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What’s New in Emacs 25.1

After a long wait Emacs 25 is finally here! It’s tradition that I take the NEWS file and talk about some of the features I find most exciting. So, no more hesitation – on with the news!

Installation Changes in Emacs 25.1

Building Emacs now requires C99 or later.

Building Emacs now requires GNU make, version 3.81 or later.

C99 compiler support should bring about many new improvements. I do wonder what feature pushed them over the edge to make it mandatory.

New configure option --with-modules.
This enables support for loading dynamic modules; see below.

This is a huge development. You can now dynamically load modules into Emacs. Useful, in particular, for things that’re hard to do well or not worth replicating in Emacs. I am thinking SQLite could be used directly, which would serve as a very fast backend for TAGS files, full text search, and more.

New configure option --with-cairo.
This builds Emacs with Cairo drawing.  As a side effect, it provides
support for built-in printing, when Emacs was built with GTK+.
The Emacs Cairo drawing is experimental and still has some known
display problems.  We encourage more testing of this build and
reporting any problems you find, but it is not recommended for
production.

This is a rather important development even if you may not want to use it now. It should provide self-sufficiency and portability as Emacs can now handle its own drawing using the Cairo library. I – and I’m pondering about the future, here – wonder if this would result in a neater and faster graphics in Emacs: better (read: nicer) support for in-lining graphics, vector primitives, simple animation and more. Cairo can render to SDL, so that raises an interesting question about a GPU-accelerated Emacs. That would in turn lead to faster rendering. But who knows? It’s early days.

But for now, at least, you get free printing even though it’s experimental.

By default, Emacs no longer works on IRIX.  We expect that Emacs
users are not affected by this, as SGI stopped supporting IRIX in
December 2013.  If you are affected, please send a bug report.  You
should be able to work around the problem either by porting the Emacs
undumping code to GCC under IRIX, or by configuring --with-wide-int,
or by sticking with Emacs 24.4.
The Emacs garbage collector assumes GC_MARK_STACK == GC_MAKE_GCPROS_NOOPS.
The GC_MAKE_GCPROS_NOOPS stack-marking variant has been the default
since Emacs 24.4, and the other variants were undocumented and were
obstacles to maintenance and development.  GC_MARK_STACK and its
related symbols have been removed from the C internals.
*configure' now prefers gnustep-config when configuring GNUstep.
If gnustep-config is not available, the old heuristics are used.
*configure' now prefers inotify to gfile for file notification,
unless gfile is explicitly requested via --with-file-notification='gfile'.
*configure' detects the kqueue file notification library on *BSD
and Mac OS X machines.
The configure option '--with-pkg-config-prog' has been removed.
Use './configure PKG_CONFIG=/full/name/of/pkg-config' if you need to.
The configure option '--with-mmdf' has been removed.
It was no longer useful, as it relied on libraries that are no longer
supported, and its presence led to confusion during configuration.
This affects only the 'movemail' utility; Emacs itself can still
process MMDF-format files as before.
The configure option '--enable-silent-rules' is now the default,
and silent rules are now quieter.  To get the old behavior where
'make' chatters a lot, configure with '--disable-silent-rules' or
build with 'make V=1'.
The configure option '--with-gameuser' now allows you to specify a
group instead of a user if its argument is prefixed by ':' (a colon).
This will cause the game score files in "${localstatedir}/games/emacs"
to be owned by that group, and the helper program for updating them to
be installed setgid.  The option now defaults to the 'games' group.

Emacs ships with many games (M-x tetris, M-x pong, M-x snake, etc.) and this change is seemingly limited to which user and group the game state is stored under. I cannot imagine too many people need to worry about this.

The 'grep-changelog' script (and its manual page) are no longer included.
It has no particular connection to Emacs and has not changed in years,
so if you want to use it, you can always take a copy from an older Emacs.

I.. never even knew about this feature. From a cursory glance it’s a Perl script that extracts Changelog entries. Not particularly useful and it is unlikely to be missed.

Emacs 25 comes with a new set of icons.
Various resolutions are available as etc/images/icons/hicolor/*/apps/emacs.png.
The old Emacs logo icons are available as 'emacs23.png' in the same location.

Very much in keeping with the fashion, this logo is in the flat style that is popular on smartphones today.

New make target 'check-expensive' to run additional tests.
This includes all tests which run via "make check", plus additional
tests which take more time to perform.

Startup Changes in Emacs 25.1

When Emacs is given a file as a command line argument and
'initial-buffer-choice' is non-nil, display both the file and
'initial-buffer-choice'.  When Emacs is given more than one file and
'initial-buffer-choice' is non-nil, show 'initial-buffer-choice'
and '*Buffer List*'.  This makes Emacs convenient to use from the
command line when 'initial-buffer-choice' is non-nil.

Useful change for the minority of people who customize initial-buffer-choice. I still recommend that people open up files through emacsclient and use the client-server architecture as it re-uses your Emacs session.

The value of 'initial-scratch-message' is now treated as a doc string
and can contain escape sequences for command keys, quotes, and the like.
The default height of GUI frames was enlarged.
This is so there's enough space in the initial window to display the
optional text about recovering crashes sessions, without losing the
splash image display.

Changes in Emacs 25.1

Xwidgets: a new feature for embedding native widgets inside Emacs buffers.
If you have gtk3 and webkitgtk3 installed, and Emacs was built with
xwidget support, you can access the embedded webkit browser with 'M-x
xwidget-webkit-browse-url'.  This opens a new buffer with the embedded
browser.  The buffer will have a new mode, 'xwidget-webkit-mode'
(similar to 'image-mode'), which supports the webkit widget.

This is a really cool feature. You can embed other widgets inside Emacs buffers. It’s a feature that Windows users will handily recognize as OLE integration – a staple feature of Windows since the 90s. This feature is more limited, however, as it only works with GTK.

For now there is just one feature that makes use of this: the Webkit browser integration. You can now browse web pages with webkit inside Emacs. The feature – having played with it a lot – is in the very early stages still. I would consider it alpha quality, but with it integrated into Emacs progress is likely to be swift.

*New functions for xwidget-webkit mode 'xwidget-webkit-insert-string',
'xwidget-webkit-adjust-size-dispatch', 'xwidget-webkit-back',
'xwidget-webkit-browse-url', 'xwidget-webkit-reload',
'xwidget-webkit-current-url', 'xwidget-webkit-scroll-backward',
'xwidget-webkit-scroll-forward', 'xwidget-webkit-scroll-down',
'xwidget-webkit-scroll-up'.
Emacs can now load shared/dynamic libraries (modules).
A dynamic Emacs module is a shared library that provides additional
functionality for use in Emacs Lisp programs, just like a package
written in Emacs Lisp would.  The functions 'load', 'require',
'load-file', etc. were extended to load such modules, as they do with
Emacs Lisp packages.  The new variable 'module-file-suffix' holds the
system-dependent value of the file-name extension ('.so' on Posix
hosts) of the module files.

A module should export a C-callable function named
'emacs_module_init', which Emacs will call as part of the call to
'load' or 'require' which loads the module.  It should also export a
symbol named 'plugin_is_GPL_compatible' to indicate that its code is
released under the GPL or compatible license; Emacs will refuse to
load modules that don't export such a symbol.

If a module needs to call Emacs functions, it should do so through the
API defined and documented in the header file 'emacs-module.h'.  Note
that any module that provides Lisp-callable functions will have to use
Emacs functions such as 'fset' and 'funcall', in order to register its
functions with the Emacs Lisp interpreter.

Modules can create 'user-ptr' Lisp objects that embed pointers to C
structs defined by the module.  This is useful for keeping around
complex data structures created by a module, to be passed back to the
module's functions.  User-ptr objects can also have associated
"finalizers" -- functions to be run when the object is GC'ed; this is
useful for freeing any resources allocated for the underlying data
structure, such as memory, open file descriptors, etc.  A new
predicate 'user-ptrp' returns non-nil if its argument is a 'user-ptr'
object.

Loadable modules in Emacs are an experimental feature, and subject to
change in future releases.  For that reason, their support is disabled
by default, and must be enabled by using the '--with-modules' option
at configure time.

As I talked about above, the new dynamic module feature is a welcome addition. However, the requirement that you must export a symbol named plugin_is_GPL_compatible does greatly limit the amount of out-of-box support for Emacs, as that symbol must be exported before the module can be loaded, even if the module is licensed under GPL already. I imagine it won’t be long before someone writes a generic shim or patch that overrides this behaviour for people who are disinclined to limit themselves to just GPL-compatible modules.

Network security (TLS/SSL certificate validity and the like) is
added via the new Network Security Manager (NSM) and controlled via
the 'network-security-level' variable.

Excellent consolidation of security control, and security alerts, in Emacs. You can now define how explicit Emacs should be when it encounters some scenarios that are potential risks, such as: unable to verify a TLS certificate; whether a self-signed certificate has changed; if a connection is suddenly unencrypted if it were previously encrypted before; and much more.

If you are security conscious you can control the level of feedback by customizing network-security.

*C-h l' now also lists the commands that were run.

The lossage function in Emacs was historically a bit anaemic; most Emacsers, myself included, never really used it much as it would only display the key and not the command. That has now changed and as a result it’s so much more useful. You can now combine it with the kmacro-edit-lossage and edit the last 300 keys and create a macro from it. Far more useful when you can see actually see the command that each key ran!

