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

src/expr.rs

@@ -2101,6 +2101,25 @@ pub(crate) fn rewrite_assign_rhs_expr<R: Rewrite>(
     rhs_kind: &RhsAssignKind<'_>,
     rhs_tactics: RhsTactics,
 ) -> Option<String> {
+    let get_lhs_last_line_shape = || {
+        let last_line = lhs.rsplitn(2, "\n").next().unwrap_or_default();
+        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 last_line.starts_with(&extra_indent_string) {
+            new_shape
+        } else {
+            shape
+        }
+    };
+    let shape = if context.config.style_edition() >= StyleEdition::Edition2024 {
+        get_lhs_last_line_shape()
+    } else {
+        shape
+    };
+
     let last_line_width = last_line_width(lhs).saturating_sub(if lhs.contains('\n') {
         shape.indent.width()
     } else {

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;
+    }
+}