This version of the text assumes you’re using Rust 1.52 or later with edition="2018" in Cargo.toml of all projects to use Rust 2018 Edition idioms. See the “Installation” section of Chapter 1 to install or update Rust, and see the new Appendix E for information on editions.
The 2018 Edition of the Rust language includes a number of improvements that make Rust more ergonomic and easier to learn. This iteration of the book contains a number of changes to reflect those improvements: