Support "replacement characters" on TTY frames

* src/nsterm.m (ns_draw_glyphless_glyph_string_foreground):
* src/pgtkterm.c (pgtk_draw_glyphless_glyph_string_foreground):
* src/haikuterm.c (haiku_draw_glyphless_glyph_string_foreground):
* src/xterm.c (x_draw_glyphless_glyph_string_foreground):
* src/w32term.c (w32_draw_glyphless_glyph_string_foreground):
* src/xdisp.c (lookup_glyphless_char_display): Handle extra-slot
of 'glyphless-char-display' that is a cons cell.
(syms_of_xdisp) <glyphless-char-display>: Update doc string.

* etc/NEWS:
* doc/lispref/display.texi (Glyphless Chars): Document the new
feature.

* lisp/faces.el (glyphless-char): Make the face stand out on TTY
frames that don't support the underline attribute.

7 files changed, 36 insertions, 13 deletions

diff --git a/src/haikuterm.c b/src/haikuterm.c
index c2d4e34ba25..df1c39974f8 100644
--- a/src/haikuterm.c
+++ b/src/haikuterm.c
@@ -1252,6 +1252,8 @@ haiku_draw_glyphless_glyph_string_foreground (struct glyph_string *s)
                    ? CHAR_TABLE_REF (Vglyphless_char_display,
                                      glyph->u.glyphless.ch)
                    : XCHAR_TABLE (Vglyphless_char_display)->extras[0]);
+              if (CONSP (acronym))
+                acronym = XCAR (acronym);
               if (STRINGP (acronym))
                 str = SSDATA (acronym);
             }
diff --git a/src/nsterm.m b/src/nsterm.m
index e3f47eb905e..6c6151701b8 100644
--- a/src/nsterm.m
+++ b/src/nsterm.m
@@ -4241,6 +4241,8 @@ ns_draw_glyphless_glyph_string_foreground (struct glyph_string *s)
                    ? CHAR_TABLE_REF (Vglyphless_char_display,
                                      glyph->u.glyphless.ch)
                    : XCHAR_TABLE (Vglyphless_char_display)->extras[0]);
+              if (CONSP (acronym))
+                acronym = XCAR (acronym);
               if (STRINGP (acronym))
                 str = SSDATA (acronym);
             }
diff --git a/src/pgtkterm.c b/src/pgtkterm.c
index b283cef7cde..491ba338821 100644
--- a/src/pgtkterm.c
+++ b/src/pgtkterm.c
@@ -1584,6 +1584,8 @@ pgtk_draw_glyphless_glyph_string_foreground (struct glyph_string *s)
                    ? CHAR_TABLE_REF (Vglyphless_char_display,
                                      glyph->u.glyphless.ch)
                    : XCHAR_TABLE (Vglyphless_char_display)->extras[0]);
+              if (CONSP (acronym))
+                acronym = XCAR (acronym);
               if (STRINGP (acronym))
                 str = SSDATA (acronym);
             }
diff --git a/src/term.c b/src/term.c
index 3bea621dbda..2e43d89232f 100644
--- a/src/term.c
+++ b/src/term.c
@@ -1862,12 +1862,24 @@ produce_glyphless_glyph (struct it *it, Lisp_Object acronym)
             acronym = CHAR_TABLE_REF (Vglyphless_char_display, it->c);
           if (CONSP (acronym))
             acronym = XCDR (acronym);
-          buf[0] = '[';
           str = STRINGP (acronym) ? SSDATA (acronym) : "";
-          for (len = 0; len < 6 && str[len] && ASCII_CHAR_P (str[len]); len++)
-            buf[1 + len] = str[len];
-          buf[1 + len] = ']';
-          len += 2;
+          /* A special kludgey feature for single-character acronyms:
+             don't put them in a box, effectively treating them as a
+             replacement character.  */
+          if (STRINGP (acronym) && SCHARS (acronym) == 1)
+            {
+              buf[0] = str[0];
+              len = 1;
+            }
+          else
+            {
+              buf[0] = '[';
+              for (len = 0;
+                   len < 6 && str[len] && ASCII_CHAR_P (str[len]); len++)
+                buf[1 + len] = str[len];
+              buf[1 + len] = ']';
+              len += 2;
+            }
         }
       else
         {
diff --git a/src/w32term.c b/src/w32term.c
index d0577efccc1..dff21489e5b 100644
--- a/src/w32term.c
+++ b/src/w32term.c
@@ -1490,6 +1490,8 @@ w32_draw_glyphless_glyph_string_foreground (struct glyph_string *s)
                    ? CHAR_TABLE_REF (Vglyphless_char_display,
                                      glyph->u.glyphless.ch)
                    : XCHAR_TABLE (Vglyphless_char_display)->extras[0]);
+              if (CONSP (acronym))
+                acronym = XCAR (acronym);
               if (STRINGP (acronym))
                 str = SSDATA (acronym);
             }
diff --git a/src/xdisp.c b/src/xdisp.c
index 86a119c0f63..70f6936dd0b 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -7842,15 +7842,14 @@ lookup_glyphless_char_display (int c, struct it *it)
       && CHAR_TABLE_EXTRA_SLOTS (XCHAR_TABLE (Vglyphless_char_display)) >= 1)
     {
       if (c >= 0)
-        {
-          glyphless_method = CHAR_TABLE_REF (Vglyphless_char_display, c);
-      if (CONSP (glyphless_method))
-        glyphless_method = FRAME_WINDOW_P (it->f)
-              ? XCAR (glyphless_method)
-              : XCDR (glyphless_method);
-        }
+        glyphless_method = CHAR_TABLE_REF (Vglyphless_char_display, c);
       else
         glyphless_method = XCHAR_TABLE (Vglyphless_char_display)->extras[0];
+
+      if (CONSP (glyphless_method))
+        glyphless_method = FRAME_WINDOW_P (it->f)
+          ? XCAR (glyphless_method)
+          : XCDR (glyphless_method);
     }
 
  retry:
@@ -37117,7 +37116,9 @@ GRAPHICAL and TEXT should each have one of the values listed above.
 The char-table has one extra slot to control the display of a character for
 which no font is found.  This slot only takes effect on graphical terminals.
 Its value should be an ASCII acronym string, `hex-code', `empty-box', or
-`thin-space'.  The default is `empty-box'.
+`thin-space'.  It could also be a cons cell of any two of these, to specify
+separate values for graphical and text terminals.
+The default is `empty-box'.
 
 If a character has a non-nil entry in an active display table, the
 display table takes effect; in this case, Emacs does not consult
diff --git a/src/xterm.c b/src/xterm.c
index c716d07adaf..e7f55255021 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -8316,6 +8316,8 @@ x_draw_glyphless_glyph_string_foreground (struct glyph_string *s)
                    ? CHAR_TABLE_REF (Vglyphless_char_display,
                                      glyph->u.glyphless.ch)
                    : XCHAR_TABLE (Vglyphless_char_display)->extras[0]);
+              if (CONSP (acronym))
+                acronym = XCAR (acronym);
               if (STRINGP (acronym))
                 str = SSDATA (acronym);
             }