path: root/doc/misc/eglot.texi

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@c %**start of header
@setfilename ../../eglot.info
@settitle Eglot: The Emacs Client for the Language Server Protocol
@include docstyle.texi
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@copying
This manual is for Eglot, the Emacs LSP client.

Copyright @copyright{} 2022--2024 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being ``A GNU Manual'',
and with the Back-Cover Texts as in (a) below.  A copy of the license
is included in the section entitled ``GNU Free Documentation License''.

(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
modify this GNU manual.''
@end quotation
@end copying

@dircategory Emacs misc features
@direntry
* Eglot: (eglot).             Language Server Protocol client for Emacs.
@end direntry

@titlepage
@sp 4
@c The title is printed in a large font.
@center @titlefont{User's Guide}
@sp 1
@center @titlefont{to}
@sp 1
@center @titlefont{Eglot: The Emacs LSP Client}
@ignore
@sp 2
@center release 1.8
@c -release-
@end ignore
@sp 3
@center Jo@~ao T@'avora & Eli Zaretskii
@c -date-

@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents

@ifnottex
@node Top
@top Eglot

@cindex LSP
@cindex language server protocol
Eglot is the Emacs client for the @dfn{Language Server Protocol}
(@acronym{LSP}).  The name ``Eglot'' is an acronym that stands for
@ifhtml
``@emph{E}macs Poly@emph{glot}''.
@end ifhtml
@ifnothtml
``Emacs polyGLOT''.
@end ifnothtml
@footnote{
A @dfn{polyglot} is a
person who is able to use several languages.
} Eglot provides infrastructure and a set of commands for enriching
the source code editing capabilities of Emacs via LSP@.  LSP is a
standardized communications protocol between source code editors (such
as Emacs) and language servers---programs external to Emacs which
analyze the source code on behalf of Emacs.  The protocol allows Emacs
to receive various source code services from the server, such as
description and location of function calls, types of variables, class
definitions, syntactic errors, etc.  This way, Emacs doesn't need to
implement the language-specific parsing and analysis capabilities in
its own code, but is still capable of providing sophisticated editing
features that rely on such capabilities, such as automatic code
completion, go-to definition of function/class, documentation of
symbol at-point, refactoring, on-the-fly diagnostics, and more.

Eglot itself is completely language-agnostic, but it can support any
programming language for which there is a language server and an Emacs
major mode.

This manual documents how to configure, use, and customize Eglot.

@insertcopying

@menu
* Quick Start::                 For the impatient.
* Eglot and LSP Servers::       How to work with language servers.
* Using Eglot::                 Important Eglot commands and variables.
* Customizing Eglot::           Eglot customization and advanced features.
* Advanced server configuration::  Fine-tune a specific language server
* Extending Eglot::             Writing Eglot extensions in Elisp
* Troubleshooting Eglot::       Troubleshooting and reporting bugs.
* GNU Free Documentation License::  The license for this manual.
* Index::
@end menu
@end ifnottex

@node Quick Start
@chapter Quick Start
@cindex quick start

This chapter provides concise instructions for setting up and using
Eglot with your programming project in common usage scenarios.  For
more detailed instructions regarding Eglot setup, @pxref{Eglot and LSP
Servers}.  @xref{Using Eglot}, for detailed description of using Eglot,
and see @ref{Customizing Eglot}, for adapting Eglot to less common use
patterns.

Here's how to start using Eglot with your programming project:

@enumerate
@item
Select and install a language server.

Eglot comes pre-configured with many popular language servers, see the
value of @code{eglot-server-programs}.  If the server(s) mentioned
there satisfy your needs for the programming language(s) with which
you want to use Eglot, you just need to make sure those servers are
installed on your system.  Alternatively, install one or more servers
of your choice and add them to the value of
@code{eglot-server-programs}, as described in @ref{Setting Up LSP
Servers}.

@item
Turn on Eglot for your project.

To start using Eglot for a project, type @kbd{M-x eglot @key{RET}} in
a buffer visiting any file that belongs to the project.  This starts
the language server configured for the programming language of that
buffer, and causes Eglot to start @dfn{managing} file-visiting buffers
related to that programming language.  This includes files that are
already visited at the time the @code{eglot} command is invoked, as
well as any files visited after this invocation.

The notion of a ``project'' used by Eglot is the same Emacs uses
(@pxref{Projects,,, emacs, GNU Emacs Manual}): in the simplest case,
the ``project'' is the single file you are editing, but it can also be
all the files in a single directory or a directory tree under some
version control system, such as Git.

There are alternate ways of starting Eglot; see @ref{Starting Eglot}
for details.

@item
Use Eglot.

Most Eglot facilities are integrated into Emacs features, such as
ElDoc, Flymake, Xref, and Imenu.  However, Eglot also provides
commands of its own, mainly to perform tasks by the language server,
such as @kbd{M-x eglot-rename} (to rename an identifier across the
entire project), @kbd{M-x eglot-format} (to reformat and reindent
code), and some others.  @xref{Eglot Commands}, for the detailed list
of Eglot commands.

@item
That's it!
@end enumerate

@node Eglot and LSP Servers
@chapter Eglot and LSP Servers

This chapter describes how to set up Eglot for your needs, and how to
start it.

@menu
* Setting Up LSP Servers::   How to configure LSP servers for your needs.
* Starting Eglot::              Ways of starting Eglot for your project.
* Shutting Down LSP Servers::
@end menu

@node Setting Up LSP Servers
@section Setting Up LSP Servers
@cindex setting up LSP server for Eglot
@cindex LSP server for Eglot, setting up
@cindex language server for Eglot

For Eglot to be useful, it must first be combined with a suitable
language server.  Usually, that means running the server program
locally as a child process of Emacs (@pxref{Processes,,, elisp, GNU
Emacs Lisp Reference Manual}) and communicating with it via the
standard input and output streams.

The language server program must be installed separately, and is not
further discussed in this manual; refer to the documentation of the
particular server(s) you want to install.

To use a language server, Eglot must know how to start it and which
programming languages each server supports.  This information is
provided by the variable @code{eglot-server-programs}.

@defvar eglot-server-programs
This variable associates major modes with names and command-line
arguments of the language server programs corresponding to the
programming language of each major mode.  It provides all the
information that Eglot needs to know about the programming language of
the source you are editing.

The value of the variable is an alist, whose elements are of the form
@w{@code{(@var{major-mode} . @var{server})}}.

The @var{major-mode} of the alist elements can be either a symbol of
an Emacs major mode or a list of the form @w{@code{(@var{mode}
:language-id @var{id})}}, with @var{mode} being a major-mode symbol
and @var{id} a string that identifies the language to the server (if
Eglot cannot by itself convert the major-mode to the language
identifier string required by the server).  In addition,
@var{major-mode} can be a list of several major modes specified in one
of the above forms -- this means a running instance of the associated
server is responsible for files of multiple major modes or languages
in the project.

The @var{server} part of the alist elements can be one of the
following:

@table @code
@item (@var{program} @var{args}@dots{})
This says to invoke @var{program} with zero or more arguments
@var{args}; the program is expected to communicate with Emacs via the
standard input and standard output streams.

