add rustfmt style edition

src/SUMMARY.md

@@ -49,6 +49,7 @@
   - [Cargo: Reject unused inherited default-features](rust-2024/cargo-inherited-default-features.md)
   - [Rustfmt: Combine all delimited exprs as last argument](rust-2024/rustfmt-overflow-delimited-expr.md)
   - [Rustfmt: Raw identifier sorting](rust-2024/rustfmt-raw-identifier-sorting.md)
+  - [Rustfmt: Style Edition](rust-2024/rustfmt-style-edition.md)
   - [`gen` keyword](rust-2024/gen-keyword.md)
   - [Macro fragment specifiers](rust-2024/macro-fragment-specifiers.md)
   - [Missing macro fragment specifiers](rust-2024/missing-macro-fragment-specifiers.md)

src/rust-2024/rustfmt-style-edition.md

@@ -0,0 +1,88 @@
+# Rustfmt: Style Edition
+
+🚧 The 2024 Edition has not yet been released and hence this section is still "under construction".
+
+More information may be found in the tracking issue at <https://github.com/rust-lang/rust/issues/123799>.
+
+## Summary
+
+User can now control which style edition to use with `rustfmt`. 
+
+## Details
+
+The default formatting produced by Rustfmt is governed
+by the rules in the [Rust Style Guide].
+
+Additionally, Rustfmt has a formatting stability guarantee that aims
+to avoid causing noisy formatting churn for users when updating a
+Rust toolchain. This stability guarantee essentially means that a newer
+version of Rustfmt cannot modify the _successfully formatted_ output
+that was produced by a previous version of Rustfmt.
+
+The combination of those two constraints had historically locked both
+the Style Guide and the default formatting behavior in Rustfmt. This
+impasse caused various challenges, such as preventing the ability to
+iterate on style improvements, and requiring Rustfmt to maintain legacy
+formatting quirks that were obviated long ago (e.g. nested tuple access).
+
+[RFC 3338] resolved this impasse by establishing a mechanism for the
+Rust Style Guide to be aligned to Rust's Edition model wherein the
+Style Guide could evolve across Editions, and `rustfmt` would allow users
+to specify their desired Edition of the Style Guide, referred to as the Style Edition. 
+
+In the 2024 Edition, `rustfmt` now supports the ability for users to control
+the Style Edition used for formatting. The 2024 Edition of the Style Guide also
+includes enhancements to the Style Guide which are detailed elsewhere in this Edition Guide.
+
+By default `rustfmt` will use the same Style Edition as the standard Rust Edition
+used for parsing, but the Style Edition can also be overridden and configured separately. 
+
+There are multiple ways to run `rustfmt` with the 2024 Style Edition:
+
+With a `Cargo.toml` file that has `edition` set to `2024`, run:
+
+```sh
+cargo fmt
+```
+
+Or run `rustfmt` directly with `2024` for the edition to use the 2024 edition
+for both parsing and the 2024 edition of the Style Guide:
+
+```sh
+rustfmt lib.rs --edition 2024
+```
+
+The style edition can also be set in a `rustfmt.toml` configuration file:
+```toml
+style_edition = "2024"
+``` 
+
+Which is then used when running `rustfmt` directly:
+```sh
+rustfmt lib.rs
+```
+
+Alternatively, the style edition can be specified directly from `rustfmt` options:
+
+```sh
+rustfmt lib.rs --style-edition 2024
+```
+
+[Rust Style Guide]: ../../style-guide/index.html
+[RFC 3338]: https://rust-lang.github.io/rfcs/3338-style-evolution.html
+
+## Migration
+
+Running `cargo fmt` or `rustfmt` with the 2024 edition or style edition will
+automatically migrate formatting over to the 2024 style edition formatting.
+
+Projects who have contributors that may utilize their editor's format-on-save
+features are also strongly encouraged to add a `.rustfmt.toml` file to their project
+that includes the corresponding `style_edition` utilized within their project, or to
+encourage their users to ensure their local editor format-on-save feature is
+configured to use that same `style_edition`.
+
+This is to ensure that the editor format-on-save output is consistent with the
+output when `cargo fmt` is manually executed by the developer, or the project's CI
+process (many editors will run `rustfmt` directly which by default uses the 2015
+edition, whereas `cargo fmt` uses the edition specified in the `Cargo.toml` file)