Display separators on tty menus with display table entries

* src/xdisp.c (display_tty_menu_separator): Lookup separator char in
standard-display-table, make a string, and display that using
display_string.
* src/xdisp.c (display_tty_menu_separator_char): New function.
(display_tty_menu_separator): Use it.
* lisp/disp-table.el (display-table): Increase from 12 to 18.
(box-double-vertical, box-double-horizontal, box-double-down-right)
(box-double-down-left, box-double-up-right, box-double-up-left): New
symbols for extra slots.
(display-table-slot, set-display-table-slot): Change doc string.
(describe-display-table): Describe new slots.
(standard-display-unicode-special-glyphs): Define new slots.
* src/disptab.h (DISP_TABLE_P): Add enumerators.
(DISP_TABLE_EXTRA_SLOTS): Define based on enum box.
* src/dispnew.c (produce_box_glyphs): Add new enumerators to switch to
make it exhaustive.
* src/xdisp.c (display_tty_menu_separator): Use BOX_DOUBLE_HORIZONTAL
for '=' if present.
* doc/lispref/display.texi: Add documentation.
* etc/NEWS: Mention in NEWS.

3 files changed, 39 insertions, 17 deletions

diff --git a/src/dispnew.c b/src/dispnew.c
index d28dc3d54fa..d1e731e69f8 100644
--- a/src/dispnew.c
+++ b/src/dispnew.c
@@ -3614,6 +3614,13 @@ produce_box_glyphs (enum box box, struct glyph_row *row, int x, int n,
     case BOX_UP_LEFT:
       dflt = '+';
       break;
+    case BOX_DOUBLE_VERTICAL:
+    case BOX_DOUBLE_HORIZONTAL:
+    case BOX_DOUBLE_DOWN_RIGHT:
+    case BOX_DOUBLE_DOWN_LEFT:
+    case BOX_DOUBLE_UP_RIGHT:
+    case BOX_DOUBLE_UP_LEFT:
+      emacs_abort ();
     }
 
   /* FIXME/tty: some face for the border.  */
diff --git a/src/disptab.h b/src/disptab.h
index 8db9a06d2f4..5ab73715e6c 100644
--- a/src/disptab.h
+++ b/src/disptab.h
@@ -28,7 +28,6 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
    && EQ (XCHAR_TABLE (obj)->purpose, Qdisplay_table)                       \
    && CHAR_TABLE_EXTRA_SLOTS (XCHAR_TABLE (obj)) == DISP_TABLE_EXTRA_SLOTS)
 
-#define DISP_TABLE_EXTRA_SLOTS 12
 #define DISP_TRUNC_GLYPH(dp) ((dp)->extras[0])
 #define DISP_CONTINUE_GLYPH(dp) ((dp)->extras[1])
 #define DISP_ESCAPE_GLYPH(dp) ((dp)->extras[2])
@@ -43,7 +42,14 @@ enum box
   BOX_DOWN_RIGHT,
   BOX_DOWN_LEFT,
   BOX_UP_RIGHT,
-  BOX_UP_LEFT
+  BOX_UP_LEFT,
+  BOX_DOUBLE_VERTICAL,
+  BOX_DOUBLE_HORIZONTAL,
+  BOX_DOUBLE_DOWN_RIGHT,
+  BOX_DOUBLE_DOWN_LEFT,
+  BOX_DOUBLE_UP_RIGHT,
+  BOX_DOUBLE_UP_LEFT
+#define DISP_TABLE_EXTRA_SLOTS (BOX_DOUBLE_UP_LEFT + 1)
 };
 
 extern Lisp_Object disp_char_vector (struct Lisp_Char_Table *, int);
diff --git a/src/xdisp.c b/src/xdisp.c
index 2676e982c9e..5b5cb3849fc 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -27272,33 +27272,42 @@ deep_copy_glyph_row (struct frame *f, struct glyph_row *to, struct glyph_row *fr
     fill_up_frame_row_with_spaces (f, to, to_used);
 }
 
+/* Return the character to be used for displaying a tty menu separator.
+   C is the character to be used by default.  BOX is the display table
+   entry for the character to be used instead.  It is looked up in
+   standard-display-table.  Value is the character to use.  */
+
+static int
+display_tty_menu_separator_char (int c, enum box box)
+{
+  if (DISP_TABLE_P (Vstandard_display_table))
+    {
+      struct Lisp_Char_Table *dp = XCHAR_TABLE (Vstandard_display_table);
+      Lisp_Object gc = dp->extras[box];
+      if (GLYPH_CODE_P (gc))
+        c = GLYPH_CODE_CHAR (gc);
+    }
+  return c;
+}
+
 /* Produce glyphs for a menu separator on a tty.
 
    FIXME: This is only a "good enough for now" implementation of menu
    separators as described in the Elisp info manual.  We should probably
-   ignore menu separators when computing the width of a menu.  Secondly,
-   optionally using Unicode characters via display table entries would
-   be nice.  Patches very welcome.  */
+   ignore menu separators when computing the width of a menu.  */
 
 static void
 display_tty_menu_separator (struct it *it, const char *label, int width)
 {
-  USE_SAFE_ALLOCA;
-  char c;
+  int c;
   if (strcmp (label, "--space") == 0)
     c = ' ';
   else if (strcmp (label, "--double-line") == 0)
-    c = '=';
+    c = display_tty_menu_separator_char ('=', BOX_DOUBLE_HORIZONTAL);
   else
-    c = '-';
-  char *sep = SAFE_ALLOCA (width);
-  memset (sep, c, width - 1);
-  sep[width -  1] = 0;
-  display_string (sep, Qnil, Qnil, 0, 0, it, width - 1, width - 1,
-                  FRAME_COLS (it->f) - 1, -1);
-  display_string (" ", Qnil, Qnil, 0, 0, it, 1, 0,
-                  FRAME_COLS (it->f) - 1, -1);
-  SAFE_FREE ();
+    c = display_tty_menu_separator_char ('-', BOX_HORIZONTAL);
+  Lisp_Object sep = Fmake_string (make_fixnum (width - 1), make_fixnum (c), Qt);
+  display_string ((char *) SDATA (sep), Qnil, Qnil, 0, 0, it, width, -1, -1, 1);
 }
 
 /* Display one menu item on a TTY, by overwriting the glyphs in the