@item (@var{program} @var{args}@dots{} :initializationOptions @var{options}@dots{})
@var{program} is invoked with @var{args} but @var{options} specifies
how to construct the @samp{:initializationOptions} JSON object to pass
the server on during the LSP handshake (@pxref{Advanced server
configuration}).

@item (@var{host} @var{port} @var{args}@dots{})
Here @var{host} is a string and @var{port} is a positive integer
specifying a TCP connection to a remote server.  The @var{args} are
passed to @code{open-network-stream}, e.g.@: if the connection needs
to use encryption or other non-default parameters (@pxref{Network,,,
elisp, GNU Emacs Lisp Reference Manual}).

@item (@var{program} @var{args}@dots{} :autoport @var{moreargs}@dots{})
@var{program} is started with a command line constructed from
@var{args} followed by an available server port and the rest of
arguments in @var{moreargs}; Eglot then establishes a TCP connection
with the server via that port on the local host.

@item @var{function}
This should be a function of a single argument: non-@code{nil} if the
connection was requested interactively (e.g., by the @code{eglot}
command), otherwise @code{nil}.  The function should return a value of
any of the forms described above.  This allows interaction with the
user for determining the program to start and its command-line
arguments.
@end table

@end defvar

Eglot comes with a fairly complete set of associations of major-modes
to popular language servers predefined.  If you need to add server
associations to the default list, use @code{add-to-list}.  For
example, if there is a hypothetical language server program
@command{fools} for the language @code{Foo} which is supported by an
Emacs major-mode @code{foo-mode}, you can add it to the alist like
this:

@lisp
(with-eval-after-load 'eglot
  (add-to-list 'eglot-server-programs
               '(foo-mode . ("fools" "--stdio"))))
@end lisp

This will invoke the program @command{fools} with the command-line
argument @option{--stdio} in support of editing source files for which
Emacs turns on @code{foo-mode}, and will communicate with the program
via the standard streams.  As usual with invoking programs, the
executable file @file{fools} should be in one of the directories
mentioned by the @code{exec-path} variable (@pxref{Subprocess
Creation,,, elisp, GNU Emacs Lisp Reference Manual}), for Eglot to be
able to find it.

Sometimes, multiple servers are acceptable alternatives for handling a
given major-mode.  In those cases, you may combine the helper function
@code{eglot-alternatives} with the functional form of
@code{eglot-server-programs}.

@lisp
(with-eval-after-load 'eglot
  (add-to-list 'eglot-server-programs
               `(foo-mode . ,(eglot-alternatives
                               '(("fools" "--stdio")
                                 ("phewls" "--fast"))))))
@end lisp

If you have @command{fools} and @command{phewls} installed, the
function produced by @code{eglot-alternatives} will prompt for the
server to use in @code{foo-mode} buffers.  Else it will use whichever
is available.

@node Starting Eglot
@section Starting Eglot
@cindex starting Eglot
@cindex activating Eglot for a project

@findex eglot
The most common way to start Eglot is to simply visit a source file of
a given language and use the command @kbd{M-x eglot}.  This starts the
language server suitable for the visited file's major-mode, and
attempts to connect to it.  If the connection to the language server
is successful, you will see the @code{[eglot:@var{project}]} indicator
on the mode line which reflects the server that was started.  If the
server program couldn't be started or connection to it failed, you
will see an error message; in that case, try to troubleshoot the
problem as described in @ref{Troubleshooting Eglot}.  Once a language
server was successfully started and Eglot connected to it, you can
immediately start using the Emacs features supported by Eglot, as
described in @ref{Eglot Features}.

A single Eglot session for a certain major-mode usually serves all the
buffers under that mode which visit files from the same project, so
you don't need to invoke @kbd{M-x eglot} again when you visit another
file from the same project which is edited using the same major-mode.
This is because Eglot uses the Emacs project infrastructure, as
described in @ref{Eglot and Buffers}, and this knows about files that
belong to the same project.  Thus, after starting an Eglot session for
some buffer, that session is automatically reused when visiting files
in the same project with the same major-mode.

@findex eglot-ensure
Alternatively, you could configure Eglot to start automatically for
one or more major-modes from the respective mode hooks.  Here's an
example for a hypothetical @code{foo-mode}:

@lisp
 (add-hook 'foo-mode-hook 'eglot-ensure)
@end lisp

@noindent
The function @code{eglot-ensure} will start an Eglot session for each
buffer in which @code{foo-mode} is turned on, if there isn't already
an Eglot session that handles the buffer.  Note that this variant of
starting an Eglot session is non-interactive, so it should be used
only when you are confident that Eglot can be started reliably for any
file which may be visited with the major-mode in question.

Note that it's often difficult to establish this confidence fully, so
it may be wise to use the interactive command @code{eglot} instead.
You only need to invoke it once per project, as all other files
visited within the same project will automatically be managed with no
further user intervention needed.

When Eglot connects to a language server for the first time in an
Emacs session, it runs the hook @code{eglot-connect-hook}
(@pxref{Eglot Variables}).

@node Shutting Down LSP Servers
@section Shutting Down LSP Servers
@cindex shutting down LSP server

When Eglot is turned on, it arranges for turning itself off
automatically if the language server process terminates.  Turning off
Eglot means that it shuts down the server connection, ceases its
management of all the buffers that use the server connection which was
terminated, deactivates its minor mode, and restores the original
values of the Emacs variables that Eglot changed when it was turned
on.  @xref{Eglot and Buffers}, for more details of what Eglot
management of a buffer entails.

@findex eglot-shutdown
You can also shut down a language server manually, by using the
command @kbd{M-x eglot-shutdown}.  This prompts for the server (unless
there's only one connection and it's used in the current buffer), and
then shuts it down.  By default, it also kills the server's events
buffer (@pxref{Troubleshooting Eglot}), but a prefix argument prevents
that.

Alternatively, you can customize the variable
@code{eglot-autoshutdown} to a non-@code{nil} value, in which case
Eglot will automatically shut down the language server process when
the last buffer served by that language server is killed.  The default
of this variable is @code{nil}, so that visiting another file would
automatically activate Eglot even when the project which started Eglot
with the server no longer has any buffer associated with it.  This
default allows you to start a server only once in each Emacs session.

@node Using Eglot
@chapter Using Eglot

This chapter describes in detail the features that Eglot provides and
how it does that.  It also provides reference sections for Eglot
commands and variables.

@menu
* Eglot Features::
* Eglot and Buffers::
* Eglot Commands::
* Eglot Variables::
@end menu

@node Eglot Features
@section Eglot Features
@cindex features in buffers supported by Eglot

While Eglot is enabled in a buffer, it is said to be @dfn{managing}
it, using LSP and the specific capabilities of the language server to
activate and enhance modern IDE features in Emacs.  Some of these
features are provided via other Emacs packages, and some via Eglot
directly (@pxref{Eglot Commands}).

Here's an overview of the main features that Eglot provides:

@itemize @bullet
@item
At-point documentation: when point is at or near a symbol or an
identifier, the information about the symbol/identifier, such as the
signature of a function or class method and server-generated
diagnostics, is made available via the ElDoc package
(@pxref{Programming Language Doc,,, emacs, GNU Emacs Manual}).  This
allows major modes to provide extensive help and documentation about
the program identifiers.

