style-guide: Format single associated type where clauses on the same line

In particular, lifetime-generic associated types often have a
`where Self: 'a` bound, which we can format on the same line.

Author: joshtriplett

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

@@ -37,6 +37,7 @@ history of the style guide. Notable changes in the Rust 2024 style edition
 include:
 
 - Miscellaneous `rustfmt` bugfixes.
+- Format single associated type `where` clauses on the same line if they fit.
 
 ## Rust 2015/2018/2021 style edition
 

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

@@ -421,9 +421,20 @@ Format associated types like type aliases. Where an associated type has a
 bound, put a space after the colon but not before:
 
 ```rust
-pub type Foo: Bar;
+type Foo: Bar;
 ```
 
+If an associated type declaration has no bounds, and has a `where` clause with
+a single entry, format the entire type declaration including the `where` clause
+on the same line if it fits:
+
+```rust
+type Item<'a> where Self: 'a;
+```
+
+If the `where` clause contains multiple entries, or the associated type has any
+bounds, format it across multiple lines as with a type alias.
+
 ## extern items
 
 When writing extern items (such as `extern "C" fn`), always specify the ABI.