2024: Assignment operator RHS indentation

src/SUMMARY.md

@@ -66,6 +66,7 @@
     - [Rustdoc nested `include!` change](rust-2024/rustdoc-nested-includes.md)
   - [Rustfmt](rust-2024/rustfmt.md)
     - [Rustfmt: Style edition](rust-2024/rustfmt-style-edition.md)
+    - [Rustfmt: Assignment operator RHS indentation](rust-2024/rustfmt-assignment-operator-rhs-indentation.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)

src/rust-2024/rustfmt-assignment-operator-rhs-indentation.md

@@ -0,0 +1,59 @@
+# Rustfmt: Assignment operator RHS indentation
+
+## Summary
+
+In the 2024 Edition, `rustfmt` now indents the right-hand side of an assignment operator relative to the last line of the left-hand side, providing a clearer delineation and making it easier to notice the assignment operator.
+
+## Details
+
+In Rust 2021 and before, if an assignment operator has a multi-line left-hand side, the indentation of the right-hand side will visually run together with the left-hand side:
+
+```rust,ignore
+impl SomeType {
+    fn method(&mut self) {
+        self.array[array_index as usize]
+            .as_mut()
+            .expect("thing must exist")
+            .extra_info =
+            long_long_long_long_long_long_long_long_long_long_long_long_long_long_long;
+
+        self.array[array_index as usize]
+            .as_mut()
+            .expect("thing must exist")
+            .extra_info = Some(ExtraInfo {
+            parent,
+            count: count as u16,
+            children: children.into_boxed_slice(),
+        });
+    }
+}
+```
+
+In the 2024 Edition, `rustfmt` now indents the right-hand side relative to the last line of the left-hand side:
+
+```rust,ignore
+impl SomeType {
+    fn method(&mut self) {
+        self.array[array_index as usize]
+            .as_mut()
+            .expect("thing must exist")
+            .extra_info =
+                long_long_long_long_long_long_long_long_long_long_long_long_long_long_long;
+
+        self.array[array_index as usize]
+            .as_mut()
+            .expect("thing must exist")
+            .extra_info = Some(ExtraInfo {
+                parent,
+                count: count as u16,
+                children: children.into_boxed_slice(),
+            });
+    }
+}
+```
+
+## 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