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

[Joel E. Denny]

I am thinking of including this patch in bison 2.4.3.  I wrote it while 
considering a complaint on comp.compilers saying that -Werror doesn't 
work:

  http://compilers.iecc.com/comparch/article/10-07-039

As far as I can tell, the only warning on which -Werror has no effect is 
the warning that a rule is useless in the parser due to conflicts.  
Because bison explicitly labels these messages as warnings, I see this 
behavior as a bug, and this patch fixes it.  But is there any backward 
compatibility problem to consider here?

Now thinking beyond 2.4.3, the only specific case of -Werror not working 
that was mentioned on comp.compilers is the case of R/R conflicts.  Bison 
currently does not label R/R conflict messages as warnings, so it is not 
as clear in this case that -Werror should have any effect.  Would treating 
parser conflicts as warnings be too contrary to traditional yacc/bison 
thinking?  Besides, bison already supports %error and %error-rr to turn 
them into errors.  On the other hand, it seems that any diagnostic printed 
on stderr is at least a warning, and I can see how the user might 
appreciate a guarantee that -Werror will prevent him from missing any such 
diagnostic.  That seems to be the thinking at comp.compilers.  Opinions?

>From 954474bfa1a875eeefe9aa1989d9c7be6f64726b Mon Sep 17 00:00:00 2001
From: Joel E. Denny <address@hidden>
Date: Sun, 1 Aug 2010 18:51:46 -0400
Subject: [PATCH] -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.
---
 ChangeLog      |   15 +++++++++++++
 NEWS           |    3 ++
 src/complain.c |   14 +++++++++--
 src/gram.c     |    9 ++++---
 src/reduce.c   |   24 +++++++-------------
 tests/local.at |   65 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 6 files changed, 106 insertions(+), 24 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 15c236e..d365830 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,18 @@
+2010-08-01  Joel E. Denny  <address@hidden>
+
+       -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.
+
 2010-07-29  Joel E. Denny  <address@hidden>
 
        maint: enable gnits only at stable releases.
diff --git a/NEWS b/NEWS
index 2ab2250..64a1ee6 100644
--- a/NEWS
+++ b/NEWS
@@ -3,6 +3,9 @@ Bison News
 
 * Changes in version 2.4.3 (????-??-??):
 
+** Bison now obeys -Werror and --warnings=error for warnings about
+   grammar rules that are useless in the parser due to conflicts.
+
 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
    been fixed.
 
diff --git a/src/complain.c b/src/complain.c
index 4266af1..496924a 100644
--- a/src/complain.c
+++ b/src/complain.c
@@ -37,7 +37,10 @@ bool complaint_issued;
  * \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
@@ -56,8 +59,13 @@ 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 81939b6..2962d98 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.  */
 
@@ -307,9 +308,9 @@ 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 4cec048..6e16ce3 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
diff --git a/tests/local.at b/tests/local.at
index 6ce7c7e..0ea4f9b 100644
--- a/tests/local.at
+++ b/tests/local.at
@@ -204,11 +204,17 @@ $2])
 #
 #   2. In the case of maintainer-xml-check, XML/XSLT output is compared with
 #      --graph and --report=all output for every working grammar.
+#
+#   3. If stderr contains a warning, -Werror and --warnings=error
+#      convert the warning to an error.
 m4_define([AT_BISON_CHECK],
 [m4_if(m4_quote($2), [0], [AT_BISON_CHECK_XML($@)],
        m4_quote($2), [], [AT_BISON_CHECK_XML($@)])
 AT_BISON_CHECK_NO_XML($@)])
 
+m4_define([AT_BISON_WERROR_MSG],
+          [[bison: warnings being treated as errors]])
+
 # AT_BISON_CHECK_NO_XML(BISON_ARGS, [OTHER_AT_CHECK_ARGS])
 # --------------------------------------------------------
 # Same as AT_BISON_CHECK except don't perform XML/XSLT checks.  This is useful
@@ -216,7 +222,64 @@ AT_BISON_CHECK_NO_XML($@)])
 # handle.
 m4_define([AT_BISON_CHECK_NO_XML],
 [AT_CHECK(m4_if(m4_quote($2), [0], [], m4_quote($2), [], [],
-                [AT_QUELL_VALGRIND ])[[bison ]]$@)])
+                [AT_QUELL_VALGRIND ])[[bison ]]$@)
+m4_if(m4_bregexp([$4], [: warning: ]), [-1], [],
+      [m4_if(m4_quote(m4_if(m4_quote($2), [], [0], [$2])), [0],
+             [[# Don't interfere with caller's files.
+              if test -f stderr; then
+                mv stderr at-bison-check-stderr.bak
+              fi
+              if test -f experr; then
+                mv experr at-bison-check-experr.bak
+              fi
+
+              # Run with -Werror.
+              ]AT_CHECK(AT_QUELL_VALGRIND[[ bison -Werror ]$1],
+                        [[1]], [$3], [stderr])[
+
+              # Build expected stderr up to and including the "warnings
+              # being treated as errors" message.
+              ]AT_DATA([[at-bison-check-warnings]], [$4])[
+              at_bison_check_first="` \
+                sed -n '/: warning: /=' at-bison-check-warnings \
+                | sed -n 1p \
+              `"
+              if test $at_bison_check_first -gt 1; then
+                sed -n "1,`expr $at_bison_check_first - 1`"p \
+                  at-bison-check-warnings > experr
+              fi
+              echo ']AT_BISON_WERROR_MSG[' >> experr
+
+              # Finish building expected stderr and check.  Unlike
+              # warnings, complaints cause bison to exit early.  Thus,
+              # with -Werror, bison does not necessarily report all
+              # warnings that it does without -Werror, but it at least
+              # reports one.
+              at_bison_check_last="`sed -n '$=' stderr`"
+              if test x"$at_bison_check_last" = x; then
+                at_bison_check_last=1
+              fi
+              at_bison_check_last="`expr $at_bison_check_last - 1`"
+              sed -n "$at_bison_check_first,$at_bison_check_last"p \
+                at-bison-check-warnings >> experr
+              ]AT_CHECK([[sed 's,.*/\(]AT_BISON_WERROR_MSG[\)$,\1,' \
+                          stderr 1>&2]], [[0]], [[]], [experr])[
+
+              # Now check --warnings=error.
+              cp stderr experr
+              ]AT_CHECK(AT_QUELL_VALGRIND[[ bison --warnings=error ]$1],
+                        [[1]], [$3], [experr])[
+
+              # Restore caller's files.
+              if test -f at-bison-check-experr.bak; then
+                mv at-bison-check-experr.bak experr
+              fi
+              if test -f at-bison-check-stderr.bak; then
+                mv at-bison-check-stderr.bak stderr
+              fi
+             ]])
+      ])
+])
 
 # AT_BISON_CHECK_XML(BISON_ARGS, [OTHER_AT_CHECK_ARGS])
 # -----------------------------------------------------
-- 
1.7.0.4