grammar: introduce %empty

Provide a means to explicitly denote empty right-hand sides of rules:
instead of

  exp:  { ... }

allow

  exp: %empty { ... }

Make sure that %empty is properly used.

With help from Joel E. Denny and Gabriel Rassoul.
http://lists.gnu.org/archive/html/bison-patches/2013-01/msg00142.html

* src/reader.h, src/reader.c (grammar_current_rule_empty_set): New.
* src/parse-gram.y (%empty): New token.
Use it.
* src/scan-gram.l (%empty): Scan it.
* src/reader.c (grammar_rule_check): Check that %empty is properly used.
* tests/actions.at (Invalid uses of %empty, Valid uses of %empty): New.

src/parse-gram.y

@@ -605,6 +605,7 @@ rhses.1:
 | rhses.1 ";"
 ;
 
+%token PERCENT_EMPTY "%empty";
 rhs:
   /* Nothing.  */
     { grammar_current_rule_begin (current_lhs_symbol, current_lhs_location,
@@ -615,6 +616,8 @@ rhs:
     { grammar_current_rule_action_append ($2, @2, $3, false); }
 | rhs "%?{...}"
     { grammar_current_rule_action_append ($2, @2, NULL, true); }
+| rhs "%empty"
+    { grammar_current_rule_empty_set (@2); }
 | rhs "%prec" symbol
     { grammar_current_rule_prec_set ($3, @3); }
 | rhs "%dprec" INT

src/reader.c

@@ -333,6 +333,12 @@ grammar_rule_check (const symbol_list *r)
       }
   }
 
+  /* Check that %empty => empty rule.  */
+  if (r->percent_empty_loc.start.file
+      && r->next && r->next->content.sym)
+    complain (&r->percent_empty_loc, complaint,
+              _("%%empty on non-empty rule"));
+
   /* 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.  */
@@ -434,6 +440,17 @@ grammar_current_rule_prec_set (symbol *precsym, location loc)
   current_rule->ruleprec = precsym;
 }
 
+/* Set %empty for the current rule. */
+
+void
+grammar_current_rule_empty_set (location loc)
+{
+  if (current_rule->percent_empty_loc.start.file)
+    complain (&loc, complaint, _("only one %s allowed per rule"), "%empty");
+  else
+    current_rule->percent_empty_loc = loc;
+}
+
 /* Attach dynamic precedence DPREC to the current rule. */
 
 void

src/reader.h

@@ -47,6 +47,8 @@ void grammar_current_rule_begin (symbol *lhs, location loc,
                                  named_ref *lhs_named_ref);
 void grammar_current_rule_end (location loc);
 void grammar_midrule_action (void);
+/* Apply %empty to the current rule.  */
+void grammar_current_rule_empty_set (location loc);
 void grammar_current_rule_prec_set (symbol *precsym, location loc);
 void grammar_current_rule_dprec_set (int dprec, location loc);
 void grammar_current_rule_merge_set (uniqstr name, location loc);

src/scan-gram.l

@@ -217,6 +217,7 @@ eqopt    ([[:space:]]*=)?
   "%defines"                        return PERCENT_DEFINES;
   "%destructor"                     return PERCENT_DESTRUCTOR;
   "%dprec"                          return PERCENT_DPREC;
+  "%empty"                          return PERCENT_EMPTY;
   "%error-verbose"                  return PERCENT_ERROR_VERBOSE;
   "%expect"                         return PERCENT_EXPECT;
   "%expect-rr"                      return PERCENT_EXPECT_RR;

src/symlist.c

@@ -44,6 +44,7 @@ symbol_list_sym_new (symbol *sym, location loc)
 
   /* Members used for LHS only.  */
   res->ruleprec = NULL;
+  res->percent_empty_loc = empty_location;
   code_props_none_init (&res->action_props);
   res->dprec = 0;
   res->merger = 0;

src/symlist.h

@@ -78,6 +78,10 @@ typedef struct symbol_list
    * each RHS are also stored here.  */
   code_props action_props;
 
+  /* The location of the first %empty for this rule, or \a
+     empty_location.  */
+  location percent_empty_loc;
+
   int dprec;
   int merger;
   location merger_declaration_location;