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.

1 files changed, 2 insertions, 0 deletions

diff --git a/gdb/testsuite/lib/gdb-utils.exp b/gdb/testsuite/lib/gdb-utils.exp
index 9741f0a..98bdd72 100644
--- a/gdb/testsuite/lib/gdb-utils.exp
+++ b/gdb/testsuite/lib/gdb-utils.exp
@@ -55,6 +55,8 @@ proc style {str style} {
 	variable { set style 36 }
 	address { set style 34 }
 	metadata { set style 2 }
+	completion-prefix { set style 2 }
+	completion-difference { set style 35 }
     }
     return "\033\\\[${style}m${str}\033\\\[m"
 }