Update heuristic in rewrite_assign_rhs

Put the rhs of assignement on the next line when putting next to `lhs = ` will
cause rhs to go multi line, but putting on the next line makes it fits
in a single line.

src/expr.rs

@@ -2937,26 +2937,21 @@ pub fn rewrite_assign_rhs<S: Into<String>>(
     let orig_shape = try_opt!(shape.offset_left(last_line_width + 1));
     let rhs = ex.rewrite(context, orig_shape);
 
-    fn count_line_breaks(src: &str) -> usize {
-        src.chars().filter(|&x| x == '\n').count()
-    }
-
     match rhs {
-        Some(ref new_str) if count_line_breaks(new_str) < 2 => {
+        Some(ref new_str) if !new_str.contains('\n') => {
             result.push(' ');
             result.push_str(new_str);
         }
         _ => {
-            // Expression did not fit on the same line as the identifier or is
-            // at least three lines big. Try splitting the line and see
-            // if that works better.
+            // Expression did not fit on the same line as the identifier.
+            // Try splitting the line and see if that works better.
             let new_shape = try_opt!(shape.block_left(context.config.tab_spaces()));
             let new_rhs = ex.rewrite(context, new_shape);
 
             // FIXME: DRY!
             match (rhs, new_rhs) {
                 (Some(ref orig_rhs), Some(ref replacement_rhs))
-                    if count_line_breaks(orig_rhs) > count_line_breaks(replacement_rhs) + 1 => {
+                    if prefer_next_line(orig_rhs, replacement_rhs) => {
                     result.push_str(&format!("\n{}", new_shape.indent.to_string(context.config)));
                     result.push_str(replacement_rhs);
                 }
@@ -2976,6 +2971,16 @@ pub fn rewrite_assign_rhs<S: Into<String>>(
     Some(result)
 }
 
+fn prefer_next_line(orig_rhs: &str, next_line_rhs: &str) -> bool {
+
+    fn count_line_breaks(src: &str) -> usize {
+        src.chars().filter(|&x| x == '\n').count()
+    }
+
+    !next_line_rhs.contains('\n') ||
+        count_line_breaks(orig_rhs) > count_line_breaks(next_line_rhs) + 1
+}
+
 fn rewrite_expr_addrof(
     context: &RewriteContext,
     mutability: ast::Mutability,