*x-select-enable-clipboard' is renamed 'select-enable-clipboard'
and 'x-select-enable-primary' is renamed 'select-enable-primary'.
Additionally they both now apply to all systems (OSX, GNUstep, Windows, you
name it), with the proviso that on some systems (e.g. Windows)
'select-enable-primary' is ineffective since the system doesn't
have the equivalent of a primary selection.

Thank you. I never could remember which setting did which and on what operating system, so having one set of variables to control the clipboard behavior is useful indeed.

New option 'switch-to-buffer-in-dedicated-window' allows you to
customize how 'switch-to-buffer' proceeds interactively when the
selected window is strongly dedicated to its buffer.

Emacs’s windows are locked in an eternal fight with their user as they fight over screen estate and what buffer is allowed to be shown where. The concept of strongly dedicated window is alien to most Emacs users as it’s fairly byzantine to figure out. This variable, which is customizable, will let you decide how buffer switching works when a window and its buffer are fundamentally entwined. I recommend you set it to prompt if you are curious about its effects, as that will prompt you to decide what to do next — no doubt annoying in the long run, but useful for figuring out how Emacs’s windowing mechanics work.

The option 'even-window-heights' has been renamed to
'even-window-sizes' and now handles window widths as well.
*terpri' gets an optional arg ENSURE to conditionally output a newline.
*insert-register' now leaves point after the inserted text
when called interactively.  A prefix argument toggles this behavior.

This is a good change but one that is likely to break macros that depend on the old behavior which is to leave the point before the inserted register text. I like this change a lot as I kept screwing up my macros because I naturally assume the point is left after inserting the register text. I am glad this is now “fixed.”

The new variable 'term-file-aliases' replaces some files from lisp/term.
The function 'tty-run-terminal-initialization' consults this variable
when deciding what terminal-specific initialization code to run.
New variable 'system-configuration-features', listing some of the
main features that Emacs was compiled with.  This is mainly intended
for use in Emacs bug reports.
A password is now hidden also when typed in batch mode.  Another
hiding character but the default '.' can be used by let-binding the
variable 'read-hide-char'.
The Emacs pseudo-random number generator can be securely seeded.
On systems where Emacs can access the system entropy or some other
cryptographically secure random stream, it now uses that when 'random'
is called with its argument t.  This allows cryptographically strong
random values; in particular, the Emacs server now uses this facility
to produce its authentication key.

Good news if you want to the Fisher-Yates shuffle algorithm with proper entropy :)

New input methods: 'tamil-dvorak', 'programmer-dvorak' and 'probhat'.

Editing Changes in Emacs 25.1

'M-x' suggests shorthands and ignores obsolete commands for completion.

Helpful for new and old users alike. I recommend you try out the package smex for command completion though.

*Successive single-char deletions are collapsed in the undo-log just like
successive char insertions.  Which commands invoke this behavior is
controlled by the new 'undo-auto-amalgamate' function.  See the node
"Undo" in the ELisp manual for more details.

This is a rather big change that is going to catch people out at first. Muscle memory will have to be re-learned, but I do think it’s a positive change. It was quite asymmetric to have insertions logged as a single item but not deletes.

*The heuristic used to insert 'undo-boundary' after each command
has changed, so that if a command causes changes in more than just the
current buffer, Emacs now calls 'undo-boundary' in every buffer
affected by the command.

This is unlikely to affect most people except hopefully for the better. I believe this change is to do with indirect buffers (a way of having blocks of text in one buffer represented in another) and this change should mean that undos now propagate correctly.

New command 'comment-line' bound to 'C-x C-;'.

In theory quite useful, but the commenting semantics are not the same as M-x comment-dwim (M-;) so I do not know how much I will end up using it myself.

New and improved facilities for inserting Unicode characters

Unicode names entered via 'C-x 8 RET' now use substring completion
by default.

Good news for those of us who insert unicode characters frequently. I quite like Helm’s helm-ucs for this though.

**C-x 8' now has shorthands for several chars, such as U+2010
(HYPHEN), U+2011 (NON-BREAKING HYPHEN), and U+2012 (FIGURE DASH).  As
before, you can type 'C-x 8 C-h' to list shorthands.

If you’ve ever written a book – believe me – you will find this useful.

*New minor mode 'electric-quote-mode' for using curved quotes as you
type.  See also the new variable 'text-quoting-style'.

This, importantly, unlike electric-pair-mode, requotes the characters you type to match the styles in text-quoting-style. Seemingly no support for guillemets, to the annoyance of continental Europeans!

New minor mode 'global-eldoc-mode' is enabled by default.

I like this change, in theory, but some major modes may not be set up right and unknowingly trigger errors. If you suddenly get strange error messages when you program in your favorite language – try disabling eldoc-mode.

Emacs now uses "bracketed paste mode" on text terminals that support it.
Bracketed paste mode causes text terminals to wrap pasted text in special
escape sequences that allow Emacs to tell the difference between text
you type and text you paste from other applications.  Emacs then
avoids interpreting each character in the pasted text as it does with
keyboard input, which results in a paste experience similar to that
under a window system, and significant performance improvements when
pasting large amounts of text.

Bracketed paste mode is disabled by default, so Emacs automatically
enables it at startup if the terminal supports it.

Good stuff if you use terminal Emacs.

Emacs now supports the latest version of the UBA.
The Emacs implementation of the Unicode Bidirectional Algorithm (UBA)
was updated to support all the latest additions and changes introduced
in Unicode Standard versions 6.3, 7.0, and the latest Unicode 8.0.
This includes full support for directional isolates and the
Bidirectional Parentheses Algorithm (BPA) specified by these Unicode
standards.

Additional code points is always a benefit.

You can access 'mouse-buffer-menu' ('C-down-mouse-1') using 'C-f10'.
New buffer-local 'electric-pair-local-mode'.
New variable 'fast-but-imprecise-scrolling' inhibits
fontification during full screen scrolling operations, giving less
hesitant operation during auto-repeat of 'C-v', 'M-v' at the cost of
possible inaccuracies in the end position.
New documentation command 'describe-symbol'.
Works for functions, variables, faces, etc.  It is bound to 'C-h o' by
default.

Handy all-in-one describe command, but I will probably never remember to use it.

New function 'custom-prompt-customize-unsaved-options' checks for
unsaved customizations and prompts user to customize (if found).  It
is intended for adding to 'kill-emacs-query-functions'.
The old 'C-x w' bindings in 'hi-lock-mode' are officially deprecated
in favor of the global 'M-s h' bindings introduced in Emacs 23.1.
They'll disappear soon.

Changes in Specialized Modes and Packages in Emacs 25.1

*New command 'checkdoc-package-keywords' checks if the
current package keywords are recognized.  Set the new option
'checkdoc-package-keywords-flag' to non-nil to make
'checkdoc-current-buffer' call this function automatically.
*New function 'checkdoc-file' checks for style errors.
It's meant for use together with 'compile':
emacs -batch --eval "(checkdoc-file \"subr.el\")"

Useful for package authors. Additional linting checks are always welcome.

*The desktop format version has been upgraded from 206 to 208.
Although Emacs 25.1 can read a version 206 desktop, earlier Emacsen
cannot read a version 208 desktop.  To upgrade your desktop file, you
must explicitly request the upgrade, by 'C-u M-x desktop-save'.  You are
recommended to do this as soon as you have firmly upgraded to Emacs
25.1 (or later).  Should you ever need to downgrade your desktop file
to version 206, you can do this with 'C-u C-u M-x desktop-save'.

If you use desktop files and you swap between 25.1 and the older one the new format is not backwards compatible.

**desktop-restore-in-current-display' now defaults to t, not nil.
That is, Emacs by default now restores frames into the current display.
New function 'bookmark-set-no-overwrite' bound to 'C-x r M'.
It raises an error if a bookmark of that name already exists,
unlike 'bookmark-set' which silently updates an existing bookmark.

Interesting that they chose to make this a new command and not simply a customizable variable for the existing C-x r m key.

Gnus

*New user options 'mm-html-inhibit-images' and 'mm-html-blocked-images'
now control how mm-* functions fetch and display images in an HTML
message.  Gnus still uses 'gnus-inhibit-images' and 'gnus-blocked-images'
for that purpose, i.e., binds mm-html- variables with those gnus-
variables, but other packages do not have to bind gnus- variables now.
**mm-inline-text-html-with-images' has been removed.
Use 'mm-html-inhibit-images' instead.  Note that the value is opposite
in meaning.

IMAP

**imap-ssl-program' has been removed, and imap.el uses the internal
GnuTLS encryption functions if possible.

JSON

**json-encode-string' now only escapes the characters it has to.
Which means that the encoded strings can contain non-ASCII characters.
**json-pretty-print' and 'json-pretty-print-buffer' now maintain
the ordering of object keys by default.

I am glad they added pretty printing of JSON. For many, many years I have lugged around a windows and Linux binary of tidy to do just that to JSON, XML and HTML files. I guess I can deprecate one of those three commands now.

*New commands 'json-pretty-print-ordered' and
'json-pretty-print-buffer-ordered' pretty prints JSON objects with
object keys sorted alphabetically.

Prettify Symbols mode

*Prettify Symbols mode supports custom composition predicates.  By
overriding the default 'prettify-symbols-compose-predicate', modes can
specify in which contexts a symbol may be displayed as some Unicode
character.  'prettify-symbols-default-compose-p' is the default which
is suitable for most programming languages such as C or Lisp (but not
(La)TeX).
*Symbols can be unprettified while point is inside them.
New variable 'prettify-symbols-unprettify-at-point' configures this.

