. Built-in support for bignums

In Emacs 27, you can now have arbitrarily long integer constants, or
operations that result in arbitrarily long integers.

#+begin_src emacs-lisp
(let ((i 100329487239873259873254987235987345987435987)) (message "%d" (1+ i)))

. HarfBuzz as the primary text-shaping engine, enabling support for
  ligatures, stylistic sets, etc.

In HarfBuzz there is a way to select styles, but we haven't yet exposed it
through Emacs Lisp. This isn't a hard job, and just requires someone to review
the HarfBuzz functionality and decide how to make it available in various
modes. For example, we'd probably like to see different ligatures in text
modes apart from programming modes, etc.

Right now we use an old feature for ligatures called static compilations,
which is something we'd like to remove from the Emacs code base entirely. For
example, the current schemes fakes ligatures by creating a string containing a
particular symbol and masking out the set of characters you'd like to replace
with the ligature. Without HarfBuzz this was the only way; but now that we
have it, we can have real ligatures. We just have to tell the HarfBuzz
library: when you see "ffi", simply legate them, and it will be replaced by
the ffi ligature.

For example, all you would have to say is, "for this text, display it as style
#7", in which case HarfBuzz can use what it knows about the font, which is
able to encode ligature replacements. We could also have a special text
property which indicates which style to use for a specific range of text.

. The Cairo build is no longer "experimental" (i.e., buggy)

This build had a lot of bugs, most of which have been fixed, so we've removed
the experimental tag in the configure script.

In the Cairo scheme, you don't use the typical server/client mode. Cairo is a
text shaping and display library, and is something that Wayland wants to see
in applications. This is the path forward for graphical Emacs on Linux
machines. It's the means for Emacs to stay relevant in the future. Just like
the move from text-only to X11, this is a evolutionary move foward for how
text is displayed by Emacs.

Importantly, to keep Emacs alive there are certain developments that need to
be tracked and supported. It's not enough to just play Lisp games and focus on
the applications that Emacs is able to run. But this will not keep Emacs alive
for years to come. As the core technologies of the system evolve around us, we
have to constantly adapt and add first class support for them. And mostly this
happens on the C side of development, not the Lisp side.

Eli could use help from people to track and understand these technology, and
help us understand which ones are important now, and which are going to become
important in the near to distant future. HarfBuzz is a good example of one of
these; Eli had noticed that Windows has abandoned Uniscribe, and chosen a
library that cannot be called from C, only C++; and on Unix, the library we
were using for complex text shaping is no longer being developed or
maintained. These are two large libraries that we depend on that we're going
to lose support for; so HarfBuzz became a necessary path forward, and it's
important for the community to understand that for all the exciting
developments that happen on the Lisp side, there are absolutely essential
changes that also need to happen on the C side for Emacs to stay relevant, or
even functional!

. Built-in tabs and tab bar

On Linux machines, and in text mode, this provides built-in tabs, just as you
have in your browser. There is a tab-bar per frame; also, per window each tab
saves a window configuration, not just the window, so that when you press a
tab it can reconfigure the entire contents of the frame.

(NOTE: This did not work for me on macOS)

. Built-in image scaling and rotation without ImageMagick

"ImageMagick is a Pandora's box" -- Eli

We see quite a few crash bugs coming from inside ImageMagick, which forces us
to be both Emacs developers and ImageMagick developers, if we want to get
these issues fixed on our schedule. By replacing our need for ImageMagick, we
shift the work -- and the guarantee of stability -- into our own code base.

. Portable dumping

This is important because modern C libraries no longer do the tricks which the
original dumper relied upon. These tricks made assumptions about how memory
allocation works, since it functioned by taking a direct dump of the heap and
then mapping it back into memory later. But this is very difficult, because
ordinarily applications are not supposed to know how memory is laid out
exactly. glibc had been giving us privileged knowledge of the heap in order to
make this possible, but made the decision to stop supporting these special
hooks, which immediately broke Emacs and result in lots of quick patches being
necessary just to keep Emacs functioning.

With the portable dumper, we can finally drop this abstraction leakage and
make ourselves immune to even which C library implementation is used. Now we
could even use MUSL, and still have support for snapshotting the load state
for fast recovery later.

. Initial support for XDG spec

~/.config, ~/.cache, ~/.local

One of the problems with this is that parts of the XDG spec are incompatible
with the original Emacs directory location assumptions, making it tricky to
maintain backward compatibility, but since this is also the way of the future,
it's good to begin introducing this support now.

. Enhanced NSM; more tight TLS security

Network Security Manager (in lisp/net) is an intermediate layer that watches
all the network connections and makes sure that the security is OK. For
example, if you start a TLS connection, NSM validates all of the certificates
involved, that the authorities are OK, that they are not expired, etc. If
anything is wrong, it prompts the user as to see they want to use. This
separates out security validation from the network transport layer.

. display-fill-column-indicator-mode as now a built-in feature

This is now native in the display engine itself, so more text repainting
tricks are needed based on hooks or change functions, etc. Also, it now
maintains compatibility with other Lisp packages that modify the buffer,
whereas previously the set of tricks needed would often collide with other
tricks being performed with other modules.

. so-long-mode now available for buffers with very long lines

This mode makes buffers with very long lines more bareable. It turns off all
kinds of features that are known to slow down long lines even more. If your
lines are very very very long, instead of waiting for a minute for this to
redisplay, you may only have to wait for a few seconds. It doesn't solve all
the problems, it just knows about features that make the problem even worse,
and turns those features. But it only knows about basic features, like
font-lock, etc.

. 'ngettext' for internationalization of messages

This is a small step in the direction of internationalization. Making Emacs
properly localized is a /very/ large job. We even name commands such that the
name is a kind of documentation -- but the name is in English!

This job is necessarily huge, but at leasts ngettext is a small step in that
direction. If a Lisp package, for example, wants to display messages based on
the locale, it can now provides strings to ngettext, and the proper string
will be chosen based on the local language.

In a sense, this is more a flag to show people that we're thinking about this
direction in the future, even though there is still so much more to be done,
and as always, volunteers are more than welcome!