libcpp: Avoid pessimizing std::move [PR106272]

std::move in a return statement can prevent the NRVO:
<https://developers.redhat.com/blog/2019/04/12/understanding-when-not-to-stdmove-in-c>

PR106272 reports that we have two such cases in class label_text's
member functions.  We have -Wpessimizing-move that's supposed to detect
problematic std::move uses, but in this case it didn't trigger.  I've filed
PR106276 to track that.

	PR preprocessor/106272

libcpp/ChangeLog:

	* include/line-map.h (class label_text): Don't std::move in a return
	statement.

diff --git a/libcpp/include/line-map.h b/libcpp/include/line-map.h
index c6379ce..c434a24 100644 (file)
--- a/libcpp/include/line-map.h
+++ b/libcpp/include/line-map.h
@@ -1873,13 +1873,13 @@ public:
      longer-lived owner.  */
   static label_text borrow (const char *buffer)
   {
-    return std::move (label_text (const_cast <char *> (buffer), false));
+    return label_text (const_cast <char *> (buffer), false);
   }
 
   /* Create a label_text instance that takes ownership of BUFFER.  */
   static label_text take (char *buffer)
   {
-    return std::move (label_text (buffer, true));
+    return label_text (buffer, true);
   }
 
   /* Take ownership of the buffer, copying if necessary.  */