Mention ‘rustc --edition=2018’

[andersk]

Rust code isn’t always compiled via Cargo, and it’s surprisingly hard to figure out that Rust 2018 is not the default for rustc and that you can run rustc --edition=2018. This ought to be documented somewhere and the Edition Guide seems like a good place.

[dclong]

And the same problem happens for edition 2021 now.