@item
On-the-fly diagnostic annotations, via the Flymake package
(@pxref{Top,,, flymake, GNU Flymake manual}).  Eglot's Flymake backend
replaces other Flymake backends while it is managing a buffer, and
enhances diagnostics with interactive server-suggested fixes
(so-called @dfn{code actions}, @pxref{Eglot Commands})

@item
Finding definitions and uses of identifiers, via Xref (@pxref{Xref,,,
emacs, GNU Emacs Manual}).  Eglot provides a backend for the Xref
capabilities which uses the language-server understanding of the
program source.  In particular, it eliminates the need to generate
tags tables (@pxref{Tags tables,,, emacs, GNU Emacs Manual}) for
languages which are only supported by the @code{etags} backend.

@item
Buffer navigation by name of function, class, method, etc., via Imenu
(@pxref{Imenu,,, emacs, GNU Emacs Manual}).  Eglot provides its own
variant of @code{imenu-create-index-function}, which generates the
index for the buffer based on language-server program source analysis.

@item
Enhanced completion of symbol at point by the
@code{completion-at-point} command (@pxref{Symbol Completion,,, emacs,
GNU Emacs Manual}).  This uses the language-server's parser data for
the completion candidates.

@item
Automatic reformatting of source code as you type it.  This is similar
to what the @code{eglot-format} command does (see below), but is
activated automatically as you type.

@item
If a completion package such as @code{company-mode}, a popular
third-party completion package (or any other completion package), is
installed, Eglot enhances it by providing completion candidates based
on the language-server analysis of the source code.
(@code{company-mode} can be installed from GNU ELPA.)

@item
If @code{yasnippet}, a popular third-party package for automatic
insertion of code templates (snippets), is installed, and the language
server supports snippet completion candidates, Eglot arranges for the
completion package to instantiate these snippets using
@code{yasnippet}.  (@code{yasnippet} can be installed from GNU ELPA.)

@item
If the popular third-party package @code{markdown-mode} is installed,
and the server provides at-point documentation formatted as Markdown
in addition to plain text, Eglot arranges for the ElDoc package to
enrich this text with fontifications and other nice formatting before
displaying it to the user.  This makes the documentation shown by
ElDoc look nicer on display.

@item
In addition to enabling and enhancing other features and packages,
Eglot also provides a number of user commands based on the
capabilities of language servers.  Examples include renaming symbols
with @kbd{eglot-rename} and asking to automatically correct problems
with @kbd{eglot-code-actions}.  @xref{Eglot Commands}.
@end itemize

Not all servers support the full set of LSP capabilities, but most of
them support enough to enable the basic set of features mentioned
above.

Conversely, some servers offer capabilities for which no equivalent
Emacs package exists yet, and so Eglot cannot (yet) expose these
capabilities to Emacs users.  However, @xref{Extending Eglot}.

Finally, it's worth noting that, by default, Eglot generally turns on
all features that it @emph{can} turn on.  It's possible to opt out of
some features via user options (@pxref{Customizing Eglot}) and a hook
that runs after Eglot starts managing a buffer (@pxref{Eglot and
Buffers}).

@node Eglot and Buffers
@section Buffers, Projects, and Eglot
@cindex buffers managed by Eglot
@cindex projects and Eglot

@cindex workspace
One of the main strong points of using a language server is that a
language server has a broad view of the program: it considers more
than just the single source file you are editing.  Ideally, the
language server should know about all the source files of your program
which are written in the language supported by the server.  In the
language-server parlance, the set of the source files of a program is
known as a @dfn{workspace}.  The Emacs equivalent of a workspace is a
@dfn{project} (@pxref{Projects,,, emacs, GNU Emacs Manual}).  Eglot
fully supports Emacs projects, and considers the file in whose buffer
Eglot is turned on as belonging to a project.  In the simplest case,
that file is the entire project, i.e.@: your project consists of a
single file.  But there are other more complex projects:

@itemize @bullet
@item
A single-directory project: several source files in a single common
directory.

@item
A VC project: source files in a directory hierarchy under some VCS,
e.g.@: a VCS repository (@pxref{Version Control,,, emacs, GNU Emacs
Manual}).

@item
An EDE project: source files in a directory hierarchy managed via the
Emacs Development Environment (@pxref{EDE,,, emacs, GNU Emacs
Manual}).
@end itemize

Eglot uses Emacs's project management infrastructure to figure out
which files and buffers belong to what project, so any kind of project
supported by that infrastructure is automatically supported by Eglot.

When Eglot starts a server program, it does so in the project's root
directory, which is usually the top-level directory of the project's
directory hierarchy.  This ensures the language server has the same
comprehensive view of the project's files as you do.

For example, if you visit the file @file{~/projects/fooey/lib/x.foo}
and @file{x.foo} belongs to a project rooted at
@file{~/projects/fooey} (perhaps because a @file{.git} directory
exists there), then @kbd{M-x eglot} causes the server program to start
with that root as the current working directory.  The server then will
analyze not only the file @file{lib/x.foo} you visited, but likely
also all the other @file{*.foo} files under the
@file{~/projects/fooey} directory.

In some cases, additional information specific to a given project will
need to be provided to the language server when starting it.  The
variable @code{eglot-workspace-configuration} (@pxref{Customizing
Eglot}) exists for that purpose.  It specifies the parameters and
their values to communicate to each language server which needs that.

When Eglot is active for a project, it performs several background
activities on behalf of the project and its buffers:

@itemize @bullet
@cindex mode-line indication of language server
@cindex mouse clicks on mode-line, and Eglot
@vindex eglot-menu
@item
All of the project's file-visiting buffers under the same major-mode
are served by a single language-server connection.  (If the project
uses several programming languages, there will usually be a separate
server connection for each group of files written in the same language
and using the same Emacs major-mode.)  Eglot adds the
@samp{[eglot:@var{project}]} indication to the mode line of
each such buffer, where @var{server} is the name of the server and
@var{project} identifies the project by its root directory.  Clicking
the mouse on the Eglot mode-line indication activates a menu with
server-specific items.

@item
For each buffer in which Eglot is active, it notifies the language
server that Eglot is @dfn{managing} the file visited by that buffer.
This tells the language server that the file's contents on disk may no
longer be up-to-date due to unsaved edits.  Eglot reports to the
server any changes in the text of each managed buffer, to make the
server aware of unsaved changes.  This includes your editing of the
buffer and also changes done automatically by other Emacs features and
commands.  Killing a buffer relinquishes its management by Eglot and
notifies the server that the file on disk is up-to-date.

@vindex eglot-managed-mode-hook
@vindex eglot-managed-p
@item
Eglot turns on a special minor mode in each buffer it manages.  This
minor mode ensures the server is notified about files Eglot manages,
and also arranges for other Emacs features supported by Eglot
(@pxref{Eglot Features}) to receive information from the language
server, by changing the settings of these features.  Unlike other
minor-modes, this special minor mode is not activated manually by the
user, but automatically, as the result of starting an Eglot session
for the buffer.  However, this minor mode provides a hook variable
@code{eglot-managed-mode-hook} that can be used to customize the Eglot
management of the buffer.  This hook is run both when the minor mode
is turned on and when it's turned off; use the variable
@code{eglot-managed-p} to tell if current buffer is still being
managed or not.  When Eglot stops managing the buffer, this minor mode
is turned off, and all the settings that Eglot changed are restored to
their original values.