Enhanced xterm support

*The new variable 'xterm-screen-extra-capabilities' for configuring xterm.
This variable tells Emacs which advanced capabilities are available in
the xterm terminal emulator used to display Emacs text-mode frames.
The default is to check each capability, and use it if available.
(This variable was introduced in Emacs 24.1, but was not announced in
its NEWS.)
*Killing text now also sets the CLIPBOARD/PRIMARY selection
in the surrounding GUI (using the OSC-52 escape sequence).  This only works
if your xterm supports it and enables the 'allowWindowOps' options (disabled
by default at least in Debian, for security reasons).

Similarly, you can yank the CLIPBOARD/PRIMARY selection (using the OSC-52
escape sequence) if your xterm has the feature enabled but for that you
additionally need to add 'getSelection' to 'xterm-extra-capabilities'.

This is excellent news for people who make use of terminal Emacs. I did not even know that you could do this.

'xterm-mouse-mode' now supports mouse-tracking (if your xterm supports it).
The way to turn on and off 'save-place' mode has changed.
It is no longer sufficient to load the saveplace library and set
'save-place' non-nil.  Instead, use the two new minor modes:
'save-place-mode' turns on saving last place in every file, and
'save-place-local-mode' does that only for the file in whose buffer it
is invoked.  The 'save-place' variable is now an obsolete alias for
'save-place-mode', which replaces it, and 'toggle-save-place' is an
obsolete alias for the new 'save-place-local-mode' command.

Wow. I’ve been an Emacs user for 13 years and I didn’t even know Emacs had this feature. Very nice if you come from an editor or IDE that has this.

ERC

ERC can now hide message types by network or channel.
'erc-hide-list' will hide all messages of the specified type, while
'erc-network-hide-list' and 'erc-channel-hide-list' will only hide the
specified message types for the respective specified targets.

That is very nice indeed. I had built something similar myself as I get sick and tired of seeing endless JOIN/PART/QUIT messages. I am glad that this is now standard in ERC.

Reconnection is now asynchronous.
Nick completion is now case-insensitive again after inadvertently
being made case-sensitive in Emacs 24.2.

Oops.

MPC

Editor’s Note: MPC Is Emacs’s built-in Music Player. What? You didn’t know Emacs had a built-in music player? Of course it does :)

New commands, key binds, and menu items.
'<' and '>' for navigating previous and next tracks in playlist
New play/pause command 'mpc-toggle-play' bound to 's'
'g' bound to new command 'mpc-seek-current' will navigate current
track.
New commands 'mpc-toggle-{consume,repeat,single,shuffle}' for
toggling playback modes.
Now supports connecting to a UNIX domain socket.
Looks at more image file names to use as album art.
Case-insensitively tries for .folder.png (freedesktop) and folder.jpg
(XP) in addition to cover.jpg.
Searches in more locations for MPD configuration files.
MPD supports the XDG base directory specification since version 0.17.6.

Midnight-mode

Editor’s Note: Midnight mode clears out disused buffers based on certain presets like last use date

midnight-mode is now a proper minor mode.
clean-buffer-*-regexps can now specify buffers via predicate functions.

package.el

New "external" package status.
An external package is any installed package that's not built-in and
not from 'package-user-dir', which usually means it's from an entry in
'package-directory-list'.  They are treated much like built-in
packages, in that they cannot be deleted through the package menu and
are not considered for upgrades.

The effect is that a user can manually place a specific version of a
package inside 'package-directory-list' and the package menu will
always respect that.

This sounds useful if you have packages that must exist outside the normal package ecosystem.

If a package is available on multiple archives and one has higher
priority (as per 'package-archive-priorities') only that one is
listed.  This can be configured with 'package-menu-hide-low-priority'.

Good news if you want packages in some order, such as MELPA-Stable -> MELPA -> ELPA.

'package-menu-toggle-hiding' now toggles the hiding of packages.
This includes the above-mentioned low-priority packages, as well as
available packages whose version is lower than the currently installed
version (which were previously impossible to display).
This allows users to downgrade a package if a lower version is
available.
When filtering the package menu, keywords starting with "arc:" or
"status:" represent package archive or status, respectively, instead
of actual keywords.
Most functions which involve downloading information now take an
ASYNC argument.  If it is non-nil, package.el performs the download(s)
asynchronously.
New variable 'package-menu-async' controls whether the
package-menu uses asynchronous downloads.
'package-install-from-buffer' and 'package-install-file' work on directories.
This follows the same rules as installing from a .tar file, except the
-pkg file is optional.
Packages which are dependencies of other packages cannot be deleted.
The FORCE argument to 'package-delete' overrides this.

Very nice change as this should stop people from accidentally deleting package dependencies.

New custom variable 'package-selected-packages' tracks packages
which were installed by the user (as opposed to installed as
dependencies).  This variable can also be manually customized.
New command 'package-install-selected-packages' installs all
packages from 'package-selected-packages' which are currently missing.

Useful if you work with multiple computers. I prefer to just use Use Package.

'package-install' function now takes a DONT-SELECT argument.  If
this function is called interactively or if DONT-SELECT is nil, add the
package being installed to 'package-selected-packages'.
New command 'package-autoremove' removes all packages which were
installed strictly as dependencies but are no longer needed.

Shell

When you invoke 'shell' interactively, the '*shell*' buffer will now
display in a new window.  However, you can customize this behavior via
the 'display-buffer-alist' variable.  For example, to get
the old behavior -- '*shell*' buffer displays in current window -- use
(add-to-list 'display-buffer-alist
     '("^\\*shell\\*$" . (display-buffer-same-window))).

I can see why they made this change but, as someone who is hopelessly used to the old way of doing things I admit I added that suggested change to my init file.

EIEIO

*Editor’s Note: EIEIO is an Emacs implemention of some of CLOS, the Common Lisp Object System. It is also funnily reminiscent of a famous Children’s Song.

The ':protection' slot option is not obeyed any more.
The 'newname' argument to constructors is optional&deprecated.
If you need your objects to be named, do it by inheriting from 'eieio-named'.
The <class>-list-p and <class>-child-p functions are declared obsolete.
The <class> variables are declared obsolete.
The <initarg> variables are declared obsolete.
defgeneric and defmethod are declared obsolete.
Use the equivalent facilities from cl-generic.el instead.
'constructor' is now an obsolete alias for 'make-instance'.
'pcase' accepts a new UPattern 'eieio'.

ido

New command 'ido-bury-buffer-at-head' bound to 'C-S-b'.
Bury the buffer at the head of 'ido-matches', analogous to how 'C-k'
kills the buffer at head.

I.. do not think I will ever need this. Also the key binding is not great, and that does not help. I do kill buffers from IDO’s switch buffer prompt, though. That is useful.

A prefix argument to 'ido-restrict-to-matches' will reverse its
meaning, and the list is restricted to those elements that do not
match the current input.

Minibuffer

You can use <UP> and <DOWN> arrow keys to move through history by lines.
The new commands 'next-line-or-history-element' and
'previous-line-or-history-element', bound to <UP> and <DOWN> in the
minibuffer, allow by-line movement through minibuffer history,
similarly to an ordinary buffer.  Only when point moves over
the bottom/top of the minibuffer it goes to the next/previous history
element.  'M-p' and 'M-n' still move directly to previous/next history
item as before.

That should make life easier for Emacs beginners who do not yet know - or use - M-p and M-n.

Search and Replace

'isearch' and 'query-replace' can now perform character folding in matches.
This is analogous to case folding, but instead of disregarding case
variants, it disregards wider classes of distinctions between similar
characters.  (Case folding is a special case of character folding.)
This means many characters in the search string will match entire
groups of characters instead of just themselves.

For instance, the ASCII double quote character " will match all
variants of double quotes, and the letter 'a' will match all of its
accented cousins, even those composed of multiple characters, as well
as many other symbols like U+249C (PARENTHESIZED LATIN SMALL LETTER
A).

Character folding is enabled by customizing 'search-default-mode' to
the value 'char-fold-to-regexp'.  You can also toggle character
folding in the middle of a search by typing 'M-s ''.

'query-replace' honors character folding if the new variable
'replace-char-fold' is customized to a non-nil value.

I like this option but it did spawn a huge discussion about the semantics of folding characters into their naive ASCII (or should we just say Anglophone?) counterparts. I still think it is a useful and thoughtful idea that, once again, shows that even little improvements in a staple like isearch can have a huge impact.

New user option 'search-default-mode'.
This option specifies the default mode for Isearch.  The default
value, nil specifies that Isearch does literal searches (however,
'case-fold-search' and 'isearch-lax-whitespace' may still be applied,
as in previous Emacs versions).
New function 'char-fold-to-regexp' can be used
by searching commands to produce a regexp matching anything that
char-folds into STRING.

Aha. That should simplify integration with third-party searchers like Swiper.

The new 'M-s M-w' key binding uses eww to search the web for the
text in the region.  The search engine to use for this is specified by
the customizable variable 'eww-search-prefix'.

Love it. EWW – Emacs’s Web Wowser, or browser, rather – is super handy for quick google and documentation searches and this will only make it more useful.

