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.

tests/actions.at

@@ -98,6 +98,7 @@ AT_DATA_GRAMMAR([[input.y]],
   static int yylex (void);
 # define YYDEBUG         1
 # define YYERROR_VERBOSE 1
+# define USE(Var)
 %}
 
 %union
@@ -112,6 +113,7 @@ AT_DATA_GRAMMAR([[input.y]],
 exp: a_1 a_2 { $<val>$ = 3; } { $<val>$ = $<val>3 + 1; } a_5
      sum_of_the_five_previous_values
     {
+       USE (($1, $2, $5));
        printf ("%d\n", $6);
     }
 ;

tests/calc.at

@@ -54,6 +54,7 @@ AT_LALR1_CC_IF(
 # define alarm(seconds) /* empty */
 #endif
 #include <ctype.h>
+#define USE(Var)
 
 /* Exercise pre-prologue dependency to %union.  */
 typedef int semantic_value;
@@ -109,7 +110,7 @@ input:
 
 line:
   '\n'
-| exp '\n'           { ]AT_PARAM_IF([*result = global_result = $1;])[ }
+| exp '\n'           { ]AT_PARAM_IF([*result = global_result = $1], [USE ($1)])[; }
 ;
 
 exp:

tests/glr-regression.at

@@ -746,13 +746,14 @@ AT_SETUP([No users destructors if stack 0 deleted])
 AT_DATA_GRAMMAR([glr-regr9.y],
 [[
 %{
-  #include <stdio.h>
-  #include <stdlib.h>
+# include <stdio.h>
+# include <stdlib.h>
   static void yyerror (char const *);
   static int yylex (void);
-  #define YYSTACKEXPANDABLE 0
+# define YYSTACKEXPANDABLE 0
   static int tokens = 0;
   static int destructors = 0;
+# define USE(Var)
 %}
 
 %glr-parser
@@ -766,7 +767,7 @@ AT_DATA_GRAMMAR([glr-regr9.y],
 %%
 
 start:
-  ambig0 'a' { destructors += 2; }
+    ambig0 'a'   { destructors += 2; USE ($2); }
   | ambig1 start { destructors += 1; }
   | ambig2 start { destructors += 1; }
   ;