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

src/expr.rs

@@ -2060,6 +2060,24 @@ 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 last_line = lhs_str.rsplit_once("\n");
+    let shape = match last_line {
+        Some((_, line)) => {
+            let tab_spaces = context.config.tab_spaces();
+            let new_shape = shape
+                .block_indent(tab_spaces)
+                .saturating_sub_width(tab_spaces);
+
+            let extra_indent_string = new_shape.to_string(&context.config).to_string();
+
+            if line.starts_with(&extra_indent_string) {
+                new_shape
+            } else {
+                shape
+            }
+        }
+        _ => shape,
+    };
     rewrite_assign_rhs(
         context,
         lhs_str,

src/shape.rs

@@ -278,6 +278,12 @@ impl Shape {
         offset_indent.to_string_inner(config, 0)
     }
 
+    pub(crate) fn to_string(&self, config: &Config) -> Cow<'static, str> {
+        let mut offset_indent = self.indent;
+        offset_indent.alignment = self.offset;
+        offset_indent.to_string_inner(config, 1)
+    }
+
     /// Creates a `Shape` with a virtually infinite width.
     pub(crate) fn infinite_width(&self) -> Shape {
         Shape {

tests/source/rust-lang/style-team#189.rs

@@ -0,0 +1,24 @@
+// 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(),
+        });
+    }
+}
+
+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;
+    }
+}

tests/target/rust-lang/style-team#189.rs

@@ -0,0 +1,24 @@
+// 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(),
+            });
+    }
+}
+
+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;
+    }
+}