'query-replace' history is enhanced.
When 'query-replace' reads the FROM string from the minibuffer, typing
'M-p' will now show previous replacements as "FROM SEP TO", where FROM
and TO are the original text and its replacement, and SEP is an arrow
string defined by the new variable 'query-replace-from-to-separator'.
To select a prior replacement, type 'M-p' until the desired
replacement appears in the minibuffer, and then exit the minibuffer by
typing RET.

Another hugely welcome improvement. I have been using a bleeding edge version of Emacs master so I have benefited from this change (and many others listed here) for quite a while but this is one that I think a lot of people will greatly benefit from.

Calc

If 'quick-calc' is called with a prefix argument, insert the
result of the calculation into the current buffer.

Quick-calc itself is great – and bound to C-x * Q, though its own “dispatcher” prevents the Emacs documentation from actually saying so – but this will make it, well, actually useful.

In Edebug, you can now set the initial mode with 'C-x C-a C-m'.
With this you can tell Edebug not to stop at the start of the first
instrumented function.

ElDoc

'eldoc-documentation-function' now defaults to 'ignore'.
'describe-char-eldoc' displays information about character at point,
and can be used as a default value of 'eldoc-documentation-function'.  It is
useful when, for example, one needs to distinguish various spaces - e.g.,
U+00A0 (NO-BREAK SPACE), U+2002 (EN SPACE), and U+2009 (THIN SPACE) - while
using mono-spaced font.

eww

HTML can now be rendered using variable-width fonts.
A new command 'F' ('eww-toggle-fonts') can be used to toggle
whether to use variable-pitch fonts or not.  The user can also
customize the 'shr-use-fonts' variable.
A new command 'R' ('eww-readable') will try do identify the main
textual parts of a web page and display only that, leaving menus and
the like off the page.

I’ve been using this feature for a long time now and it’s excellent. It almost always does exactly what it should and only extract and display the pertinent part of an article. Believe it or not, but this greatly improves EWW’s usability as a browser (sorry, wowser) as it’ll cut out all the crud and leaves naught but the text.

A new command 'D' ('eww-toggle-paragraph-direction') allows you to
toggle the paragraph direction between left-to-right and right-to-left.
You can now use several eww buffers in parallel by renaming eww
buffers you want to keep separate.

Much-welcome change.

Partial state of the eww buffers (the URIs and the titles of the
pages visited) is now preserved in the desktop file.
'eww-after-render-hook' is now called after eww has rendered
the data in the buffer.
The 'eww-reload' command now takes a prefix to not reload via
the net, but just use the local copy of the HTML.
The DOM shr and eww uses has been changed to the general Emacs
xml.el/libxml2 DOM, and a new package dom.el has been added to
interact with this DOM.  See the Emacs Lisp manual for interface
details.
'mailcap-mime-data' is now consulted when displaying PDF files.
The new 'S' command will list all eww buffers, and allow managing
them.
https pages with valid certificates have headers marked in green, while
invalid certificates are marked in red.

Message mode

text/html messages that contain inline image parts will be
transformed into multipart/related messages before sending.
In Show Paren Mode, a parenthesis can be highlighted when point
stands inside it, and certain parens can be highlighted when point is
at BOL or EOL, or in whitespace there.  To enable these, customize,
respectively, 'show-paren-when-point-inside-paren' or
'show-paren-when-point-in-periphery'.
If gpg2 exists on the system, it is now used as the default value
of 'epg-gpg-program' (instead of gpg).

GPG 1 is ancient yet the prevailing version on most Debian systems. But annoyingly Emacs did not recognize if GPG 2 was installed.

Lisp mode

Strings after ':documentation' are highlighted as docstrings.
This enhances Lisp mode fontification to handle documentation of the
form '(:documentation "the doc string")' used in Common Lisp code for
CLOS class and slot documentation.

Rectangle editing

Rectangle Mark mode can have corners past EOL or in the middle of a TAB.

A welcome change as rectangle mode didn’t support this until now, rendering it rather less useful than it could have been.

'C-x C-x' in 'rectangle-mark-mode' now cycles through the four corners.

C-x C-x is one of those commands that I use all the time, and that it also supports rectangle mode is even better. This will be especially useful for macros.

'string-rectangle' provides on-the-fly preview of the result.

Another small quality-of-life improvement.

New font-lock functions 'font-lock-ensure' and 'font-lock-flush'.
These should be used in preference to 'font-lock-fontify-buffer' when
called from Lisp.
Macro 'minibuffer-with-setup-hook' can optionally append a function
to 'minibuffer-setup-hook'.

If the first argument of the macro is of the form '(:append FUN)',
then FUN will be appended to 'minibuffer-setup-hook', instead of
prepending it.

cl-lib

New functions 'cl-fresh-line', 'cl-digit-char-p', and 'cl-parse-integer'.
'pcase' accepts the new UPattern 'cl-struct'.

Calendar and diary

The default 'diary-file' is now located in "~/.emacs.d".
New commands to insert diary entries with Chinese dates:
'diary-chinese-insert-anniversary-entry' 'diary-chinese-insert-entry'
'diary-chinese-insert-monthly-entry', 'diary-chinese-insert-yearly-entry'.
The calendar can now list and mark diary entries with Chinese dates.
See 'diary-chinese-list-entries' and 'diary-chinese-mark-entries'.
The option 'calendar-mode-line-format' can now be nil,
which means to do nothing special with the mode line in calendars.
New option 'calendar-weekend-days'.
The option customizes which day headers receive the
'calendar-weekend-header' face.

Useful for countries or regions where the weekend is not Saturday/Sunday. :

New optional args N and STRING for 'holiday-greek-orthodox-easter'.
Many items obsolete since at least version 23.1 have been removed.
The majority were function/variable/face aliases, too numerous to list here.
The remainder were:
Functions 'calendar-one-frame-setup', 'calendar-only-one-frame-setup',
'calendar-two-frame-setup', 'european-calendar', 'american-calendar'.
Hooks 'cal-menu-load-hook', 'cal-x-load-hook'.
Macro 'calendar-for-loop'.
Variables 'european-calendar-style', 'diary-face', 'hebrew-holidays-{1,4}'.
The nil and list forms of 'diary-display-function'.
New ERT function 'ert-summarize-tests-batch-and-exit'.
If the output of ERT tests in batch mode execution can be saved to a
log file, then it can be passed as an argument to the above function
to produce a neat summary.
New js.el option 'js-indent-first-init'.

Info

Info mode now displays symbol names in fixed-pitch font.
If you want to get the old behavior back, customize the 'Info-quoted'
face to use the same definitions as the default face.
'Info-fontify-maximum-menu-size' can be t for no limit.
'info-display-manual' can now be given a prefix argument which (any
non-nil value) directs the command to limit the completion
alternatives to currently visited manuals.
ntlm.el has support for NTLM2.

Rmail

The Rmail commands 'd', 'C-d' and 'u' take optional repeat counts
to delete or undelete multiple messages.
Rmail can now render HTML mail messages if your Emacs was built with
libxml2 or if you have the Lynx browser installed.  By default, Rmail
will display the HTML version of a mail message that has both HTML and
plain text parts, if display of HTML email is possible; customize the
'rmail-mime-prefer-html' option to nil if you don't want that.
In the commands that make summaries by subject, recipients, or senders,
you can no longer use commas to separate regular expressions.
SES now supports local printer functions; see 'ses-define-local-printer'.

Shell-script Mode

In sh-mode you can now use 'sh-shell' as a file-local variable to
specify the type of shell in use (bash, csh, etc).

Useful if you don’t use /usr/bin/env bash (or similar) as the shebang command.

New value 'always' for 'sh-indent-after-continuation'.
This provides old-style ("dumb") indentation of continued lines.
See the doc string of 'sh-indent-after-continuation' for details.

TLS

Fatal TLS errors are now silent by default.
If Emacs isn't built with TLS support, an external TLS-capable
program is used instead.  This program used to be run in --insecure
mode by default, but has now changed to be secure instead, and will
fail if you try to connect to non-verifiable hosts.  This is
controlled by the 'tls-program' variable.

URL

The URL package accepts now the protocols "ssh", "scp" and "rsync".
When 'url-handler-mode' is enabled, file operations for these
protocols as well as for "telnet" and "ftp" are passed to Tramp.
The URL package allows customizing the 'url-user-agent' string.
The new 'url-user-agent' variable can be customized to be a string or
a function.
The new interface variable 'url-request-noninteractive' can be used
to specify that we're running in a noninteractive context, and that
we should not be queried about things like TLS certificate validity.
'url-mime-accept-string' can now be used as in "interface"
variable, meaning you can bind it around an 'url-retrieve' call.
If URL is used with a https connection, the first callback argument
PLIST will contain a :peer element that has the output of
'gnutls-peer-status' (if Emacs is built with GnuTLS support).

Tramp

New connection method "afp", which allows you to access Mac OS X
volumes via the Apple Filing Protocol.
New connection method "nc", which allows you to access dumb
busyboxes.
Method-specific parameters can be overwritten now with variable
'tramp-connection-properties'.
Handler for 'file-notify-valid-p' for remote machines that support
filesystem notifications.

Wow. This is great news as auto-revert-mode and auto-revert-tail-mode uses inotify (or similar) to provide that for your local environment. Now TRAMP will use it, too, for remote ones. Great news if you want to “watch” log files remotely in Emacs.

SQL mode

New user variable 'sql-default-directory' enables remote
connections using Tramp.

