; * etc/TODO: Update.

etc/TODO

@@ -219,6 +219,54 @@ https://lists.gnu.org/r/emacs-devel/2013-11/msg00515.html
     processing.  That is why we added text properties and variable
     width fonts.  However, more features are still needed to achieve this.
 
+** Support ligatures out of the box
+For the list of typographical ligatures, see
+
+  https://en.wikipedia.org/wiki/Orthographic_ligature#Ligatures_in_Unicode_(Latin_alphabets)
+
+For Text and derived modes, the job is to figure out which ligatures
+we want to support, how to let the user customize that, and probably
+define a minor mode for automatic ligation (as some contexts might not
+want, say, "fi" or "ff" always yield a ligature, and also because it
+might slow down redisplay, because character composition goes through
+Lisp).
+
+For ligature support in programming language modes, one can look at
+the various add-on packages out there that provide the feature via
+prettify-symbols-mode.  We need to figure out which ligatures are
+needed for each programming language, and provide user options to turn
+this on and off.
+
+The implementation should use the infrastructure for character
+compositions, i.e., we should define appropriate regexp-based rules
+for character sequences that need to be composed into ligatures, and
+populate composition-function-table with those rules.  See
+composite.el for examples of this, and also grep lisp/language/*.el
+for references to composition-function-table.
+
+The prettify-symbols-mode should be deprecated once ligature support
+is in place.
+
+** Support for Stylistic Sets
+This will allow using "alternate glyphs" supported by modern fonts.
+For an overview of this feature, see
+
+  https://www.typography.com/faq/157
+  https://glyphsapp.com/tutorials/stylistic-sets
+
+HarfBuzz supports this, see this discussion:
+
+  https://lists.freedesktop.org/archives/harfbuzz/2019-September/007434.html
+
+One possible way of letting Lisp program support this would be to
+introduce a new text property 'stylistic-set' whose value will be the
+set name(s), a symbol or a list of symbols.  Characters that have this
+property should be processed specially by 'get_glyph_face_and_encoding':
+instead of calling the 'encode_char' method of the font driver, we
+should invoke the 'shape' method.  'hbfont_shape' should be extended
+to pass to 'hb_shape_full' the required array of features, as
+mentioned in the above HarfBuzz discussion.
+
 ** Extend text-properties and overlays
 *** Several text-property planes
 This would get us rid of font-lock-face property (and I'd be happy to
@@ -529,10 +577,6 @@ from the emacsclient process.
 ** Optionally make the cursor a little thinner at the end of a line
   or the end of the buffer.
 
-** Port the conservative stack marking code of Emacs's garbage collector
-  to more systems, so that we can completely get rid of GCPROs.  Note
-  that Boehm garbage collector provides this.
-
 ** Reorder defcustom's in each package so that the more important
   options come first in the Customize buffers.  This could be done by
   either rearranging the file (since options are shown in the order
@@ -1469,17 +1513,25 @@ presence of multi-file documents.
 ** Replace linum.el with nlinum.el
    https://lists.gnu.org/r/emacs-devel/2013-08/msg00379.html
 
+   (Since Emacs 26 introduced native line numbers, this item is
+   probably obsolete.)
+
 ** Merge sendmail.el and messages.el.
    Probably not a complete merge, but at least arrange for messages.el to be
    a derived mode of sendmail.el.  Or arrange for messages.el to be split
    into a small core and "the rest" so that we use less resources as long as
    we stick to the features provided in sendmail.el.
 
+   (Probably obsolete, as Emacs 24 switched to message.el as the
+   default mail composer.)
+
 ** Replace gmalloc.c with the modified Doug Lea code from the current
    GNU libc so that the special mmapping of buffers can be removed --
    that apparently loses under Solaris, at least. [fx has mostly done
    this.]
 
+   (Obsolete, since gmalloc.c is nowadays only used on MS-DOS.)
+
 ** Rewrite make-docfile to be clean and maintainable.
    It might be better to replace it with Lisp, using the byte compiler.
    https://lists.gnu.org/r/emacs-devel/2012-06/msg00037.html