--update: when used, do not generate the output files

It is inconvenient that we also generate the output files when we
update the grammar file, and it's somewhat unexpected.  Let's not do
that.

* src/main.c (main): Skip generation when --update is passed.
* src/getargs.c (usage): Update the help message.
* doc/bison.texi (Bison Options): Likewise.
* tests/input.at: Check that we don't generate the output.

doc/bison.texi

@@ -10272,8 +10272,8 @@ Print the name of the directory containing skeletons and XSLT.
 @item -u
 @item --update
 Update the grammar file (remove duplicates, update deprecated directives,
-etc.).  Leaves a backup of the original file with a @code{~} appended.  For
-instance:
+etc.) and exit (i.e., do not generate any of the output files).  Leaves a
+backup of the original file with a @code{~} appended.  For instance:
 
 @example
 @group

src/getargs.c

@@ -287,7 +287,7 @@ Operation modes:\n\
                              and exit\n\
       --print-datadir        output directory containing skeletons and XSLT\n\
                              and exit\n\
-  -u, --update               apply fixes to the source grammar file\n\
+  -u, --update               apply fixes to the source grammar file and exit\n\
   -y, --yacc                 emulate POSIX Yacc\n\
   -W, --warnings[=CATEGORY]  report the warnings falling in CATEGORY\n\
   -f, --feature[=FEATURE]    activate miscellaneous features\n\

src/main.c

@@ -155,31 +155,34 @@ main (int argc, char *argv[])
 
   print_precedence_warnings ();
 
-  /* Output file names. */
-  compute_output_file_names ();
-
-  /* Output the detailed report on the grammar.  */
-  if (report_flag)
+  if (!update_flag)
     {
-      timevar_push (tv_report);
-      print_results ();
-      timevar_pop (tv_report);
-    }
+      /* Output file names. */
+      compute_output_file_names ();
 
-  /* Output the graph.  */
-  if (graph_flag)
-    {
-      timevar_push (tv_graph);
-      print_graph ();
-      timevar_pop (tv_graph);
-    }
+      /* Output the detailed report on the grammar.  */
+      if (report_flag)
+        {
+          timevar_push (tv_report);
+          print_results ();
+          timevar_pop (tv_report);
+        }
 
-  /* Output xml.  */
-  if (xml_flag)
-    {
-      timevar_push (tv_xml);
-      print_xml ();
-      timevar_pop (tv_xml);
+      /* Output the graph.  */
+      if (graph_flag)
+        {
+          timevar_push (tv_graph);
+          print_graph ();
+          timevar_pop (tv_graph);
+        }
+
+      /* Output xml.  */
+      if (xml_flag)
+        {
+          timevar_push (tv_xml);
+          print_xml ();
+          timevar_pop (tv_xml);
+        }
     }
 
   /* Stop if there were errors, to avoid trashing previous output
@@ -193,9 +196,12 @@ main (int argc, char *argv[])
   timevar_pop (tv_free);
 
   /* Output the tables and the parser to ftable.  In file output.  */
-  timevar_push (tv_parser);
-  output ();
-  timevar_pop (tv_parser);
+  if (!update_flag)
+    {
+      timevar_push (tv_parser);
+      output ();
+      timevar_pop (tv_parser);
+    }
 
   timevar_push (tv_free);
   nullable_free ();

tests/input.at

@@ -2500,7 +2500,7 @@ AT_DATA_GRAMMAR([[input.y]],
 %no-default_prec
 %no_default-prec
 %no_lines
-%output = "foo"
+%output = "output.c"
 %pure_parser
 %token_table
 %error-verbose
@@ -2559,7 +2559,9 @@ AT_BISON_CHECK([[-ffixit input.y]], [], [], [experr])
 AT_CHECK([[sed -e '/^fix-it:/d' errors-all >experr]])
 AT_BISON_CHECK([[input.y]], [], [], [experr])
 
-# Update the input file.
+
+# Update the input file.  Make sure we generated nothing.
+AT_CHECK([rm -f output.c])
 AT_CHECK([cp input.y input.y.orig])
 AT_CHECK([sed -e '/fix-it/d' <errors-all >experr])
 AT_CHECK([echo "bison: file 'input.y' was updated (backup: 'input.y~')" >>experr])
@@ -2568,6 +2570,9 @@ AT_BISON_CHECK([[--update input.y]], [], [[]], [experr])
 # Check the backup.
 AT_CHECK([diff input.y.orig input.y~])
 
+# Check we did not generate any file.
+AT_CHECK([test ! -f output.c])
+
 # Check the update.
 AT_CHECK([cat input.y], [],
 [[%code top {
@@ -2588,7 +2593,7 @@ AT_CHECK([cat input.y], [],
 %no-default-prec
 %no-default-prec
 %no-lines
-%output "foo"
+%output "output.c"
 %pure-parser
 %token-table
 %glr-parser