Support font driver supersession

* src/font.c (font_update_drivers): If argument NEW_DRIVERS is t, then don't
use superseded drivers.
(syms_of_font) <Qfont_driver_superseded_by>: New DEFSYM.

* src/ftfont.c (syms_of_ftfont) [HAVE_HARFBUZZ]:
* src/ftcrfont.c (syms_of_ftcrfont) [HAVE_HARFBUZZ]:
* src/w32font.c (syms_of_w32font) [HAVE_HARFBUZZ]:
* src/xftfont.c (syms_of_xftfont) [HAVE_HARFBUZZ]: Make Harfbuzz variants
supersede non-Harfbuzz ones.

* src/w32fns.c (Fx_create_frame, w32_create_tip_frame): Remove font backend
determination code.

1 files changed, 29 insertions, 6 deletions

diff --git a/src/font.c b/src/font.c
index 5705758b99f..964028ccf00 100644
--- a/src/font.c
+++ b/src/font.c
@@ -3518,7 +3518,10 @@ free_font_driver_list (struct frame *f)
 
 /* Make the frame F use font backends listed in NEW_DRIVERS (list of
    symbols, e.g. xft, x).  If NEW_DRIVERS is t, make F use all
-   available font drivers.  If NEW_DRIVERS is nil, finalize all drivers.
+   available font drivers that are not superseded by another driver.
+   (A font driver SYMBOL is superseded by the driver specified by
+   SYMBOL's 'font-driver-superseded-by property if it is a non-nil
+   symbol.)  If NEW_DRIVERS is nil, finalize all drivers.
 
    A caller must free all realized faces if any in advance.  The
    return value is a list of font backends actually made used on
@@ -3527,16 +3530,33 @@ free_font_driver_list (struct frame *f)
 Lisp_Object
 font_update_drivers (struct frame *f, Lisp_Object new_drivers)
 {
-  Lisp_Object active_drivers = Qnil;
+  Lisp_Object active_drivers = Qnil, default_drivers = Qnil;
   struct font_driver_list *list;
 
+  /* Collect all unsuperseded driver symbols into
+     `default_drivers'.  */
+  Lisp_Object all_drivers = Qnil;
+  for (list = f->font_driver_list; list; list = list->next)
+    all_drivers = Fcons (list->driver->type, all_drivers);
+  for (Lisp_Object rest = all_drivers; CONSP (rest); rest = XCDR (rest))
+    {
+      Lisp_Object superseded_by
+        = Fget (XCAR (rest), Qfont_driver_superseded_by);
+
+      if (NILP (superseded_by)
+          || NILP (Fmemq (superseded_by, all_drivers)))
+        default_drivers = Fcons (XCAR (rest), default_drivers);
+    }
+
+  if (EQ (new_drivers, Qt))
+    new_drivers = default_drivers;
+
   /* At first, turn off non-requested drivers, and turn on requested
      drivers.  */
   for (list = f->font_driver_list; list; list = list->next)
     {
       struct font_driver const *driver = list->driver;
-      if ((EQ (new_drivers, Qt) || ! NILP (Fmemq (driver->type, new_drivers)))
-          != list->on)
+      if ((! NILP (Fmemq (driver->type, new_drivers))) != list->on)
         {
           if (list->on)
             {
@@ -3559,8 +3579,7 @@ font_update_drivers (struct frame *f, Lisp_Object new_drivers)
 
   if (NILP (new_drivers))
     return Qnil;
-
-  if (! EQ (new_drivers, Qt))
+  else
     {
       /* Re-order the driver list according to new_drivers.  */
       struct font_driver_list **list_table, **next;
@@ -3599,6 +3618,8 @@ font_update_drivers (struct frame *f, Lisp_Object new_drivers)
             {
               struct font_driver const *driver = list->driver;
               eassert (! list->on);
+              if (NILP (Fmemq (driver->type, default_drivers)))
+                continue;
               if (! driver->start_for_frame
                   || driver->start_for_frame (f) == 0)
                 {
@@ -5359,6 +5380,8 @@ syms_of_font (void)
   DEFSYM (QL2R, "L2R");
   DEFSYM (QR2L, "R2L");
 
+  DEFSYM (Qfont_driver_superseded_by, "font-driver-superseded-by");
+
   scratch_font_spec = Ffont_spec (0, NULL);
   staticpro (&scratch_font_spec);
   scratch_font_prefer = Ffont_spec (0, NULL);