-Werror: fix for rules useless in parser after conflicts.

* NEWS (2.4.3): Document fix.
* src/complain.c (error_message): Extend to handle incomplete
error messages so warn and warn_at can be used in more cases.
* src/gram.c (grammar_rules_useless_report): Use warn_at so that
-Werror is always obeyed.
* src/reduce.c (reduce_print): Use warn so that the "warnings
being treated as errors" message is printed consistently before
the first warning message.  This makes testing easier.
* tests/local.at (AT_BISON_WERROR_MSG): New macro.
(AT_BISON_CHECK_NO_XML): Extend to check -Werror and
--warnings=error when warnings appear in bison's stderr.
(cherry picked from commit 954474bfa1a875eeefe9aa1989d9c7be6f64726b)

3 files changed, 24 insertions, 23 deletions

diff --git a/src/complain.c b/src/complain.c
index d5d17b7c..22ad4248 100644
--- a/src/complain.c
+++ b/src/complain.c
@@ -38,7 +38,10 @@ static unsigned *indent_ptr = 0;
  * \param loc     the location, defaulting to the current file,
  *                or the program name.
  * \param prefix  put before the message (e.g., "warning").
- * \param message the error message, a printf format string.
+ * \param message the error message, a printf format string.  Iff it
+ *                ends with ": ", then no trailing newline is printed,
+ *                and the caller should print the remaining
+ *                newline-terminated message to stderr.
  * \param args    the arguments of the format string.
  */
 static
@@ -68,8 +71,13 @@ error_message (location *loc,
     fprintf (stderr, "%s: ", prefix);
 
   vfprintf (stderr, message, args);
-  putc ('\n', stderr);
-  fflush (stderr);
+  {
+    size_t l = strlen (message);
+    if (l < 2 || message[l-2] != ':' || message[l-1] != ' ') {
+      putc ('\n', stderr);
+      fflush (stderr);
+    }
+  }
 }
 
 /** Wrap error_message() with varargs handling. */
diff --git a/src/gram.c b/src/gram.c
index 10a1953a..74ead640 100644
--- a/src/gram.c
+++ b/src/gram.c
@@ -23,11 +23,12 @@
 
 #include <quotearg.h>
 
+#include "complain.h"
 #include "gram.h"
+#include "print-xml.h"
 #include "reader.h"
 #include "reduce.h"
 #include "symtab.h"
-#include "print-xml.h"
 
 /* Comments for these variables are in gram.h.  */
 
@@ -308,9 +309,9 @@ grammar_rules_useless_report (const char *message)
   for (r = 0; r < nrules ; ++r)
     if (!rules[r].useful)
       {
-	location_print (stderr, rules[r].location);
-	fprintf (stderr, ": %s: %s: ", _("warning"), message);
-	rule_print (&rules[r], stderr);
+        warn_at (rules[r].location, "%s: ", message);
+        rule_print (&rules[r], stderr);
+        fflush (stderr);
       }
 }
 
diff --git a/src/reduce.c b/src/reduce.c
index 4cec048e..6e16ce31 100644
--- a/src/reduce.c
+++ b/src/reduce.c
@@ -383,23 +383,15 @@ static void
 reduce_print (void)
 {
   if (nuseless_nonterminals > 0)
-    {
-      fprintf (stderr, "%s: %s: ", grammar_file, _("warning"));
-      fprintf (stderr, ngettext ("%d nonterminal useless in grammar",
-                                 "%d nonterminals useless in grammar",
-                                 nuseless_nonterminals),
-               nuseless_nonterminals);
-      fprintf (stderr, "\n");
-    }
+    warn (ngettext ("%d nonterminal useless in grammar",
+                    "%d nonterminals useless in grammar",
+                    nuseless_nonterminals),
+          nuseless_nonterminals);
   if (nuseless_productions > 0)
-    {
-      fprintf (stderr, "%s: %s: ", grammar_file, _("warning"));
-      fprintf (stderr, ngettext ("%d rule useless in grammar",
-                                 "%d rules useless in grammar",
-                                 nuseless_productions),
-               nuseless_productions);
-      fprintf (stderr, "\n");
-    }
+    warn (ngettext ("%d rule useless in grammar",
+                    "%d rules useless in grammar",
+                    nuseless_productions),
+          nuseless_productions);
 }
 
 void