@item
When you visit a file under the same project, whether an existing or a
new file, its buffer is automatically added to the set of buffers
managed by Eglot, and the server which supports the buffer's
major-mode is notified about that.  Thus, visiting a non-existent file
@file{/home/joe/projects/fooey/lib/y.foo} in the above example will
notify the server of the @file{*.foo} files' language that a new file
was added to the project, even before the file appears on disk.  The
special Eglot minor mode is also turned on automatically in the buffer
visiting the file.
@end itemize

@node Eglot Commands
@section Eglot Commands
@cindex commands, Eglot

This section provides a reference for the most commonly used Eglot
commands:

@ftable @code
@item M-x eglot
This command adds the current buffer and the file it visits to the
group of buffers and files managed by Eglot on behalf of a suitable
language server.  If a language server for the buffer's
@code{major-mode} (@pxref{Major Modes,,, emacs, GNU Emacs Manual}) is
not yet running, it will be started; otherwise the buffer and its file
will be added to those managed by an existing server session.

The command attempts to figure out the buffer's major mode and the
suitable language server; in case it fails, it might prompt for the
major mode to use and for the server program to start.  If invoked
with @kbd{C-u}, it always prompts for the server program, and if
invoked with @kbd{C-u C-u}, it also prompts for the major mode.

If the language server is successfully started and contacted, this
command arranges for any other buffers belonging to the same project
and using the same major mode to use the same language-server session.
That includes any buffers created by visiting files after this command
succeeds to connect to a language server.

All the Emacs features that are capable of using Eglot services
(@pxref{Eglot Features}) are automatically configured by this command
to start using the language server via Eglot.  To customize which
Emacs features will be configured to use Eglot, use the
@code{eglot-stay-out-of} option (@pxref{Customizing Eglot}).

@item M-x eglot-reconnect
This command shuts down the current connection to the language
server and immediately restarts it using the same options used
originally.  This can sometimes be useful to unclog a partially
malfunctioning server connection.

@item M-x eglot-shutdown
This command shuts down a language server.  It prompts for a language
server to shut down (unless there's only one server session, and it
manages the current buffer).  Then the command shuts down the server
and stops managing the buffers the server was used for.  Emacs
features (@pxref{Eglot Features}) that Eglot configured to work with
the language server are restored back to their original configuration.

Normally, this command kills the buffers used for communicating with
the language server, but if invoked with a prefix argument @kbd{C-u},
the command doesn't kill those buffers, allowing them to be used for
diagnostics and problem reporting (@pxref{Troubleshooting Eglot}).

@item M-x eglot-shutdown-all
This command shuts down all the language servers active in the current
Emacs session.  As with @code{eglot-shutdown}, invoking this command
with a prefix argument avoids killing the buffers used for
communications with the language servers.

@item M-x eglot-rename
This command renames the program symbol (a.k.a.@: @dfn{identifier}) at
point to another name.  It prompts for the new name of the symbol, and
then modifies all the files in the project which are managed by the
language server of the current buffer to implement the renaming.

@item M-x eglot-format
This command reformats the active region according to the
language-server rules.  If no region is active, it reformats the
entire current buffer.

@item M-x eglot-format-buffer
This command reformats the current buffer, in the same manner as
@code{eglot-format} does.

@cindex code actions
@item M-x eglot-code-actions
@itemx M-x eglot-code-action-organize-imports
@itemx M-x eglot-code-action-quickfix
@itemx M-x eglot-code-action-extract
@itemx M-x eglot-code-action-inline
@itemx M-x eglot-code-action-rewrite
These commands allow you to invoke the so-called @dfn{code actions}:
requests for the language server to provide editing commands for
correcting, refactoring or beautifying your code.  These commands may
affect more than one visited file belonging to the project.

The command @code{eglot-code-actions} asks the server if there are any
code actions for any point in the buffer or contained in the active
region.  If there are, you have the choice to execute one of them via
the minibuffer.

A common use of code actions is fixing the Flymake error diagnostics
issued by Eglot (@pxref{Top,,, flymake, GNU Flymake manual}).
Clicking on a diagnostic with @kbd{mouse-2} invokes
@code{eglot-code-actions-at-mouse} which pops up a menu of available
code actions.  The variable @code{eglot-diagnostics-map} can be used
to control the mouse binding.

Other commands execute a specific code action.  For example,
@code{eglot-code-action-organize-imports} rearranges the program's
@dfn{imports}---declarations of modules whose capabilities the program
uses.

@cindex inlay hints
@item M-x eglot-inlay-hints-mode
This command toggles LSP @dfn{inlay hints} on and off for the current
buffer.  Inlay hints are small text annotations to specific parts of
the whole buffer, not unlike diagnostics, but designed to help
readability instead of indicating problems.  For example, a C++
language server can serve hints about positional parameter names in
function calls and a variable's automatically deduced type.  Inlay
hints help the user not have to remember these things by heart.
@end ftable

The following Eglot commands are used less commonly, mostly for
diagnostic and troubleshooting purposes:

@ftable @code
@item M-x eglot-events-buffer
This command pops up the events buffer used for communication with the
language server of the current buffer.

@item M-x eglot-stderr-buffer
This command pops up the buffer with the debug info printed by the
language server to its standard error stream.

@item M-x eglot-forget-pending-continuations
Forget pending requests for the server of the current buffer.
@c FIXME: Better description of the need.

@item M-x eglot-signal-didChangeConfiguration
This command updates the language server configuration according to
the current value of the variable @code{eglot-workspace-configuration}
(@pxref{Customizing Eglot}).

@item M-x eglot-clear-status
Clear the last JSONRPC error for the server of the current buffer.
Eglot keeps track of erroneous situations encountered by the server in
its mode-line indication so that the user may inspect the
communication leading up to it (@pxref{Troubleshooting Eglot}).  If
the situation is deemed uninteresting or temporary, this command can
be used to ``forget'' the error.  Note that the command @code{M-x
eglot-reconnect} can sometimes be used to unclog a temporarily
malfunctioning server.
@end ftable

As described in @ref{Eglot Features} most features associated with
Eglot are actually provided by other Emacs packages and features, and
Eglot only enhances them by allowing them to use the information
coming from the language servers.  For completeness, here's the list
of commands of those other packages that are very commonly used in
Eglot-managed buffers:

@c Not @ftable, because the index entries should mention Eglot
@table @code
@cindex eldoc, and Eglot
@cindex documentation using Eglot
@item M-x eldoc
Ask the ElDoc system for help at point.

@cindex flymake, and Eglot
@cindex on-the-fly diagnostics using Eglot
@item M-x flymake-show-buffer-diagnostics
Ask Flymake system to display diagnostics for the current buffer.

@item M-x flymake-show-project-diagnostics
Ask Flymake to list diagnostics for all the files in the current
project.

@cindex xref, and Eglot
@cindex finding definitions of identifiers using Eglot
@item M-x xref-find-definitions
Ask Xref to go the definition of the identifier at point.

@cindex imenu navigation using Eglot
@item M-x imenu
Let the user navigate the program source code using buffer index,
categorizing program elements by syntactic class (class, method,
variable, etc.) and offering completion.