Another major win for TRAMP. It still astounds me just how much stuff you can do seamlessly with it.

New command 'sql-send-line-and-next'.
This command, bound to 'C-c C-n' by default, sends the current line to
the SQL process and advances to the next line, skipping whitespace and
comments.
Added support for Vertica SQL.
Basic push support, via 'vc-push', bound to 'C-x v P'.
Implemented for Bzr, Git, Hg.  As part of this change, the pre-existing
(undocumented) command 'vc-hg-push' now behaves slightly differently.

I am glad VC is catching up as it’s fallen a bit behind now that DVCSes are so popular

The new command 'vc-region-history' shows the log+diff of the active region.

That is a very cool feature. I still use C-x v l to show the file log even though I mostly use Git (and therefore Magit).

You can refresh the VC state of a file buffer with 'M-x vc-refresh-state'.
This command is useful when you perform version control commands
outside Emacs (e.g., from the shell prompt), or if you switch the VC
back-end for the buffer's file, or remove it from version control.
New option 'vc-annotate-background-mode' controls whether
the color range from 'vc-annotate-color-map' is applied to the
background or to the foreground.
New options for customizing encoding of Git commit log messages.
The new user options 'vc-git-commits-coding-system' and
'vc-git-log-output-coding-system' specify the encoding of log messages
sent to Git when committing, and the decoding of log messages read
from Git history commands.  These options default to UTF-8; if
customized, they should be consistent with the Git config variables
i18n.commitEncoding and i18n.logOutputEncoding.
('vc-git-commits-coding-system' existed previously, but was a
variable, not a user option.)
'compare-windows' now compares text with the most recently selected window
instead of the next window.  If you want the previous behavior of
comparing with the next window, customize the new option
'compare-windows-get-window-function' to the value
'compare-windows-get-next-window'.
Two new faces 'compare-windows-removed' and 'compare-windows-added'
replace the face 'compare-windows', which is now an obsolete alias for
'compare-windows-added'.
The VC state indicator in the mode line now has different faces
corresponding to each of the possible states.  See the 'vc-faces'
customization group.
'log-edit-insert-changelog' converts "(tiny change)" to
"Copyright-paperwork-exempt: yes".  Set 'log-edit-rewrite-tiny-change'
nil to disable this.
vc-mcvs.el has been removed.
VHDL mode now supports VHDL'08.

Calculator

Decimal display mode uses "," groups, so it's more
fitting for use in money calculations
Factorial works with non-integer inputs.

Hide-IfDef mode

Hide-IfDef mode now support full C/C++ expressions in macros,
macro argument expansion, interactive macro evaluation and automatic
scanning of #define'd symbols.

This sounds really, really neat and advanced, but – I don’t write C any more. I am hoping someone will tell me if this is useful to them or not. It certainly sounds cool.

New command 'hif-evaluate-macro', bound to 'C-c @ e', displays the
result of evaluating a macro.
New command 'hif-clear-all-ifdef-define', bound to 'C-c @ C', clears
all defined symbols in 'hide-ifdef-env'.
New custom variable 'hide-ifdef-header-regexp' to define C/C++ header
file name patterns.  Defaults to files whose extension is one of '.h',
'.hh', '.hpp', '.hxx', or '.h++', matched case-insensitively.
New custom variable 'hide-ifdef-expand-reinclusion-protection' to prevent
reinclusion protected (a.k.a. "idempotent") header files from being hidden.
(This could happen when an idempotent header file is visited again,
when its guard symbol is already defined.)  Defaults to t.
New custom variable 'hide-ifdef-exclude-define-regexp' to define symbol
name patterns (e.g. all "FOR_DOXYGEN_ONLY_*") to be ignored when
looking for macro definitions.  By default, no symbols are ignored.

TeX mode

New custom variable 'tex-print-file-extension' to help users who
use PDF instead of DVI.
TeX mode now supports Prettify Symbols mode.  When enabling
'prettify-symbols-mode' in a tex-mode buffer, \alpha ... \omega, and
many other math macros are displayed using unicode characters.

Glad to see prettify-symbols-mode extended to support (La)TeX and its extensive commands.

New 'big-indent' style in 'whitespace-mode' highlights deep indentation.
By default, 32 consecutive spaces or four consecutive TABs are
considered to be too deep, but the new variable
'whitespace-big-indent-regexp' can be customized to change that.
New options in 'tildify-mode'.
New options 'tildify-space-string', 'tildify-pattern', and
'tildify-foreach-region-function' variables make
'tildify-string-alist', 'tildify-pattern-alist', and
'tildify-ignored-environments-alist' variables (as well as a few
helper functions) obsolete.

Another mode rears its head that I have never used. And this one’s been around since 1997! It automagically inserts &nbsp;, etc. where needed to when you press space, given certain patterns.

New package Xref replaces Etags's front-end and UI.

The new package Xref provides a generic framework and new commands to
find and move to definitions of functions, macros, data structures
etc., as well as go back to the location where you were before moving
to a definition.  It supersedes and obsoletes many Etags commands,
while still using the etags.el code that reads the TAGS tables as one
of its back-ends.

The command 'xref-find-definitions' replaces 'find-tag' and provides
an interface to pick one definition among several.
'tags-loop-continue' is now unbound.  'xref-pop-marker-stack' replaces
'pop-tag-mark', but has a keybinding ('M-,') different from the one
'pop-tag-mark' used.

'xref-find-definitions-other-window' replaces 'find-tag-other-window'.
'xref-find-definitions-other-frame' replaces 'find-tag-other-frame'.
'xref-find-apropos' replaces 'find-tag-regexp'.

As a result of this, the following commands are now obsolete:
'find-tag-other-window', 'find-tag-other-frame', 'find-tag-regexp',
'tags-apropos'.

'tags-loop-continue' is not obsolete because it's still useful in
'tags-search' and 'tags-query-replace', for which there are no direct
replacements yet.

This is a huge change and a great one. Emacs has a problem of too many back-ends and front-ends competing for attention so hopefully this will serve as a unified platform for indexing and jumping to things.

Variants of 'tags-search' and 'tags-query-replace' in Dired were also
replaced by xref-style commands, see the "Dired" section below.

New variables

'find-tag-marker-ring-length' is now an obsolete alias for
'xref-marker-ring-length'.  'find-tag-marker-ring' is now an obsolete
alias for a private variable.  'xref-push-marker-stack' and
'xref-pop-marker-stack' should be used instead to manipulate the stack
of searches for definitions.
'xref-find-definitions' and 'describe-function' now display
information about mode local overrides (defined by cedet/mode-local.el
'define-overloadable-function' 'define-mode-local-overrides').

framework's Lisp API is still experimental and can change in major,
backward-incompatible ways.

New package Project

The new package Project provides generic infrastructure for dealing
with projects.  The main commands included in it are
'project-find-file' and 'project-find-regexp'.

The Lisp API of this package is still experimental.

Twenty years too late? Emacs has historically fared poorly with any project that is not a singular file; see for instance M-x compile, et al. that insist you save all unsaved buffers in case one of them is part of the compile command you are running.

Still, this is positive news, even in the face of very popular third-party project management packages. I hope the community will come together to improve the default version so package authors can themselves depend on this feature and build their own packages with it in mind.

EUDC

Editor’s Note: EUDC is a unified interface for querying directory servers like LDAP, etc.

EUDC's LDAP backend has been improved.
EUDC supports LDAP-over-SSL URLs (ldaps://).
EUDC passes LDAP passwords through a pipe to the ldapsearch
subprocess instead of on the command line.
EUDC handles LDAP wildcards automatically so the user shouldn't
need to configure this manually anymore.
The LDAP configuration section of EUDC's manual has been
rewritten.
New custom variable 'eudc-server-hotlist' to allow specifying
multiple EUDC servers in init file.
Custom variable 'eudc-inline-query-format' defaults to completing
on email and firstname instead of surname.
Custom variable 'eudc-expansion-overwrites-query' defaults to nil
to avoid interfering with the kill ring.
Custom variable 'eudc-inline-expansion-format' defaults to
"Firstname Surname <mail-address>".
Custom variable 'eudc-options-file' defaults to
"~/.emacs.d/eudc-options".
New custom variable 'ldap-ldapsearch-password-prompt-regexp' to
allow overriding the regular expression that recognizes the ldapsearch
command line's password prompt.
EUDC's BBDB backend now supports BBDB 3.
EUDC's PH backend (eudcb-ph.el) is obsolete.

Eshell

The new built-in command 'clear' can scroll window contents out of sight.
If provided with an optional non-nil argument, the scrollback contents
will be cleared.

I have a hacky version that I use for shell that does something similar. Glad to see it make it in.

New buffer syntax '#<buffer-name>', which is equivalent to
'#<buffer buffer-name>'.  This shorthand makes interacting with
buffers from eshell more convenient.  Custom variable
'eshell-buffer-shorthand', which has been broken for a while, has been
removed.

Excellent change.

By default, eshell "visual" program buffers (created by
'eshell-visual-commands' and similar custom vars) are no longer killed
when their processes die.  This fixes issues with short-lived commands
and makes visual programs more useful in general.  For example, if
"git log" is a visual command, it will always show the visual command
buffer, even if the "git log" process dies.  For the old behavior,
make the new option 'eshell-destroy-buffer-when-process-dies' non-nil.

Very self-explanatory and a useful improvement all around.

