Update per comments

src/expr.rs

@@ -2124,21 +2124,21 @@ pub(crate) fn rewrite_assign_rhs_expr<R: Rewrite>(
     rhs_kind: &RhsAssignKind<'_>,
     rhs_tactics: RhsTactics,
 ) -> RewriteResult {
-    let get_lhs_last_line_shape = || {
-        let last_line = lhs.rsplitn(2, "\n").next().unwrap_or_default();
+    let get_rhs_shape = || {
+        let last_line = lhs.lines().last().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) {
+        let extra_indent_string = new_shape.to_string(&context.config);
+        if last_line.starts_with(extra_indent_string.as_ref()) {
             new_shape
         } else {
             shape
         }
     };
     let shape = if context.config.style_edition() >= StyleEdition::Edition2024 {
-        get_lhs_last_line_shape()
+        get_rhs_shape()
     } else {
         shape
     };

src/shape.rs

@@ -278,6 +278,7 @@ impl Shape {
         offset_indent.to_string_inner(config, 0)
     }
 
+    /// similar to to_string_with_newline, except the result does not start with a new line
     pub(crate) fn to_string(&self, config: &Config) -> Cow<'static, str> {
         let mut offset_indent = self.indent;
         offset_indent.alignment = self.offset;