The Rust "edition" flag is set on a per-library ("crate") basis, not per source file. It basically just controls a few small differences at the parser level. I

The Rust "edition" flag is set on a per-library ("crate") basis, not per source file. It basically just controls a few small differences at the parser level. I'm really happy about the ability to mix older Rust 2015 and newer Rust 2018 code in the same project, and the tools for automatically migrating code to the 2018 edition. It took us like 3 hours to migrate a large code base at work, and I think I needed to file a bug report about exactly one upstream library. Everything else was seamless. And we can still use all our Rust 2015 dependencies.