rust-lang/style-team#189: rhs-should-use-indent-of-last-line-of-lhs

src/expr.rs

@@ -2060,12 +2060,38 @@ fn rewrite_assignment(
     let lhs_shape = shape.sub_width(operator_str.len() + 1)?;
     let lhs_str = format!("{} {}", lhs.rewrite(context, lhs_shape)?, operator_str);
 
+    let lhs_lines: Vec<&str> = lhs_str.split("\n").collect();
+
+    let mut rhs_shape = shape.clone();
+
+    for line in lhs_lines.into_iter().rev() {
+        let mut indent_width = 0;
+        let mut first_char = ' ';
+        for char in line.chars() {
+            if char != ' ' {
+                first_char = char;
+                break;
+            } else {
+                indent_width += 1;
+            }
+        }
+
+        if first_char != '/' {
+            let indent = Indent::from_width(&context.config, indent_width);
+            rhs_shape = Shape::indented(indent, &context.config);
+            break;
+        }
+    }
+
+    println!("config={:?}", context.config.max_width());
+    println!("old shape={shape:?}");
+    println!("new shape={rhs_shape:?}");
     rewrite_assign_rhs(
         context,
         lhs_str,
         rhs,
         &RhsAssignKind::Expr(&rhs.kind, rhs.span),
-        shape,
+        rhs_shape,
     )
 }
 

tests/source/issue-189.rs

@@ -0,0 +1,14 @@
+// rustfmt-style_edition: 2024
+
+impl SomeType {
+    fn method(&mut self) {
+        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(),
+        });
+    }
+}

tests/target/issue-189.rs

@@ -0,0 +1,14 @@
+// rustfmt-style_edition: 2024
+
+impl SomeType {
+    fn method(&mut self) {
+        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(),
+            });
+    }
+}