style-guide: Document style editions, start 2024 style edition

Link to a snapshot for the 2015/2018/2021 style edition.

src/doc/style-guide/src/SUMMARY.md

@@ -9,4 +9,5 @@
 - [Other style advice](advice.md)
 - [`Cargo.toml` conventions](cargo.md)
 - [Guiding principles and rationale](principles.md)
+- [Rust style editions](editions.md)
 - [Nightly-only syntax](nightly.md)

src/doc/style-guide/src/editions.md

@@ -0,0 +1,44 @@
+# Rust style editions
+
+The default Rust style evolves over time, as Rust does. However, to avoid
+breaking established code style, and CI jobs checking code style, changes to
+the default Rust style only appear in *style editions*.
+
+Code written in a given Rust edition uses the corresponding Rust style edition
+by default. To make it easier to migrate code style separately from the
+semantic changes between Rust editions, formatting tools such as `rustfmt`
+allow updating the style edition separately from the Rust edition.
+
+The current version of the style guide describes the latest Rust style edition.
+Each distinct past style will have a corresponding archived version of the
+style guide.
+
+Note that archived versions of the style guide do not document formatting for
+newer Rust constructs that did not exist at the time that version of the style
+guide was archived. However, each style edition will still format all
+constructs valid in that Rust edition, with the style of newer constructs
+coming from the first subsequent style edition providing formatting rules for
+that construct (without any of the systematic/global changes from that style
+edition).
+
+Not all Rust editions have corresponding changes to the Rust style. For
+instance, Rust 2015, Rust 2018, and Rust 2021 all use the same style edition.
+
+## Rust 2024 style edition
+
+This style guide describes the Rust 2024 style edition. The Rust 2024 style
+edition is currently nightly-only and may change before the release of Rust
+2024.
+
+For a full history of changes in the Rust 2024 style edition, see the git
+history of the style guide. Notable changes in the Rust 2024 style edition
+include:
+
+- Miscellaneous `rustfmt` bugfixes.
+
+## Rust 2015/2018/2021 style edition
+
+The archived version of the style guide at
+<https://github.com/rust-lang/rust/tree/37343f4a4d4ed7ad0891cb79e8eb25acf43fb821/src/doc/style-guide/src>
+describes the style edition corresponding to Rust 2015, Rust 2018, and Rust
+2021.