The release of Rust 1.0 established "stability without stagnation" as a core Rust deliverable. Ever since the 1.0 release, the rule for Rust has been that once a feature has been released on stable, we are committed to supporting that feature for all future releases.
There are times, however, when it is useful to be able to make small changes
to the language that are not backwards compatible.
The most obvious example is introducing a new keyword,
which would invalidate variables with the same name.
For example, the first version of Rust did not have the
Suddenly changing those words to keywords in a later version would've broken code like
let async = 1;.
Editions are the mechanism we use to solve this problem.
When we want to release a feature that would otherwise be backwards incompatible,
we do so as part of a new Rust edition.
Editions are opt-in, and so existing crates do
not see these changes until they explicitly migrate over to the new edition.
This means that even the latest version of Rust will still not treat
async as a keyword,
unless edition 2018 or later is chosen.
This choice is made per crate as part of its
New crates created by
cargo new are always configured to use the latest stable edition.
The most important rule for editions is that crates in one edition can interoperate seamlessly with crates compiled in other editions. This ensures that the decision to migrate to a newer edition is a "private one" that the crate can make without affecting others.
The requirement for crate interoperability implies some limits on the kinds of changes that we can make in an edition. In general, changes that occur in an edition tend to be "skin deep". All Rust code, regardless of edition, is ultimately compiled to the same internal representation within the compiler.
Our goal is to make it easy for crates to upgrade to a new edition.
When we release a new edition,
we also provide tooling to automate the migration.
It makes minor changes to your code necessary to make it compatible with the new edition.
For example, when migrating to Rust 2018, it changes anything named
async to use the equivalent
raw identifier syntax:
The automated migrations are not necessarily perfect: there might be some corner cases where manual changes are still required. The tooling tries hard to avoid changes to semantics that could affect the correctness or performance of the code.
In addition to tooling, we also maintain this Edition Migration Guide that covers the changes that are part of an edition. This guide describes each change and gives pointers to where you can learn more about it. It also covers any corner cases or details you should be aware of. This guide serves both as an overview of the edition and as a quick troubleshooting reference if you encounter problems with the automated tooling.