Warn about unused values.

* src/symlist.h, src/symlist.c (symbol_list, symbol_list_new): Add
a `used' member.
(symbol_list_n_get, symbol_list_n_used_set): New.
(symbol_list_n_type_name_get): Use symbol_list_n_get.
* src/scan-gram.l (handle_action_dollar): Flag used symbols.
* src/reader.c (grammar_current_rule_check): Check that values are
used.
* src/symtab.c (symbol_print): Accept 0.
* tests/existing.at: Remove the type information.
Empty the actions.
Remove useless actions (beware of mid-rule actions: perl -000
-pi -e 's/s*{}(?=[ns]*[|;])//g').
* tests/actions.at (Exotic Dollars): Use unused values.
* tests/calc.at: Likewise.
* tests/glr-regression.at (No users destructors if stack 0 deleted):
Likewise.
* src/gram.c (rule_useful_p, rule_never_reduced_p): Use
rule_useful_p.

src/symlist.h

@@ -25,9 +25,10 @@
 # include "location.h"
 # include "symtab.h"
 
+/* A list of symbols, used during the parsing to store the rules.  */
 typedef struct symbol_list
 {
-  struct symbol_list *next;
+  /* The symbol.  */
   symbol *sym;
   location location;
 
@@ -35,9 +36,16 @@ typedef struct symbol_list
   const char *action;
   location action_location;
 
+  /* Whether this symbol's value is used in the current action. */
+  bool used;
+
+  /* Precedence/associativity.  */
   symbol *ruleprec;
   int dprec;
   int merger;
+
+  /* The list.  */
+  struct symbol_list *next;
 } symbol_list;
 
 
@@ -48,18 +56,24 @@ symbol_list *symbol_list_new (symbol *sym, location loc);
 void symbol_list_print (symbol_list *l, FILE *f);
 
 /* Prepend SYM at LOC to the LIST.  */
-symbol_list *symbol_list_prepend (symbol_list *list,
+symbol_list *symbol_list_prepend (symbol_list *l,
 				  symbol *sym,
 				  location loc);
 
 /* Free the LIST, but not the symbols it contains.  */
-void symbol_list_free (symbol_list *list);
+void symbol_list_free (symbol_list *l);
 
 /* Return its length. */
-unsigned int symbol_list_length (symbol_list *list);
+unsigned int symbol_list_length (symbol_list *l);
+
+/* Get symbol N in symbol list L.  */
+symbol_list *symbol_list_n_get (symbol_list *l, int n);
 
 /* Get the data type (alternative in the union) of the value for
    symbol N in rule RULE.  */
-uniqstr symbol_list_n_type_name_get (symbol_list *rp, location loc, int n);
+uniqstr symbol_list_n_type_name_get (symbol_list *l, location loc, int n);
+
+/* The symbol N in symbol list L is USED.  */
+void symbol_list_n_used_set (symbol_list *l, int n, bool used);
 
 #endif /* !SYMLIST_H_ */