2024: Add chapter on single-line where clauses

Author: joshtriplett

src/SUMMARY.md

@@ -67,5 +67,6 @@
   - [Rustfmt](rust-2024/rustfmt.md)
     - [Rustfmt: Style edition](rust-2024/rustfmt-style-edition.md)
     - [Rustfmt: Combine all delimited exprs as last argument](rust-2024/rustfmt-overflow-delimited-expr.md)
+    - [Rustfmt: Single-line `where` clauses](rust-2024/rustfmt-single-line-where-clauses.md)
     - [Rustfmt: Raw identifier sorting](rust-2024/rustfmt-raw-identifier-sorting.md)
     - [Rustfmt: Version sorting](rust-2024/rustfmt-version-sorting.md)

src/rust-2024/rustfmt-single-line-where-clauses.md

@@ -0,0 +1,43 @@
+# Rustfmt: Single-line where clauses
+
+## Summary
+
+When an associated type or method declaration has a single bound in the `where` clause, it can now sometimes be formatted on one line.
+
+## Details
+
+In general, the [Rust Style Guide](../../style-guide/) states to wrap `where` clauses onto subsequent lines, with the keyword `where` on a line of its own and then the individual clauses indented on subsequent lines.
+
+However, in the 2024 Style Edition, when writing an associated type declaration or a method declaration (with no body), a short `where` clause can appear on the same line.
+
+This is particularly useful for generic associated types (GATs), which often need `Self: Sized` bounds.
+
+In Rust 2021 and before, this would look like:
+
+```rust
+trait MyTrait {
+    fn new(&self) -> Self
+    where
+        Self: Sized;
+
+    type Item<'a>: Send
+    where
+        Self: 'a;
+}
+```
+
+In the 2024 Edition, `rustfmt` now produces:
+
+```rust
+trait MyTrait {
+    fn new(&self) -> Self where Self: Sized;
+
+    type Item<'a>: Send where Self: 'a;
+}
+```
+
+## Migration
+
+The change can be applied automatically by running `cargo fmt` or `rustfmt` with the 2024 Edition. See the [Style edition] chapter for more information on migrating and how style editions work.
+
+[Style edition]: rustfmt-style-edition.md