diagnostics: Add new option -fdiagnostics-plain-output

Adds the new option -fdiagnostics-plain-output, which is an alias for
several others:

    -fno-diagnostics-show-caret
    -fno-diagnostics-show-line-numbers
    -fdiagnostics-color=never
    -fdiagnostics-urls=never

The idea is that in the future, if the default behavior of diagnostics is
changed to add some fancy feature or other, then the
-fdiagnostics-plain-output option will also be changed accordingly so that
the old behavior is preserved in the presence of this option. This allows
us to use -fdiagnostics-plain-output in in the testsuite, such that the
testsuite (specifically the setting of TEST_ALWAYS_FLAGS in prune.exp)
does not need to be touched whenever diagnostics get a new look. This also
removes the need to add workarounds to compat.exp for every new option
that may be needed in a newer version of the compiler, but is not
supported in older versions.

gcc/ChangeLog:

	* common.opt: Add new option -fdiagnostics-plain-output.
	* doc/invoke.texi: Document it.
	* opts-common.c (decode_cmdline_options_to_array): Implement it.
	(decode_cmdline_option): Add missing const qualifier to argv.

libstdc++-v3/ChangeLog:

	* testsuite/lib/libstdc++.exp: Use the new option
	-fdiagnostics-plain-output.

gcc/testsuite/ChangeLog:

	* lib/prune.exp: Change TEST_ALWAYS_FLAGS to use -fdiagnostics-plain-output.
	* lib/c-compat.exp: Adapt to the prune.exp change.

1 files changed, 37 insertions, 2 deletions

diff --git a/gcc/opts-common.c b/gcc/opts-common.c
index de9510a..237e4ce 100644
--- a/gcc/opts-common.c
+++ b/gcc/opts-common.c
@@ -529,7 +529,7 @@ add_misspelling_candidates (auto_vec<char *> *candidates,
    consumed.  */
 
 static unsigned int
-decode_cmdline_option (const char **argv, unsigned int lang_mask,
+decode_cmdline_option (const char *const *argv, unsigned int lang_mask,
 		       struct cl_decoded_option *decoded)
 {
   size_t opt_index;
@@ -944,7 +944,8 @@ decode_cmdline_options_to_array (unsigned int argc, const char **argv,
   struct cl_decoded_option *opt_array;
   unsigned int num_decoded_options;
 
-  opt_array = XNEWVEC (struct cl_decoded_option, argc);
+  int opt_array_len = argc;
+  opt_array = XNEWVEC (struct cl_decoded_option, opt_array_len);
 
   opt_array[0].opt_index = OPT_SPECIAL_program_name;
   opt_array[0].warn_message = NULL;
@@ -981,6 +982,40 @@ decode_cmdline_options_to_array (unsigned int argc, const char **argv,
 	  argv[++i] = replacement;
 	}
 
+      /* Expand -fdiagnostics-plain-output to its constituents.  This needs
+	 to happen here so that prune_options can handle -fdiagnostics-color
+	 specially.  */
+      if (!strcmp (opt, "-fdiagnostics-plain-output"))
+	{
+	  /* If you have changed the default diagnostics output, and this new
+	     output is not appropriately "plain" (e.g., the change needs to be
+	     undone in order for the testsuite to work properly), then please do
+	     the following:
+		 1.  Add the necessary option to undo the new behavior to
+		     the array below.
+		 2.  Update the documentation for -fdiagnostics-plain-output
+		     in invoke.texi.  */
+	  const char *const expanded_args[] = {
+	    "-fno-diagnostics-show-caret",
+	    "-fno-diagnostics-show-line-numbers",
+	    "-fdiagnostics-color=never",
+	    "-fdiagnostics-urls=never",
+	  };
+	  const int num_expanded = ARRAY_SIZE (expanded_args);
+	  opt_array_len += num_expanded - 1;
+	  opt_array = XRESIZEVEC (struct cl_decoded_option,
+				  opt_array, opt_array_len);
+	  for (int j = 0, nj; j < num_expanded; j += nj)
+	    {
+	      nj = decode_cmdline_option (expanded_args + j, lang_mask,
+					  &opt_array[num_decoded_options]);
+	      num_decoded_options++;
+	    }
+
+	  n = 1;
+	  continue;
+	}
+
       n = decode_cmdline_option (argv + i, lang_mask,
 				 &opt_array[num_decoded_options]);
       num_decoded_options++;