@cindex symbol completion using Eglot
@item M-x completion-at-point
Request completion of the symbol at point.
@end table

@node Eglot Variables
@section Eglot Variables
@cindex variables, Eglot

This section provides a reference for the Eglot user options.

@vtable @code
@item eglot-autoreconnect
This option controls the ability to reconnect automatically to the
language server when Eglot detects that the server process terminated
unexpectedly.  The default value @code{3} means to attempt reconnection only
if the previous successful connection lasted for more than that number
of seconds; a different positive value changes the minimal length of
the connection to trigger reconnection.  A value of @code{t} means
always reconnect automatically, and @code{nil} means never reconnect
(in which case you will need to reconnect manually using @kbd{M-x
eglot}).

@item eglot-connect-timeout
This specifies the number of seconds before connection attempt to a
language server times out.  The value of @code{nil} means never time
out.  The default is 30 seconds.

@item eglot-sync-connect
This setting is mainly important for connections which are slow to
establish.  Whereas the variable @code{eglot-connect-timeout} controls
how long to wait for, this variable controls whether to block Emacs's
user interface while waiting.  The default value is @code{3}; a positive
value means block for that many seconds, then wait for the connection
in the background.  The value of @code{t} means block during the whole
waiting period.  The value of @code{nil} or @code{0} means don't block at
all during the waiting period.

@item eglot-events-buffer-config
This configures the size and format of the Eglot events buffer.
@xref{Eglot Commands, eglot-events-buffer}, for how to access that
buffer.  If the value is changed, the connection should be restarted
using @kbd{M-x eglot-reconnect} for the new value to take effect.
@c FIXME: Shouldn't the defcustom do this by itself using the :set
@c attribute?  Maybe not because reconnecting is a complex task.
@xref{Troubleshooting Eglot}, for when this could be useful.

@item eglot-autoshutdown
If this is non-@code{nil}, Eglot shuts down a language server when the
last buffer managed by it is killed.  @xref{Shutting Down LSP Servers}.
The default is @code{nil}; if you want to shut down a server, use
@kbd{M-x eglot-shutdown} (@pxref{Eglot Commands}).

@item eglot-confirm-server-edits
Various Eglot commands and code actions result in the language server
sending editing commands to Emacs.  If this option's value is
non-@code{nil}, Eglot will ask for confirmation before performing
edits proposed by the language server.  This option's value can be
crafted to require this confirmation for specific commands or only
when the edit affects files not yet visited by the user.  Consult this
option's docstring for more information.

@item eglot-ignored-server-capabilities
This variable's value is a list of language server capabilities that
Eglot should not use.  The default is @code{nil}: Eglot uses all of
the capabilities supported by each server.

@item eglot-extend-to-xref
If this is non-@code{nil}, and @kbd{M-.}
(@code{xref-find-definitions}) lands you in a file outside of your
project, such as a system-installed library or header file,
transiently consider that file as managed by the same language server.
That file is still outside your project (i.e. @code{project-find-file}
won't find it), but Eglot and the server will consider it to be part
of the workspace.  The default is @code{nil}.

@item eglot-mode-map
This variable is the keymap for binding Eglot-related command.  It is
in effect only as long as the buffer is managed by Eglot.  By default,
it is empty, with the single exception: @kbd{C-h .} is remapped to
invoke @code{eldoc-doc-buffer}.  You can bind additional commands in
this map.  For example:

@lisp
  (define-key eglot-mode-map (kbd "C-c r") 'eglot-rename)
  (define-key eglot-mode-map (kbd "C-c o") 'eglot-code-action-organize-imports)
  (define-key eglot-mode-map (kbd "C-c h") 'eldoc)
  (define-key eglot-mode-map (kbd "<f6>") 'xref-find-definitions)
@end lisp

@end vtable

Additional variables, which are relevant for customizing the server
connections, are documented in @ref{Customizing Eglot}.

@node Customizing Eglot
@chapter Customizing Eglot
@cindex customizing Eglot

Eglot itself has a relatively small number of customization options.
A large part of customizing Eglot to your needs and preferences should
actually be done via options of the Emacs packages and features which
Eglot supports and enhances (@pxref{Eglot Features}).  For example:

@itemize @bullet
@item
To configure the face used for server-derived errors and warnings,
customize the Flymake faces @code{flymake-error} and
@code{flymake-warning}.

@item
To configure the amount of space taken up by documentation in the
echo area, customize the ElDoc variable
@code{eldoc-echo-area-use-multiline-p}.

@item
To completely change how ElDoc displays the at-point documentation
destination, customize the ElDoc variable
@code{eldoc-display-functions}.
@end itemize

For this reason, this manual describes only how to customize
Eglot's own operation, which mainly has to do with the server
connections and the server features to be used by Eglot.

@c @table, not @vtable, because some of the variables are indexed
@c elsewhere
@table @code
@item eglot-server-programs
This variable determines which language server to start for each
supported major mode, and how to invoke that server's program.
@xref{Setting Up LSP Servers}, for the details.

@vindex eglot-strict-mode
@item eglot-strict-mode
This is @code{nil} by default, meaning that Eglot is generally lenient
about non-conforming servers.  If you need to debug a server, set this
to @w{@code{(disallow-non-standard-keys enforce-required-keys)}}.

@vindex eglot-server-initialized-hook
@item eglot-server-initialized-hook
A hook run after the server object is successfully initialized.

@vindex eglot-connect-hook
@item eglot-connect-hook
A hook run after connection to the server is successfully
established.  @xref{Starting Eglot}.

@item eglot-managed-mode-hook
A hook run after Eglot started or stopped managing a buffer.
@xref{Eglot and Buffers}, for details of its usage.

@vindex eglot-stay-out-of
@item eglot-stay-out-of
This variable's value lists Emacs features that Eglot shouldn't
automatically try to manage on the user's behalf.  It is useful, for
example, when you need to use non-LSP Flymake or Company back-ends.
To have Eglot stay away from some Emacs feature, add that feature's
symbol or a regexp that will match a symbol's name to the list: for
example, the symbol @code{xref} to leave Xref alone, or the string
@samp{company} to stay away from your Company customizations.  Here's an
example:

@lisp
(add-to-list 'eglot-stay-out-of 'flymake)
@end lisp

Note that you can still configure the excluded Emacs features manually
to use Eglot in your @code{eglot-managed-mode-hook} or via some other
mechanism.

@vindex eglot-report-progress
@cindex progress
@item eglot-report-progress
Set this variable to true if you'd like progress notifications coming
from the language server to be handled as Emacs's progress reporting
facilities.
@end table

@node Advanced server configuration
@chapter Advanced server configuration

Though many language servers work well out-of-the-box, most allow
fine-grained control of their operation via specific configuration
options that are transmitted over the LSP protocol and vary from
server to server.  A small number of servers require such special
configuration to work acceptably, or even to work at all.

