that for a long time the language now known as Raku was squatting on the next major version, "6", hence the need to jump to "7" to avoid confusion with Raku.