Browse-url

Support for the Google Chrome web browser.
Support for the Conkeror web browser.
Support for several ancient browsers is now officially obsolete.
'tar-mode': new 'tar-new-entry' command, allowing for new members to
be added to the archive.

Autorevert

Dired buffers are also auto-reverted via file notifications, if
Emacs is compiled with file notification support.

Super! I am a deft hand at typing g (refresh) when I switch into a buffer even though Emacs has had the capability of forcefully doing this for a long time. Using file notification is much better though.

'auto-revert-use-notify' is set to nil in 'global-auto-revert-mode'.
See <http://debbugs.gnu.org/22814>.

File Notifications

The kqueue library is integrated for *BSD and Mac OS X machines.
The new event 'stopped' signals, that a file notification watch is
not active any longer.
The new function 'file-notify-valid-p' checks, whether a file
notification descriptor still corresponds to an activate watch.

Dired

The command 'dired-do-compress', bound to 'Z', now can compress
directories and decompress zip files.

Oh good. That command never worked as anyone expected it to. I get burnt by it frequently and I end up with a hundred files all gzipped individually.

New command 'dired-do-compress-to', bound to 'c', can be used to
compress many marked files into a single named archive.  The
compression command is determined from the new
'dired-compress-files-alist' variable.

Another lifesaver.

New user interface for the 'A' and 'Q' commands.
These keys, now bound to 'dired-do-find-regexp' and
'dired-do-find-regexp-and-replace', work similarly to 'xref-find-apropos'
and 'xref-query-replace-in-results': they present the matches
in the '*xref*' buffer and let you move through the matches.  No need
to use 'tags-loop-continue' to resume the search or replace loop.  The
previous commands, 'dired-do-search' and
'dired-do-query-replace-regexp', are still available, but not bound to
keys; rebind 'A' and 'Q' to invoke them if you want the old behavior
back.  We intend to obsolete the old commands in a future release.

I use Q a lot for bulk replace-regexp but this is a great move towards unifying the many disparate ways of doing the same thing in Emacs.

Tabulated List Mode

It is now safe for a mode that derives 'tabulated-list-mode' to not
call 'tabulated-list-init-header', in which case it will have no
header.
'tabulated-list-print' takes a second optional argument, UPDATE,
which specifies an alternative printing method which is faster when
few or no entries have changed.

Obsolete packages

gulp.el
landmark.el (moved to elpa.gnu.org)

New Modes and Packages in Emacs 25.1

pinentry.el allows GnuPG passphrase to be prompted through the
minibuffer instead of a graphical dialog, depending on whether the gpg
command is called from Emacs (i.e., INSIDE_EMACS environment variable
is set).  This feature requires newer versions of GnuPG (2.1.5 or
later) and Pinentry (0.9.5 or later).  To use this feature, add
"allow-emacs-pinentry" to "~/.gnupg/gpg-agent.conf" and reload the
configuration with "gpgconf --reload gpg-agent".

I wrote about this annoying dialog in a recent article about Keeping Secrets in Emacs with GnuPG and Auth Sources. With formal support – but only for GPG 2.x, sadly – this is still a welcome feature.

cl-generic.el provides CLOS-style multiple-dispatch generic functions.
The main entry points are 'cl-defgeneric' and 'cl-defmethod'.  See the
node "Generic Functions" in the Emacs Lisp manual for more details.
'scss-mode' (a minor variant of 'css-mode') is a major mode for editing
SCSS (Sassy CSS) files.
'let-alist' is a new macro (and a package) that allows one to easily
let-bind the values stored in an alist.
'tildify-mode' allows automatic insertion of hard spaces as one
types the text.  Breaking line after a single-character words is
forbidden by Czech and Polish typography (and may be discouraged in
other languages), so 'auto-tildify-mode' makes it easier to create
a typographically-correct documents.
The 'seq' library adds sequence manipulation functions and macros
that complement basic functions provided by subr.el.  All functions
are prefixed with 'seq-' and work on lists, strings and vectors.
'pcase' accepts a new Upattern 'seq'.

Good. Emacs is full of useful stuff but it does lack a lot of primitives that operate on lists, et al.

The 'map' library provides map-manipulation functions that work on
alists, hash-table and arrays.  All functions are prefixed with
'map-'.  'pcase' accepts a new UPattern 'map'.

Again. Also very welcome.

The 'thunk' library provides functions and macros to control the
evaluation of forms.
'js-jsx-mode' (a minor variant of 'js-mode') provides indentation
support for JSX, an XML-like syntax extension to ECMAScript.

Incompatible Lisp Changes in Emacs 25.1

'setq' and 'setf' must now be called with an even number of
arguments.  The earlier behavior of silently supplying a nil to the
last variable when there was an odd number of arguments has been
eliminated.
'syntax-begin-function' is declared obsolete.
Removed 'font-lock-beginning-of-syntax-function' and the SYNTAX-BEGIN
slot in 'font-lock-defaults'.
The new implementation of Subword mode affects word movement everywhere.
When Subword mode is turned on, 'forward-word', 'backward-word', and
everything that uses them will move by sub-words, effectively
overriding the buffer's syntax table.  Lisp programs that shouldn't be
affected by Subword mode should call the new functions
'forward-word-strictly' and 'backward-word-strictly' instead.
'package-initialize' now sets 'package-enable-at-startup' to nil if
called during startup.  Users who call this function in their init
file and still expect it to be run after startup should set
'package-enable-at-startup' to t after the call to
'package-initialize'.
':global' minor mode use 'setq-default' rather than 'setq'.
This means that you can't use 'make-local-variable' and expect them to
"magically" become buffer-local.
'track-mouse' no longer freezes the shape of the mouse pointer.
The 'track-mouse' form no longer refrains from changing the shape of
the mouse pointer for the entire time the body of that form is
executed.  Lisp programs that use 'track-mouse' for dragging across
large portions of the Emacs display, and want to avoid changes in the
pointer shape during dragging, should bind the variable 'track-mouse'
to the special value 'dragging' in the body of the form.
The optional PREDICATE argument of 'lisp-complete-symbol' no longer
has any effect.  (This change was made in Emacs 24.4 but was not
advertised at the time.)
'indirect-function' does not signal 'void-function' any more.
This is mostly a bug-fix, since this change was missed back in 24.4 when
'symbol-function' was changed not to signal 'void-function' any more.
As a consequence, the second arg of 'indirect-function' is now obsolete.
'M-x shell' and 'M-x compile' no longer set the EMACS environment variable.
This avoids clashing when other programs use the variable for other purposes.
Although 'M-x term' still sets EMACS for compatibility with Bash 4.3
and earlier, this is deprecated and will be phased out when Bash 4.4
or later takes over.  Use the INSIDE_EMACS environment variable instead.
'save-excursion' does not save&restore the mark any more.
Use 'save-mark-and-excursion' if you want the old behavior.
'read-buffer' and 'read-buffer-function' can now be called with a 4th
argument (PREDICATE).
'completion-table-dynamic' by default stays in the minibuffer.
The minibuffer will be the current buffer when the function is called.
If you want the old behavior of calling the function in the buffer
from which the minibuffer was entered, use the new argument
SWITCH-BUFFER to 'completion-table-dynamic'.
window-configurations no longer record the buffers' marks.
'inhibit-modification-hooks' now also inhibits lock-file checks, as
well as active region handling.
'deactivate-mark' is now buffer-local.
'cl-the' now asserts that its argument is of the given type.
'process-running-child-p' may now return a numeric process
group ID instead of t.
Mouse click events on mode line or header line no longer include
any reference to a buffer position.  The 6th member of the mouse
position list returned for such events is now nil.
Menu items in keymaps do not support the "key shortcut cache" any more.
These slots used to hold key-shortcut data, but have been obsolete since
Emacs 21.
Emacs no longer downcases the first letter of a system diagnostic
when signaling a file error.  For example, it now reports "Permission
denied" instead of "permission denied".  The old behavior was problematic
in languages like German where downcasing rules depend on grammar.
New variable 'text-quoting-style' to control how Emacs translates quotes.
Set it to 'curve' for curved single quotes, to 'straight' for straight
apostrophes, and to 'grave' for grave accent and apostrophe.  The
default value nil acts like 'curve' if curved single quotes are
displayable, and like 'grave' otherwise.  The new variable affects
display of diagnostics and help, but not of info.  As the variable is
not intended for casual use, it is not a user option.
Message-issuing functions like 'message' and 'error' now translate
various sorts of single quotes in their format strings according to
the value of 'text-quoting-style' (see above).  This translation
cannot be disabled.  To get the old behavior, use 'format', which is
not affected by 'text-quoting-style', e.g., (message "%s" (format
"...." foo bar)).
'substitute-command-keys' now replaces quotes.
That is, it converts documentation strings' quoting style as per the
value of 'text-quoting-style'.  Doc strings in source code can use
either curved single quotes or grave accents and apostrophes.  As
before, characters preceded by \= are output as-is.
The character classes [:alpha:] and [:alnum:] in regular expressions
now match multibyte characters using Unicode character properties.
If you want the old behavior where they matched any character with
word syntax, use '\sw' instead.

This is arguably more correct but if you depend on the old behavior you may want to alter your regular expressions!

