[oconnor663]

Editions are only necessary for otherwise-backwards-incompatible changes, to avoid breaking existing code. Even relatively major language features like the ? operator were shipped without an edition bump. But for example I think the await keyword itself was added as part of an edition, because it was incompatible with existing code that had a method named await.

The messaging around the 2018 edition was a little different here, with some attempts to use the edition release as an opportunity to advertise and summarize all the features that had shipped since 2015. But I think folks decided that was confusing, and with the 2021 edition the messaging was toned down.

[jen20]

Yes, that's a fair point about the actual purpose of editions being to handle forwards-compatibility.

Informally though, I think there are certainly a subset of people that use "Rust 2021" to mean all of the features included up to that point, even if it's not the precise definition.