add -fsyntax-only

When debugging Bison itself, this is very handy, especially when
tweaking the frontend badly enough to break the backends. It can also
be used to check a grammar.

* src/getargs.h, src/getargs.c (feature_syntax_only): New.
(feature_args, feature_types): Adjust.
* src/main.c (main): Use it.

src/getargs.c

@@ -231,6 +231,7 @@ static const char * const feature_args[] =
   "none",
   "caret", "diagnostics-show-caret",
   "fixit", "diagnostics-parseable-fixits",
+  "syntax-only",
   "all",
   0
 };
@@ -240,6 +241,7 @@ static const int feature_types[] =
   feature_none,
   feature_caret, feature_caret,
   feature_fixit_parsable, feature_fixit_parsable,
+  feature_syntax_only,
   feature_all
 };
 
@@ -362,6 +364,8 @@ FEATURES is a list of comma separated words that can include:\n\
     show errors with carets\n\
   'fixit', 'diagnostics-parseable-fixits'\n\
     show machine-readable fixes\n\
+  'syntax-only'\n\
+    do not generate any file\n\
   'all'\n\
     all of the above\n\
   'none'\n\
@@ -698,6 +702,7 @@ getargs (int argc, char *argv[])
 
       case 'u':
         update_flag = true;
+        feature_flag |= feature_syntax_only;
         break;
 
       case 'v':

src/getargs.h

@@ -114,10 +114,11 @@ extern int trace_flag;
 
 enum feature
   {
-    feature_none           = 0,       /**< No additional feature.  */
-    feature_caret          = 1 << 0,  /**< Output errors with carets.  */
-    feature_fixit_parsable = 1 << 1,  /**< Issue instructions to fix the sources.  */
-    feature_all            = ~0       /**< All above features.  */
+    feature_none             = 0,      /**< No additional feature.  */
+    feature_caret            = 1 << 0, /**< Output errors with carets.  */
+    feature_fixit_parsable   = 1 << 1, /**< Issue instructions to fix the sources.  */
+    feature_syntax_only      = 1 << 2, /**< Don't generate output.  */
+    feature_all              = ~0      /**< All above features.  */
   };
 /** What additional features to use.  */
 extern int feature_flag;

src/main.c

@@ -155,7 +155,10 @@ main (int argc, char *argv[])
 
   print_precedence_warnings ();
 
-  if (!update_flag)
+  /* Whether to generate output files.  */
+  bool generate = !(feature_flag & feature_syntax_only);
+
+  if (generate)
     {
       /* Output file names. */
       compute_output_file_names ();
@@ -196,7 +199,7 @@ main (int argc, char *argv[])
   timevar_pop (tv_free);
 
   /* Output the tables and the parser to ftable.  In file output.  */
-  if (!update_flag)
+  if (generate)
     {
       timevar_push (tv_parser);
       output ();