The character classes [:graph:] and [:print:] in regular expressions
no longer match every multibyte character.  Instead, Emacs now
consults the Unicode character properties to determine which
characters are graphic or printable.  In particular, surrogates and
unassigned codepoints are now rejected.  If you want the old behavior,
use [:multibyte:] instead.
The 'diff' command uses the unified format now.  To restore the old
behavior, set 'diff-switches' to '-c'.
'grep-template' and 'grep-find-template' values don't include the

–color argument anymore. It’s added at the <C> place holder position dynamically. Any third-party code that changes these templates should be updated accordingly.

'(/ N)' is now equivalent to '(/ 1 N)' rather than to '(/ N 1)'.
The new behavior is compatible with Common Lisp and with XEmacs.
This change does not affect Lisp code intended to be portable to
Emacs 24.2 and earlier, which did not support unary '/'.
The 'default-directory' value doesn't have to end slash.  To make
that happen, 'unhandled-file-name-directory' now defaults to calling
'file-name-as-directory'.
The URL package now insists on sending only unibyte strings to server
This means packages that use URL cannot bind 'url-request-data' to
multibyte strings.  If non-ASCII characters should be part of the URL
payload, then 'url-request-data' should be encoded to become a unibyte
string.

Lisp Changes in Emacs 25.1

pcase

New UPatterns 'quote', 'app'.
New UPatterns can be defined with 'pcase-defmacro'.
New vector QPattern.
'syntax-propertize' is now automatically called on-demand during forward
parsing functions like 'forward-sexp'.
New hooks 'prefix-command-echo-keystrokes-functions' and
'prefix-command-preserve-state-hook' allow the definition of prefix
commands other than the predefined 'C-u'.
New functions 'filepos-to-bufferpos' and 'bufferpos-to-filepos'.
These allow conversion between buffer positions and the corresponding
file byte offsets, given the file's encoding.
The default value of 'load-read-function' is now 'read'.
Previously, the default value of nil implied using 'read'.
New hook 'pre-redisplay-functions'.
It is a bit easier to use than 'pre-redisplay-function'.
The second arg of 'looking-back' should always be provided explicitly.
Previously, it was an optional argument, now it's mandatory.
Text properties 'intangible', 'point-entered', and 'point-left' are obsolete.
Replaced by properties 'cursor-intangible' and 'cursor-sensor-functions',
implemented by the new 'cursor-intangible-mode' and
'cursor-sensor-mode' minor modes.
'inhibit-point-motion-hooks' now defaults to t and is obsolete.
Use the new minor modes 'cursor-intangible-mode' and
'cursor-sensor-mode' instead.
New process type 'pipe', which can be used in combination with the
':stderr' keyword of 'make-process' to handle standard error output
of subprocess.
New function 'make-process' provides an alternative interface to
'start-process'.  It allows programs to set process parameters such as
process filter, sentinel, etc., through keyword arguments (similar to
'make-network-process').
Subprocesses are automatically told about changes in window dimensions.
The new option 'window-adjust-process-window-size-function' controls
how subprocesses are told to adapt their logical window sizes to
changes in the Emacs window configuration.  Its default value calls
'set-process-window-size' with the smallest dimensions of all the
windows that display the subprocess's buffer.
A new function 'directory-files-recursively' returns all matching
files (recursively) under a directory.
New variable 'inhibit-message', when bound to non-nil, inhibits
'message' and related functions from displaying messages in the echo
area.  The output is still logged to the '*Messages*' buffer.
A new text property 'inhibit-read-only' can be used in read-only
buffers to allow certain parts of the text to be writable.
A new variable 'comment-end-can-be-escaped' is useful in languages
such as C and C++ where line comments with escaped newlines are
continued to the next line.
New macro 'define-advice'.
Emacs Lisp now supports generators.
See the "Generators" section of the ELisp manual for the details.
New finalizer facility for running code when objects become unreachable.
See the "Finalizer Type" subsection in the ELisp manual for the
details.
Lexical closures can use '(:documentation FORM)' to build their docstring.
It should be placed right where the docstring would be, and FORM is then
evaluated (and should return a string) when the closure is built.
'define-inline' provides a new way to define inlinable functions.
New function 'macroexpand-1' to perform a single step of macro expansion.
::

Some “x-*” functions were obsoleted and/or renamed:

‘x-select-text’ is renamed ‘gui-select-text’.

‘x-selection-value’ is renamed ‘gui-selection-value’.

‘x-get-selection’ is renamed ‘gui-get-selection’.

‘x-get-clipboard’ and ‘x-clipboard-yank’ are marked obsolete.

‘x-get-selection-value’ is renamed to ‘gui-get-primary-selection’.

‘x-set-selection’ is renamed to ‘gui-set-selection’.

New function 'string-greaterp', which return the opposite result of
'string-lessp'.
The new functions 'string-collate-lessp' and 'string-collate-equalp'
preserve the collation order as defined by the system's locale(1)
environment.  For the time being this is implemented for modern POSIX
systems and for MS-Windows, for other systems they fall back to their
counterparts 'string-lessp' and 'string-equal'.
The ls-lisp package uses 'string-collate-lessp' to sort file names.
The effect is that, on systems that use ls-lisp for Dired, the default
sort order of the files in Dired is now different from what it was in
previous versions of Emacs.  In particular, the file names are sorted
disregarding punctuation, accents, and diacritics, and letter case is
ignored.  For example, files whose name begin with a period will no
longer appear near the beginning of the directory listing.  If you
want the old, locale-independent sorting, customize the new option
'ls-lisp-use-string-collate' to the nil value.
The MS-Windows specific variable 'w32-collate-ignore-punctuation',
if set to a non-nil value, causes the above 2 functions to ignore
symbol and punctuation characters when collating strings.  This
emulates the behavior of modern Posix platforms when the locale's
codeset is "UTF-8" (as in "en_US.UTF-8").  This is needed because
MS-Windows doesn't support UTF-8 as codeset in its locales.
New function 'alist-get', which is a generalized variable
suitable for use with 'setf'.
New function 'funcall-interactively', which works like 'funcall'
but makes 'called-interactively-p' treat the function as (you guessed it)
called interactively.
New function 'function-put' to use instead of 'put' for function properties.
The new function 'bidi-find-overridden-directionality' allows you to
find characters whose directionality was, perhaps maliciously,
overridden by directional override control characters.  Lisp programs
can use this to detect potential phishing of URLs and other links that
exploits bidirectional display reordering.
The new function 'buffer-substring-with-bidi-context' allows you to
copy a portion of a buffer into a different location while preserving
the visual appearance both of the copied text and the text at
destination, even when the copied text includes mixed bidirectional
text and directional control characters.
New properties that can be specified with 'declare':

'(interactive-only INSTEAD)', says to use INSTEAD for non-interactive use.
'(pure VAL)', if VAL is non-nil, indicates the function is pure.
'(side-effect-free VAL)', if VAL is non-nil, indicates the function does not
have side effects.
New macro 'with-file-modes', for evaluating expressions with default file
permissions set to temporary values (e.g., for creating private files).
You can access the slots of structures using 'cl-struct-slot-value'.
Function 'sort' can deal with vectors.
Function 'system-name' now returns an updated value if the current
system's name has changed or if the Emacs process has changed systems.
To avoid long waits it no longer consults DNS to canonicalize the name
(in some cases this may affect generated message-id headers - customize
'message-user-fqdn' if this bothers you). The variable 'system-name'
is now obsolete.
Function 'write-region' no longer outputs "Wrote FILE" in batch mode.
If 'pwd' is called with a prefix argument, insert the current default
directory at point.
New functions return extended information about fonts and faces.
The function 'font-info' now returns more details about a font.
In particular, it now returns the average width of the font's
characters, which can be used for geometry-related calculations.
A new function 'default-font-width' returns the average width of a
character in the current buffer's default font.  If the default face
is remapped (see 'face-remapping-alist'), the value for the remapped
face is returned.  This function complements the existing function
'default-font-height'.
New functions 'window-font-height' and 'window-font-width' return
the height and average width of characters in a specified face and
window.  If FACE is remapped (see 'face-remapping-alist'), the
function returns the information for the remapped face.
A new function 'window-max-chars-per-line' returns the maximal
number of characters that can be displayed on one line.  If a face
and/or window are provided, these values are used for the
calculation.  This function is different from 'window-body-width' in
that it accounts for (i) continuation glyphs, (ii) the size of the
font, and (iii) the specified window.

