Add completion styling

Readline has a styling feature for completion -- if it is enabled, the
common prefix of completions will be displayed in a different style.
This doesn't work in gdb, because gdb implements its own completer.

This patch implements the feature.  However, it doesn't directly use
the Readline feature, because gdb can do a bit better: it can let the
user control the styling using the existing mechanisms.

This version incorporates an Emacs idea, via Eli: style the prefix,
the "difference character", and the suffix differently.

gdb/ChangeLog
2020-05-23  Tom Tromey  <tom@tromey.com>

	* NEWS: Add entry for completion styling.
	* completer.c (_rl_completion_prefix_display_length): Move
	declaration earlier.
	(gdb_fnprint): Use completion_style.
	(gdb_display_match_list_1): Likewise.
	* cli/cli-style.c (completion_prefix_style)
	(completion_difference_style, completion_suffix_style): New
	globals.
	(_initialize_cli_style): Register new globals.
	* cli/cli-style.h (completion_prefix_style)
	(completion_difference_style, completion_suffix_style): Declare.

gdb/doc/ChangeLog
2020-05-23  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Output Styling): Mention completion styling.
	(Editing): Mention readline completion styling.

gdb/testsuite/ChangeLog
2020-05-23  Tom Tromey  <tom@tromey.com>

	* gdb.base/style.exp: Add completion styling test.
	* lib/gdb-utils.exp (style): Add completion styles.

2 files changed, 51 insertions, 0 deletions

diff --git a/gdb/cli/cli-style.c b/gdb/cli/cli-style.c
index a0c3cc5..b16b800 100644
--- a/gdb/cli/cli-style.c
+++ b/gdb/cli/cli-style.c
@@ -98,6 +98,21 @@ cli_style_option metadata_style ("metadata", ui_file_style::DIM);
 
 /* See cli-style.h.  */
 
+cli_style_option completion_prefix_style ("completion-prefix",
+					  ui_file_style::DIM);
+
+/* See cli-style.h.  */
+
+cli_style_option completion_difference_style ("completion-difference",
+					      ui_file_style::MAGENTA);
+
+/* See cli-style.h.  */
+
+cli_style_option completion_suffix_style ("completion-suffix",
+					  ui_file_style::NONE);
+
+/* See cli-style.h.  */
+
 cli_style_option::cli_style_option (const char *name,
 				    ui_file_style::basic_color fg)
   : changed (name),
@@ -366,6 +381,33 @@ your data, for example \"<unavailable>\""),
 				       &style_set_list, &style_show_list,
 				       false);
 
+  completion_prefix_style.add_setshow_commands (no_class, _("\
+Completion prefix display styling.\n\
+Configure completion prefix colors and display intensity\n\
+The \"completion-prefix\" style is used when GDB displays the shared\n\
+prefix common to the possible completions."),
+						&style_set_list,
+						&style_show_list,
+						false);
+
+  completion_difference_style.add_setshow_commands (no_class, _("\
+Completion difference display styling.\n\
+Configure completion difference colors and display intensity\n\
+The \"completion-difference\" style is used when GDB displays the\n\
+character that differs between the possible completions."),
+						&style_set_list,
+						&style_show_list,
+						false);
+
+  completion_suffix_style.add_setshow_commands (no_class, _("\
+Completion suffix display styling.\n\
+Configure completion suffix colors and display intensity\n\
+The \"completion-suffix\" style is used when GDB displays the suffix\n\
+of the possible completions."),
+						&style_set_list,
+						&style_show_list,
+						false);
+
   tui_border_style.add_setshow_commands (no_class, _("\
 TUI border display styling.\n\
 Configure TUI border colors\n\
diff --git a/gdb/cli/cli-style.h b/gdb/cli/cli-style.h
index 6422e52..c2e0df1 100644
--- a/gdb/cli/cli-style.h
+++ b/gdb/cli/cli-style.h
@@ -124,6 +124,15 @@ extern cli_style_option tui_border_style;
 /* The border style of a TUI window that does have the focus.  */
 extern cli_style_option tui_active_border_style;
 
+/* The style for the common prefix of completions.  */
+extern cli_style_option completion_prefix_style;
+
+/* The style for the difference character of completions.  */
+extern cli_style_option completion_difference_style;
+
+/* The style for the suffix of completions.  */
+extern cli_style_option completion_suffix_style;
+
 /* True if source styling is enabled.  */
 extern bool source_styling;