After having setup a server executable program in
@code{eglot-server-programs} (@pxref{Setting Up LSP Servers}) and
ensuring Eglot can invoke it, you may want to take advantage of some
of these options.  You should first distinguish two main kinds of
server configuration:

@itemize @bullet
@item
Project-specific, applying to a specific project;

@item
User-specific, applying to all projects the server is used for.
@end itemize

When you have decided which kind you need, the following sections
teach how Eglot's user variables can be used to achieve it:

@menu
* Project-specific configuration::
* User-specific configuration::
* JSONRPC objects in Elisp::
@end menu

It's important to note that not all servers allow both kinds of
configuration, nor is it guaranteed that user options can be copied
over to project options, and vice-versa.  When in doubt, consult your
language server's documentation.

It's also worth noting that some language servers can read these
settings from configuration files in the user's @code{HOME} directory
or in a project's directory.  For example, the @command{pylsp} Python
server reads the file @file{~/.config/pycodestyle} for user
configuration.  The @command{clangd} C/C++ server reads both
@file{~/.config/clangd/config.yaml} for user configuration and
@file{.clangd} for project configuration.  It may be advantageous to
use these mechanisms instead of Eglot's, as this will probably work
with other LSP clients and may be easier to debug than options riding
on the LSP wire.

@node Project-specific configuration
@section Project-specific configuration
@vindex eglot-workspace-configuration
@cindex workspace configuration

To set project-specific settings, which the LSP specification calls
@dfn{workspace configuration}, the variable
@code{eglot-workspace-configuration} may be used.

This variable is a directory-local variable (@pxref{Directory
Variables, , Per-directory Local Variables, emacs, The GNU Emacs
Manual}).  It's important to recognize that this variable really only
makes sense when set directory-locally.  It usually does not make
sense to set it file-locally or in a major-mode hook.

The most common way to set @code{eglot-workspace-configuration } is
using a @file{.dir-locals.el} file in the root of your project.  If
you can't do that, you may also set it from Elisp code via the
@code{dir-locals-set-class-variables} function.  (@pxref{Directory
Local Variables,,, elisp, GNU Emacs Lisp Reference Manual}).

However you choose to set it, the variable's value is a plist
(@pxref{Property Lists,,, elisp, GNU Emacs Lisp Reference Manual}) with
the following format:

@lisp
 (@var{:server1} @var{plist1} @var{:server2} @var{plist2} @dots{})
@end lisp

@noindent
Here, @var{:server1} and @var{:server2} are keywords whose names
identify the LSP language servers to target.  Consult server
documentation to find out what name to use.  @var{plist1} and
@var{plist2} are plists of options, possibly nesting other plists.

@findex eglot-show-workspace-configuration
When experimenting with workspace settings, you can use the command
@kbd{M-x eglot-show-workspace-configuration} to inspect and debug the
value of this variable in its final JSON form, ready to be sent to the
server (@pxref{JSONRPC objects in Elisp}).  This helper command works
even before actually connecting to the server.

These variable's value doesn't take effect immediately.  That happens
upon establishing the connection, in response to an explicit query
from the server, or when issuing the command @kbd{M-x
eglot-signal-didChangeConfiguration} which notifies the server during
an ongoing Eglot session.

@subsection Examples

For some users, setting @code{eglot-workspace-configuration} is a
somewhat daunting task.  One of the reasons is having to manage the
general Elisp syntax of per-mode directory-local variables, which uses
alists (@pxref{Association Lists,,, elisp, GNU Emacs Lisp Reference
Manual}), and the specific syntax of Eglot's variable, which uses
plists.  Some examples are useful.

Let's say you want to configure two language servers to be used in a
project written in a combination of the Python and Go languages.  You
want to use the @command{pylsp} and @command{gopls} languages
servers.  In the documentation of the servers in question (or in some
other editor's configuration file, or in some blog article), you find
the following configuration options in informal dotted-notation
syntax:

@example
pylsp.plugins.jedi_completion.include_params: true
pylsp.plugins.jedi_completion.fuzzy: true
pylsp.pylint.enabled: false
gopls.usePlaceholders: true
@end example

To apply this to Eglot, and assuming you chose the
@file{.dir-locals.el} file method, the contents of that file could be:

@lisp
((nil
  . ((eglot-workspace-configuration
      . (:pylsp (:plugins (:jedi_completion (:include_params t
                                             :fuzzy t)
                           :pylint (:enabled :json-false)))
         :gopls (:usePlaceholders t)))))
 (python-base-mode . ((indent-tabs-mode . nil)))
 (go-mode          . ((indent-tabs-mode . t))))
@end lisp

@noindent
This sets the value of @code{eglot-workspace-configuration} in all the
buffers inside the project; each server will use only the section of
the parameters intended for that server, and ignore the rest.  Note
how alists are used for associating Emacs mode names with alists
associating variable names with variable values.  Then notice how
plists are used inside the value of
@code{eglot-workspace-configuration}.

This following form may also be used:

@lisp
((python-base-mode
  . ((eglot-workspace-configuration
      . (:pylsp (:plugins (:jedi_completion (:include_params t
                                             :fuzzy t)
                           :pylint (:enabled :json-false)))))
     (indent-tabs-mode . nil)))
 (go-mode
  . ((eglot-workspace-configuration
      . (:gopls (:usePlaceholders t)))
     (indent-tabs-mode . t))))
@end lisp

@noindent
This sets up the value of @code{eglot-workspace-configuration}
separately depending on the major mode of each of that project's
buffers.  @code{python-base-mode} buffers will have the variable set to
@code{(:pylsp (:plugins ...))}.  @code{go-mode} buffers will have the
variable set to @code{(:gopls (:usePlaceholders t))}.

Some servers will issue workspace configuration for specific files
inside your project.  For example, if you know @code{gopls} is asking
about specific files in the @code{src/imported} subdirectory and you
want to set a different option for @code{gopls.usePlaceholders} , you
may use something like:

@lisp
((python-base-mode
  . ((eglot-workspace-configuration
      . (:pylsp (:plugins (:jedi_completion (:include_params t
                                             :fuzzy t)
                           :pylint (:enabled :json-false)))))
     (indent-tabs-mode nil)))
 (go-mode
  . ((eglot-workspace-configuration
      . (:gopls (:usePlaceholders t)))
     (indent-tabs-mode t)))
 ("src/imported"
   . ((eglot-workspace-configuration
      . (:gopls (:usePlaceholders nil))))))
@end lisp

Finally, if one needs to determine the workspace configuration based
on some dynamic context, @code{eglot-workspace-configuration} can be
set to a function.  The function is called with the
@code{eglot-lsp-server} instance of the connected server (if any) and
with @code{default-directory} set to the root of the project.  The
function should return a plist suitable for use as the variable's
value.

@node User-specific configuration
@section User-specific configuration
@cindex initializationOptions
@cindex command-line arguments

This kind of configuration applies to all projects the server is used
for.  Here, there are a number of ways to do this inside Eglot.

A common way is to pass command-line options to the server invocation
via @code{eglot-server-programs}.  Let's say we want to configure
where the @command{clangd} server reads its
@code{compile_commands.json} from.  This can be done like so:

@lisp
(with-eval-after-load 'eglot
  (add-to-list 'eglot-server-programs
               `(c++-mode . ("clangd" "--compile-commands-dir=/tmp"))))

@end lisp

@noindent
Another way is to have Eglot pass a JSON object to the server during
the LSP handshake.  This is done using the
@code{:initializationOptions} syntax of @code{eglot-server-programs}:

@lisp
(with-eval-after-load 'eglot
  (add-to-list 'eglot-server-programs
               `(c++-mode . ("clangd" :initializationOptions
                                      (:compilationDatabasePath "/tmp")))))
@end lisp

@noindent
The argument @code{(:compilationDatabasePath "/tmp")} is Emacs's
representation in plist format of a simple JSON object
@code{@{"compilationDatabasePath": "/tmp"@}}.  To learn how to
represent more deeply nested options in this format, @pxref{JSONRPC
objects in Elisp}.

