No you’re misunderstanding the ecosystem. Rust 2024 code can call 2021 code without issue (and vice versa I think although could be wrong on the vice versa). So you can progressively update code of individual components as you want or not at all and still continue using it just fine in new code using later editions. Thats the very definition of back compat, something you really really shouldn’t do with C++ (every file should be compiled with the same language version target although practically that may be less of an issue depending on the specific stdlib implementation and if they break ABI)
There’s also automated migration tools to convert 2021 code to 2024. It might fail on some translations but generally it’s pretty automatic.
So huge difference both in the migration mechanism and the bc story.
Right. A good mental model would be to imagine every crate of Rust source has an associated Edition and there's a sort of "pre-processing" step where we translate that into the latest version of Rust seamlessly, preserving its exact meaning.
So e.g. 2018 Edition said r# at the start of an identifier now marks a "raw" identifier. Keywords promise never to start this way, so r#foo is the same as foo but r#foo even works if some lunatic makes foo a keyword whereas just foo would become a keyword if that happened. As a result if you write
let async = 5;
... in Rust 1.0 that translator treats it exactly as though you'd written
let r#async = 5;
... in a modern Rust edition because these days the keyword async exists.
There’s also automated migration tools to convert 2021 code to 2024. It might fail on some translations but generally it’s pretty automatic.
So huge difference both in the migration mechanism and the bc story.