New utilities in subr-x.el:

New macros 'if-let' and 'when-let' allow defining bindings and to
execute code depending whether all values are true.
New macros 'thread-first' and 'thread-last' allow threading a form
as the first or last argument of subsequent forms.
Documentation strings now support quoting with curved single quotes
in addition to the old style with grave accent and apostrophe.  The
new style looks better on today's displays.  In the new Electric Quote
mode, you can enter curved single quotes into documentation by typing
grave accent and apostrophe.  Outside Electric Quote mode, you can
enter them by typing 'C-x 8 [' and 'C-x 8 ]', or (if your Alt key
works) by typing 'A-[' and 'A-]'.  As described above under
'text-quoting-style', the user can specify how to display doc string
quotes.
New function 'format-message' is like 'format' and also converts
curved single quotes, grave accents and apostrophes as per
'text-quoting-style'.
'show-help-function's arg is converted via 'substitute-command-keys'
before being passed to the function.  Help strings, help-echo
properties, etc. can therefore contain command key escapes and
quotation marks.
Time-related changes:
Time conversion functions now accept an optional ZONE argument
that specifies the time zone rules for conversion.  ZONE is omitted or
nil for Emacs local time, t for Universal Time, 'wall' for system wall
clock time, or a string as in the TZ environment variable.  The
affected functions are 'current-time-string', 'current-time-zone',
'decode-time', and 'format-time-string'.  The function 'encode-time',
which already accepted a simple time zone rule argument, has been
extended to accept all the new forms.
Incompatible change in the third argument of 'format-time-string'.
Previously, any non-nil argument was interpreted as specifying Universal Time.
This is no longer true; packages that want Universal Time should pass t
as the third argument.
Time-related functions now consistently accept numbers
(representing seconds since the epoch) and nil (representing the
current time) as well as the usual list-of-integer representation.
Affected functions include 'current-time-string', 'current-time-zone',
'decode-time', 'float-time', 'format-time-string', 'seconds-to-time',
'time-add', 'time-less-p', 'time-subtract', 'time-to-day-in-year',
'time-to-days', and 'time-to-seconds'.
The 'encode-time-value' and 'with-decoded-time-value' macros have
been obsoleted.
'calendar-next-time-zone-transition', 'time-add', and
'time-subtract' no longer return time values in the obsolete and
undocumented integer-pair format.  Instead, they return a list of two
integers.
New function 'set-binary-mode' allows switching a standard stream
of the Emacs process to binary I/O mode.
The new function 'directory-name-p' can be used to check whether a file
name (as returned from, for instance, 'file-name-all-completions') is
a directory file name.  It returns non-nil if the last character in
the name is a directory separator character (forward slash on GNU and
Unix systems, forward- or backslash on MS-Windows and MS-DOS).
ASCII approximations to curved quotes are put in 'standard-display-table'
if the terminal cannot display curved quotes.
Standard output and error streams now transliterate characters via
'standard-display-table', and encode output using 'locale-coding-system'.
To force a specific encoding, bind 'coding-system-for-write' to the
coding-system of your choice when invoking functions like 'prin1' and
'message'.
New var 'truncate-string-ellipsis' to choose how to indicate truncation.
New possible value for 'system-type': 'nacl'.
This is used by Google's Native Client (NaCl).
Miscellaneous name change

For consistency with the usual Emacs spelling, the Lisp variable
'hfy-optimisations' has been renamed to 'hfy-optimizations'.
The old name should still work, as an obsolescent alias.
Changes in Frame- and Window- Handling
Emacs can now draw horizontal scroll bars on some platforms that
provide toolkit scroll bars, namely Gtk+, Lucid, Motif and Windows.
Horizontal scroll bars are turned off by default.
New function 'horizontal-scroll-bars-available-p' telling whether
horizontal scroll bars are available on the underlying system.
New mode 'horizontal-scroll-bar-mode' to toggle horizontal scroll
bars on all existing and future frames.
New function 'toggle-horizontal-scroll-bar' to toggle horizontal
scroll bars on the selected frame.
New frame parameters 'horizontal-scroll-bars' and
'scroll-bar-height' to set horizontal scroll bars and their height
for individual frames and in 'default-frame-alist'.
The 'horizontal-scroll-bars' parameter was already present and non-nil
by default in Emacs 24 and before (although it didn't have any
effect).  This could cause a problem if you share your desktop files
with older versions of Emacs: saving desktop in Emacs before v25.1,
then restoring it in v25.1 would turn on horizontal scroll bars in all
buffers.  To resolve this issue, put this in your ~/.emacs init file:

(modify-all-frames-parameters '((horizontal-scroll-bars . nil)))
New functions 'frame-scroll-bar-height' and
'window-scroll-bar-height' return the height of horizontal scroll
bars on a specific frame or window.
'set-window-scroll-bars' now accepts five parameters where the last
two specify height and type of the window's horizontal scroll bar.
'window-scroll-bars' now returns type and sizes of horizontal scroll
bars too.
New buffer-local variables 'horizontal-scroll-bar' and
'scroll-bar-height'.
New functions 'frame-geometry' and 'frame-edges' give access to a
frame's geometry.
New functions 'mouse-absolute-pixel-position' and
'set-mouse-absolute-pixel-position' get/set screen coordinates of the
mouse cursor.
The function 'window-edges' now accepts three additional arguments to
retrieve body, absolute and pixel edges of the window.
The functions 'window-inside-edges', 'window-inside-pixel-edges' and
'window-inside-absolute-pixel-edges' have been renamed to respectively
'window-body-edges', 'window-body-pixel-edges' and
'window-absolute-body-pixel-edges'.  The old names are kept as aliases.
New function 'window-absolute-pixel-position' to get the screen
coordinates of a visible buffer position.
The height of a frame's menu and tool bar are no longer counted in the
frame's text height.  This means that the text height stands only for
the height of the frame's root window plus that of the echo area (if
present).  This was already the behavior for frames with external tool
and menu bars (like in the Gtk builds) but has now been extended to all
builds.
Frames now do not necessarily preserve the number of columns or lines
they display when setting default font, menu bar, fringe width, or
scroll bars.  In particular, maximized and fullscreen frames are
conceptually never resized if such settings change.  For fullheight and
fullwidth frames, the behavior may depend on the toolkit used.
New option 'frame-inhibit-implied-resize' if non-nil, means that
setting default font, menu bar, fringe width, or scroll bars of a
specific frame does not resize that frame in order to preserve the
number of columns or lines it displays.
New function 'window-preserve-size' allows you to preserve the size of
a window without "fixing" it.  It's supported by 'fit-window-to-buffer',
'temp-buffer-resize-mode' and 'display-buffer'.
New 'display-buffer' action function 'display-buffer-use-some-frame'.
This displays the buffer in an existing frame other than the current
frame, and allows the caller to specify a frame predicate to exclude
frames.
New minor mode 'window-divider-mode' and options
'window-divider-default-places', 'window-divider-default-bottom-width'
and 'window-divider-default-right-width'.
When a window is shrunk horizontally its margins are no longer removed
automatically.  Rather, Emacs refuses to split or resize windows when
this would cause margins to no longer fit into the width reserved for the
corresponding window.  An application can override this behavior for a
particular window by setting that window's 'min-margins' parameter.  As
a consequence, the application becomes fully responsible for trimming
the margin sizes of that window and any window inheriting these margins.
The window displaying the '*Completions*' buffer with minibuffer
completion candidates is now shown at the bottom of the selected
frame.  The size of that window is always as large as required to
display all the candidates, except when limited by the minimum size
of the other windows on that frame; those other windows are resized
to provide space for the '*Completions*' display.  The Emacs manual
describes how to customize 'display-buffer-alist' to get back the old
behavior, see the node "Temporary Displays" there.
Tearoff menus and detachable toolbars for Gtk+ have been removed.
Those features have been deprecated in Gtk+ for a long time.

Etags

etags no longer qualifies class members by default.

By default, 'etags' will not qualify class members for Perl and C-like
object-oriented languages with their class names and namespaces, and
will remove qualifications used explicitly in the code from the tag
names it puts in TAGS files.  This is so the etags.el back-end for
'xref-find-definitions' is more accurate and produces less false
positives.

Use --class-qualify (-Q) if you want the old default behavior of
qualifying class members in C++, Java, Objective C, and Perl.  Note
that using -Q might make some class members become "unknown" to 'M-.'
('xref-find-definitions'); if so, you can use 'C-u M-.' to specify the
qualified names by hand.
New language Ruby

Names of modules, classes, methods, functions, and constants are
tagged.  Overloaded operators are also tagged.
New language Go
Names of packages, functions, and types are tagged.
Improved support for Lua

Etags now tags functions even if the "function" keyword follows some
whitespace at line beginning.

Changes in Emacs 25.1 on Non-Free Operating Systems

MS-Windows specific Emacs build scripts are no longer in the distribution.
This includes the makefile.w32-in files in various subdirectories, and
the support files.  The file nt/configure.bat now just tells the user
to use the procedure described in nt/INSTALL, by running the Posix
'configure' script in the top-level directory.
Building Emacs for MS-Windows requires at least Windows XP
or Windows Server 2003.  The built binaries still run on all versions
of Windows starting with Windows 9X.
Emacs running on MS-Windows now supports the daemon mode.
The byte counts in etags-generated TAGS files are now the same on
MS-Windows as they are on other platforms.
On OS X, configure creates a Cocoa ("Nextstep") build by default.
Pass '--without-ns' to configure to create an X11 build, the old default.
OS X 10.5 or older is no longer supported.
OS X on PowerPC is no longer supported.
New variable 'ns-use-fullscreen-animation' controls animation for
non-native NS fullscreen.  The default is nil.  Set to t to enable
animation when entering and leaving fullscreen.  For native OSX fullscreen
this has no effect.
On the OS X Cocoa ("Nextstep") port, multicolor font (such as color
emoji) display is disabled.  This feature was accidentally added when
Emacs 24.4 included the new Core Text based font backend code that was
originally implemented for a non-mainline port.  This will be enabled
again once it is also implemented in Emacs on free operating systems.
If some symbols, such as emoji, do not display, we suggest to install
an appropriate font, such as Symbola; then they will be displayed,
albeit without the color effects.
The new function 'w32-application-type' returns the type of an
MS-Windows application given the name of its executable program file.
New variable 'w32-pipe-buffer-size'.
It can be used to tune the size of the buffer of pipes created for
communicating with subprocesses, when the program run by a subprocess
exhibits unusual buffering behavior.  Default is zero, which lets the
OS use its default size.