Use emoji to indicate errors and warnings

This patch adds, at long last, some emoji output to gdb.  In
particular, warnings are indicated with the U+26A0 (WARNING SIGN), and
errors with U+274C (CROSS MARK).

There is a new setting to control whether emoji output can be used.
It defaults to "auto", which means emoji will be used if the host
charset is UTF-8.  Note that disabling styling will also disable
emoji, handy for traditionalists.

I've refactored mingw console output a little, so that emoji will not
be printed to the console.  Note the previous code here was a bit
strange in that it assumed that the first use of gdb_console_fputs
would be to stdout.

This version lets the user control the prefixes directly, so different
emoji can be chosen if desired.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Reviewed-By: Keith Seitz <keiths@redhat.com>
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>

gdb/cli/cli-style.c

@@ -23,6 +23,7 @@
 #include "cli/cli-style.h"
 #include "source-cache.h"
 #include "observable.h"
+#include "charset.h"
 
 /* True if styling is enabled.  */
 
@@ -42,6 +43,10 @@ bool source_styling = true;
 
 bool disassembler_styling = true;
 
+/* User-settable variable controlling emoji output.  */
+
+static auto_boolean emoji_styling = AUTO_BOOLEAN_AUTO;
+
 /* Names of intensities; must correspond to
    ui_file_style::intensity.  */
 static const char * const cli_intensities[] = {
@@ -410,6 +415,85 @@ show_style_disassembler (struct ui_file *file, int from_tty,
     gdb_printf (file, _("Disassembler output styling is disabled.\n"));
 }
 
+/* Implement 'show style emoji'.  */
+
+static void
+show_emoji_styling (struct ui_file *file, int from_tty,
+		    struct cmd_list_element *c, const char *value)
+{
+  if (emoji_styling == AUTO_BOOLEAN_TRUE)
+    gdb_printf (file, _("CLI emoji styling is enabled.\n"));
+  else if (emoji_styling == AUTO_BOOLEAN_FALSE)
+    gdb_printf (file, _("CLI emoji styling is disabled.\n"));
+  else
+    gdb_printf (file, _("CLI emoji styling is automatic (currently %s).\n"),
+		emojis_ok () ? _("enabled") : _("disabled"));
+}
+
+/* See cli-style.h.  */
+
+bool
+emojis_ok ()
+{
+  if (!cli_styling || emoji_styling == AUTO_BOOLEAN_FALSE)
+    return false;
+  if (emoji_styling == AUTO_BOOLEAN_TRUE)
+    return true;
+  return strcmp (host_charset (), "UTF-8") == 0;
+}
+
+/* See cli-style.h.  */
+
+void
+no_emojis ()
+{
+  emoji_styling = AUTO_BOOLEAN_FALSE;
+}
+
+/* Emoji warning prefix.  */
+static std::string warning_prefix = "⚠️ ";
+
+/* Implement 'show warning-prefix'.  */
+
+static void
+show_warning_prefix (struct ui_file *file, int from_tty,
+		     struct cmd_list_element *c, const char *value)
+{
+  gdb_printf (file, _("Warning prefix is \"%s\".\n"),
+	      warning_prefix.c_str ());
+}
+
+/* See cli-style.h.  */
+
+void
+print_warning_prefix (ui_file *file)
+{
+  if (emojis_ok ())
+    gdb_puts (warning_prefix.c_str (), file);
+}
+
+/* Emoji error prefix.  */
+static std::string error_prefix = "❌️ ";
+
+/* Implement 'show error-prefix'.  */
+
+static void
+show_error_prefix (struct ui_file *file, int from_tty,
+		     struct cmd_list_element *c, const char *value)
+{
+  gdb_printf (file, _("Error prefix is \"%s\".\n"),
+	      error_prefix.c_str ());
+}
+
+/* See cli-style.h.  */
+
+void
+print_error_prefix (ui_file *file)
+{
+  if (emojis_ok ())
+    gdb_puts (error_prefix.c_str (), file);
+}
+
 void _initialize_cli_style ();
 void
 _initialize_cli_style ()
@@ -431,6 +515,13 @@ If enabled, output to the terminal is styled."),
 			   set_style_enabled, show_style_enabled,
 			   &style_set_list, &style_show_list);
 
+  add_setshow_auto_boolean_cmd ("emoji", no_class, &emoji_styling, _("\
+Set whether emoji output is enabled."), _("\
+Show whether emoji output is enabled."), _("\
+If enabled, emojis may be displayed."),
+			   nullptr, show_emoji_styling,
+			   &style_set_list, &style_show_list);
+
   add_setshow_boolean_cmd ("sources", no_class, &source_styling, _("\
 Set whether source code styling is enabled."), _("\
 Show whether source code styling is enabled."), _("\
@@ -627,4 +718,23 @@ coming from your source code."),
 		 &style_disasm_set_list);
   add_alias_cmd ("symbol", function_prefix_cmds.show, no_class, 0,
 		 &style_disasm_show_list);
+
+  add_setshow_string_cmd ("warning-prefix", no_class,
+			  &warning_prefix,
+			  _("Set the warning prefix text."),
+			  _("Show the warning prefix text."),
+			  _("\
+The warning prefix text is displayed before any warning, when\n\
+emoji output is enabled."),
+			  nullptr, show_warning_prefix,
+			  &style_set_list, &style_show_list);
+  add_setshow_string_cmd ("error-prefix", no_class,
+			  &error_prefix,
+			  _("Set the error prefix text."),
+			  _("Show the error prefix text."),
+			  _("\
+The error prefix text is displayed before any error, when\n\
+emoji output is enabled."),
+			  nullptr, show_error_prefix,
+			  &style_set_list, &style_show_list);
 }