In this case, the two examples achieve exactly the same, but notice
how the option's name has changed between them.

@vindex eglot-workspace-configuration
Finally there is another way to do user-specific configuration of
language servers, which may be used if the methods above are not
supported.  It consists of @emph{globally} setting
@code{eglot-workspace-configuration}, a variable originally intended
for project-specific configuration.  This has the same effect as
giving all your projects a certain default configuration, as described
in @ref{Project-specific configuration}.  Here is an example:

@lisp
(setq-default eglot-workspace-configuration
              '(:pylsp (:plugins (:jedi_completion (:include_params t
                                                    :fuzzy t)
                                  :pylint (:enabled :json-false)))
                :gopls (:usePlaceholders t)))
@end lisp

Note that the global value of @code{eglot-workspace-configuration} is
always overridden if a directory-local value is detected.

@node JSONRPC objects in Elisp
@section JSONRPC objects in Elisp

Emacs's preferred way of representing JSON is via Lisp lists.  In
Eglot, the syntax of this list is the simplest possible (the one with
fewer parenthesis), a plist (@pxref{Property Lists,,, elisp, GNU Emacs
Lisp Reference Manual}).

The plist may be arbitrarily complex, and generally containing other
keyword-value property sub-plists corresponding to JSON sub-objects.

For representing the JSON leaf values @code{true}, @code{false},
@code{null} and @code{@{@}}, you can use the Lisp values @code{t},
@code{:json-false}, @code{nil}, and @code{eglot-@{@}}, respectively.
JSON arrays are represented as Elisp vectors surrounded by square brackets
(@pxref{Vectors,,,elisp,GNU Emacs Lisp Reference Manual}).

For example, the plist

@lisp
(:pylsp (:plugins (:jedi_completion (:include_params t
                                     :fuzzy t
                                     :cache_for ["pandas" "numpy"])
                   :pylint (:enabled :json-false)))
 :gopls (:usePlaceholders t))
@end lisp

@noindent
is serialized by Eglot to the following JSON text:

@example
@{
  "pylsp": @{
    "plugins": @{
      "jedi_completion": @{
        "include_params": true,
        "fuzzy": true,
        "cache_for": [ "pandas", "numpy" ]
      @},
      "pylint": @{
        "enabled": false
      @}
    @}
  @},
  "gopls": @{
    "usePlaceholders": true
  @}
@}
@end example

@node Extending Eglot
@chapter Extending Eglot

Sometimes it may be useful to extend existing Eglot functionality
using Elisp its public methods.  A good example of when this need may
arise is adding support for a custom LSP protocol extension only
implemented by a specific server.

The best source of documentation for this is probably Eglot source
code itself, particularly the section marked ``API''.

Most of the functionality is implemented with Common-Lisp style
generic functions (@pxref{Generics,,,eieio,EIEIO}) that can be easily
extended or overridden.  The Eglot code itself is an example on how to
do this.

The following is a relatively simple example that adds support for the
@code{inactiveRegions} experimental feature introduced in version 17
of the @command{clangd} C/C++ language server++.

Summarily, the feature works by first having the server detect the
Eglot's advertisement of the @code{inactiveRegions} client capability
during startup, whereupon the language server will report a list of
regions of inactive code for each buffer.  This is usually code
surrounded by C/C++ @code{#ifdef} macros that the preprocessor removes
based on compile-time information.

The language server reports the regions by periodically sending a
@code{textDocument/inactiveRegions} notification for each managed
buffer (@pxref{Eglot and Buffers}). Normally, unknown server
notifications are ignored by Eglot, but we're going change that.

Both the announcement of the client capability and the handling of the
new notification is done by adding methods to generic functions.

@itemize @bullet
@item
The first method extends @code{eglot-client-capabilities} using a
simple heuristic to detect if current server is @command{clangd} and
enables the @code{inactiveRegion} capability.

@lisp
(cl-defmethod eglot-client-capabilities :around (server)
  (let ((base (cl-call-next-method)))
    (when (cl-find "clangd" (process-command
                              (jsonrpc--process server))
                   :test #'string-match)
      (setf (cl-getf (cl-getf base :textDocument)
                     :inactiveRegionsCapabilities)
            '(:inactiveRegions t)))
    base))
@end lisp

Notice we use an internal function of the @code{jsonrpc.el} library,
and a regexp search to detect @command{clangd}.  An alternative would
be to define a new EIEIO subclass of @code{eglot-lsp-server}, maybe
called @code{eglot-clangd}, so that the method would be simplified:

@lisp
(cl-defmethod eglot-client-capabilities :around ((_s eglot-clangd))
  (let ((base (cl-call-next-method)))
    (setf (cl-getf (cl-getf base :textDocument)
                     :inactiveRegionsCapabilities)
            '(:inactiveRegions t))))
@end lisp

However, this would require that users tweak
@code{eglot-server-program} to tell Eglot instantiate such sub-classes
instead of the generic @code{eglot-lsp-server} (@pxref{Setting Up LSP
Servers}). For the purposes of this particular demonstration, we're
going to use the more hacky regexp route which doesn't require that.

Note, however, that detecting server versions before announcing new
capabilities is generally not needed, as both server and client are
required by LSP to ignore unknown capabilities advertised by their
counterparts.

@item
The second method implements @code{eglot-handle-notification} to
process the server notification for the LSP method
@code{textDocument/inactiveRegions}.  For each region received it
creates an overlay applying the @code{shadow} face to the region.
Overlays are recreated every time a new notification of this kind is
received.

To learn about how @command{clangd}'s special JSONRPC notification
message is structured in detail you could consult that server's
documentation.  Another possibility is to evaluate the first
capability-announcing method, reconnect to the server and peek in the
events buffer (@pxref{Eglot Commands, eglot-events-buffer}).  You
could find something like:

@lisp
[server-notification] Mon Sep  4 01:10:04 2023:
(:jsonrpc "2.0" :method "textDocument/inactiveRegions" :params
          (:textDocument
           (:uri "file:///path/to/file.cpp")
           :regions
           [(:start (:character 0 :line 18)
             :end (:character 58 :line 19))
            (:start (:character 0 :line 36)
             :end (:character 1 :line 38))]))
@end lisp

This reveals that the @code{textDocument/inactiveRegions} notification
contains a @code{:textDocument} property to designate the managed
buffer and an array of LSP regions under the @code{:regions} property.
Notice how the message (originally in JSON format), is represented as
Elisp plists (@pxref{JSONRPC objects in Elisp}).

The Eglot generic function machinery will automatically destructure
the incoming message, so these two properties can simply be added to
the new method's lambda list as @code{&key} arguments.  Also, the
@code{eglot-uri-to-path} and @code{eglot-range-region} may be used to
easily parse the LSP @code{:uri} and @code{:start ... :end ...}
objects to obtain Emacs objects for file names and positions.

