diagnostics: %empty enables -Wempty-rule

* src/complain.h, src/complain.c (warning_is_unset): New.
* src/reader.c (grammar_current_rule_empty_set): If enabled -Wempty-rule,
if not disabled.
* tests/actions.at (Implicitly empty rule): Check this feature.
Also check that -Wno-empty-rule does disable this warning.

src/complain.c

@@ -196,6 +196,15 @@ warning_severity (warnings flags)
     }
 }
 
+bool
+warning_is_unset (warnings flags)
+{
+  size_t b;
+  for (b = 0; b < warnings_size; ++b)
+    if (flags & 1 << b && warnings_flag[b] != severity_unset)
+      return false;
+  return true;
+}
 
 /** Display a "[-Wyacc]" like message on \a f.  */
 

src/complain.h

@@ -101,6 +101,9 @@ typedef enum
     Wall              = ~complaint & ~fatal & ~silent
   } warnings;
 
+/** Whether the warnings of \a flags are all unset.
+    (Never enabled, never disabled). */
+bool warning_is_unset (warnings flags);
 
 /** Make a complaint, with maybe a location.  */
 void complain (location const *loc, warnings flags, char const *message, ...)

src/reader.c

@@ -451,6 +451,10 @@ grammar_current_rule_prec_set (symbol *precsym, location loc)
 void
 grammar_current_rule_empty_set (location loc)
 {
+  /* If %empty is used and -Wno-empty-rule is not, then enable
+     -Wempty-rule.  */
+  if (warning_is_unset (Wempty_rule))
+    warning_argmatch ("empty-rule", 0, 0);
   if (current_rule->percent_empty_loc.start.file)
     complain (&loc, complaint, _("only one %s allowed per rule"), "%empty");
   else