-Wempty-rule: diagnose empty rules without %empty

* src/complain.h, src/complain.c (warning_empty_rule, Wempty_rule):
New warning category.
(warnings_args, warnings_types): Adjust.
* src/reader.c (grammar_rule_check): Check the empty rules are
flagged by %empty.
* tests/actions.at (Implicitly empty rule): New.
* tests/existing.at: Add expected warnings.

src/complain.c

@@ -63,6 +63,7 @@ static const char * const warnings_args[] =
   "conflicts-sr",
   "conflicts-rr",
   "deprecated",
+  "empty-rule",
   "precedence",
   "other",
   "all",
@@ -78,6 +79,7 @@ static const int warnings_types[] =
   Wconflicts_sr,
   Wconflicts_rr,
   Wdeprecated,
+  Wempty_rule,
   Wprecedence,
   Wother,
   Wall,

src/complain.h

@@ -35,6 +35,7 @@ typedef enum
     warning_yacc,           /**< POSIXME.  */
     warning_conflicts_sr,   /**< S/R conflicts.  */
     warning_conflicts_rr,   /**< R/R conflicts.  */
+    warning_empty_rule,     /**< Implicitly empty rules.  */
     warning_deprecated,     /**< Obsolete constructs.  */
     warning_precedence,     /**< Useless precedence and associativity.  */
     warning_other,          /**< All other warnings.  */
@@ -85,6 +86,7 @@ typedef enum
     Wconflicts_sr     = 1 << warning_conflicts_sr,
     Wconflicts_rr     = 1 << warning_conflicts_rr,
     Wdeprecated       = 1 << warning_deprecated,
+    Wempty_rule       = 1 << warning_empty_rule,
     Wprecedence       = 1 << warning_precedence,
     Wother            = 1 << warning_other,
 

src/getargs.c

@@ -327,6 +327,7 @@ Warning categories include:\n\
   `conflicts-sr'      S/R conflicts (enabled by default)\n\
   `conflicts-rr'      R/R conflicts (enabled by default)\n\
   `deprecated'        obsolete constructs\n\
+  `empty-rule'        empty rules without %empty\n\
   `precedence'        useless precedence and associativity\n\
   `other'             all other warnings (enabled by default)\n\
   `all'               all the warnings\n\

src/reader.c

@@ -339,6 +339,12 @@ grammar_rule_check (const symbol_list *r)
     complain (&r->percent_empty_loc, complaint,
               _("%%empty on non-empty rule"));
 
+  /* Check that empty rule => %empty.  */
+  if (!(r->next && r->next->content.sym)
+      && !r->midrule_parent_rule
+      && !r->percent_empty_loc.start.file)
+    complain (&r->location, Wempty_rule, _("empty rule without %%empty"));
+
   /* See comments in grammar_current_rule_prec_set for how POSIX
      mandates this complaint.  It's only for identifiers, so skip
      it for char literals and strings, which are always tokens.  */