The remainder of the implementation consists of standard Elisp
techniques to loop over arrays, manage buffers and overlays.

@lisp
(defvar-local eglot-clangd-inactive-region-overlays '())

(cl-defmethod eglot-handle-notification
  (_server (_method (eql textDocument/inactiveRegions))
           &key regions textDocument &allow-other-keys)
  (if-let* ((path (expand-file-name (eglot-uri-to-path
                                     (cl-getf textDocument :uri))))
            (buffer (find-buffer-visiting path)))
      (with-current-buffer buffer
        (mapc #'delete-overlay eglot-clangd-inactive-region-overlays)
        (cl-loop
         for r across regions
         for (beg . end) = (eglot-range-region r)
         for ov = (make-overlay beg end)
         do
         (overlay-put ov 'face 'shadow)
         (push ov eglot-clangd-inactive-region-overlays)))))
@end lisp

@end itemize

After evaluating these two additions and reconnecting to the
@command{clangd} language server (version 17), the result will be that
all the inactive code in the buffer will be nicely grayed out using
the LSP server knowledge about current compile time preprocessor
defines.

@node Troubleshooting Eglot
@chapter Troubleshooting Eglot
@cindex troubleshooting Eglot

This chapter documents commands and variables that can be used to
troubleshoot Eglot problems.  It also provides guidelines for
reporting Eglot bugs in a way that facilitates their resolution.

When you encounter problems with Eglot, try first using the commands
@kbd{M-x eglot-events-buffer} and @kbd{M-x eglot-stderr-buffer}.  They
pop up special buffers that can be used to inspect the communications
between the Eglot and language server.  In many cases, this will
indicate the problems or at least provide a hint.

@menu
* Performance::
* Getting the latest version::
* Reporting bugs::
@end menu

@node Performance
@section Performance
@cindex performance problems, with Eglot
A common and easy-to-fix cause of performance problems in Eglot
(especially in older versions) is its events buffer, since it
represents additional work that Eglot must do (@pxref{Eglot Commands,
eglot-events-buffer}).  If you find Eglot is operating correctly but
slowly, try to customize the variable
@code{eglot-events-buffer-config} (@pxref{Eglot Variables}) and set
its @code{:size} property to 0.  This will disable recording any
events and may speed things up.

In other situations, the cause of poor performance lies in the
language server itself.  Servers use aggressive caching and other
techniques to improve their performance.  Often, this can be tweaked
by changing the server configuration (@pxref{Advanced server
configuration}).

@node Getting the latest version
@section Getting the latest version
@cindex upgrading Eglot

To install the latest Eglot in an Emacs version that does not bundle
Eglot, use @kbd{M-x package-install}.

Often, a newer Eglot version exists that has fixed a longstanding bug,
has more LSP features, or just better supports a particular language
server.  Recent Eglot versions can self-update via the command
@kbd{M-x eglot-upgrade-eglot}.  This will replace any currently
installed version with the newest one available from the ELPA archives
configured in @code{package-archives}.

You can also update Eglot through other methods, such as
@code{use-package} (@pxref{Installing packages,,, use-package,
use-package User Manual}), @code{package-install},
@code{list-packages} or the newer @code{package-upgrade}
(@pxref{Packages,,, emacs, GNU Emacs Manual}).  However, do read the
docstrings of the command you intend to use before you use it, as some
of them may not work in exactly the same way across Emacs versions,
meaning your configuration may be not portable.

@node Reporting bugs
@section Reporting bugs
@cindex bug reports

If you think you have found a bug, we want to hear about it.  Before
reporting a bug, keep in mind that interaction with language servers
represents a large quantity of unknown variables.  Therefore, it is
generally both @emph{difficult} and @emph{absolutely essential} that
the maintainers reproduce bugs exactly as they happened to you, the
user.

To report an Eglot bug, send e-mail to @email{bug-gnu-emacs@@gnu.org}.

To understand how to write this email, get acquainted with Emacs's bug
reporting guidelines (@pxref{Bugs,,, emacs, GNU Emacs Manual}).  Then,
follow this Eglot-specific checklist:

@enumerate
@item
Include the transcript of JSONRPC events obtained from the buffer
popped up by @kbd{M-x eglot-events-buffer}.  You may narrow down the
transcript if you are sure of where the problematic exchange is, but
it's safer to include the whole transcript, either attached or inline.

@item
If Emacs signaled an error (an error message was seen or heard), make
sure to repeat the process after turning on @code{debug-on-error} via
@kbd{M-x toggle-debug-on-error}.  This normally produces a backtrace
of the error that should also be attached to the bug report.

@item
Include a description of how the maintainer should obtain, install,
and configure the language server you used.  Maintainers usually have
access to GNU/Linux systems, though not necessarily the distribution
that you may be using.  If possible, try to replicate the problem with
the C/C@t{++} or Python servers, as these are very easy to install.

@item
Describe how to setup a @emph{minimal} project directory where Eglot
should be started for the problem to happen.  Describe each file's
name and its contents.  Alternatively, you can supply the address of a
public Git repository.

@item
Include versions of the software used.  The Emacs version can be
obtained with @kbd{M-x emacs-version}.

We welcome bug reports about all Eglot versions, but it is helpful to
first check if the problem isn't already fixed in the latest version
(@pxref{Getting the latest version}).

It's also essential to include the version of ELPA packages that are
explicitly or implicitly loaded.  The optional but popular Company or
Markdown packages are distributed as GNU ELPA packages, not to mention
Eglot itself in some situations.  Some major modes (Go, Rust, etc.)
are provided by ELPA packages.  It's sometimes easy to miss these,
since they are usually implicitly loaded when visiting a file in that
language.

ELPA packages usually live in @code{~/.emacs.d/elpa} (or what is in
@code{package-user-dir}).  Including a listing of files in that
directory is a way to tell the maintainers about ELPA package
versions.

@item
Include a recipe to replicate the problem with @emph{a clean Emacs
run}.  The invocation @code{emacs -Q -f package-initialize} starts
Emacs with no configuration and initializes the ELPA packages.  A very
minimal @file{.emacs} initialization file (10 lines or less) is also
acceptable and good means to describe changes to variables.

There is usually no need to include @code{require} statements in the
recipe, as Eglot's functionality uses autoloads.

Likewise, there is rarely the need to use things like
@code{use-package} or @code{eglot-ensure}.  This just makes the recipe
harder to follow.  Prefer setting variables with @code{setq} and
adding to hooks with @code{add-hook}.  Prefer starting Eglot with
@code{M-x eglot}.

@item
Make sure to double check all the above elements and re-run the recipe
to see that the problem is reproducible.  Following the recipe should
produce event transcript and error backtraces that are very similar to
the ones you included.  If the problem only happens sometimes, mention
this in your report.
@end enumerate

Please keep in mind that some problems reported against Eglot may
actually be bugs in the language server or the Emacs feature/package
that used Eglot to communicate with the language server.  Eglot is, in
many cases, just a frontend to that functionality.

@node GNU Free Documentation License
@appendix GNU Free Documentation License
@include doclicense.texi

@node Index
@unnumbered Index
@printindex cp

@bye