bug#69683: 28.2; eglot overrides imenu without fallback even when unsupported by server

[Phil Hagelberg]

When invoking M-x imenu in a buffer with eglot enabled, it uses eglot's
LSP-aware implementation of imenu. However, not all LSP servers handle
that functionality. When connected to a server that does not offer this
functionality, eglot signals an error:

  jsonrpc-error: "request id=11 failed:", (jsonrpc-error-code . -32601), 
(jsonrpc-error-message . "\"textDocument/documentSymbol\" is not in the 
request-handlers table"), (jsonrpc-error-data)

However, usually there is a perfectly good functional imenu
implementation that the user wants to invoke; they just can't do it
because eglot took over the imenu command.

Ideally eglot should only add its functionality to the imenu command
when it's connected to a server that actually supports the endpoint
needed to handle it.

You can reproduce this problem with fennel-mode and fennel-ls.

* Install fennel-mode: https://git.sr.ht/~technomancy/fennel-mode
  Cloning and running M-x eval-buffer on fennel-mode.el is sufficient.

* Install fennel-ls: https://git.sr.ht/~xerool/fennel-ls
  Clone and run `make install PREFIX=$WHATEVER', then evaluate this:
  (with-eval-after-load 'eglot
    (add-to-list 'eglot-server-programs '(fennel-mode . ("fennel-ls"))))

* Open a new file, say /tmp/scratch.fnl and insert: (fn abc [])

* Run M-x imenu, and see the error.

* Run M-x eglot-shutdown.

* Run M-x imenu, and it succeeds.

I'm sure you could reproduce the same problem in another language where
the Emacs mode supports imenu but the language server doesn't. But I
don't know what other language fits that criteria.


In GNU Emacs 28.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.37, cairo 
version 1.16.0)
 of 2023-05-14, modified by Debian built on x86-ubc-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12101007
System Description: Debian GNU/Linux 12 (bookworm)

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/libexec
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-libsystemd --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/28.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/28.2/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils
 --with-native-compilation --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/libexec
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-libsystemd --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/28.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/28.2/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils
 --with-native-compilation --with-cairo --with-x=yes
 --with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
 -ffile-prefix-map=/build/emacs-mPr7Vr/emacs-28.2+1=. -fstack-protector-strong
 -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM GTK3 ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Fennel

Minor modes in effect:
  recentf-mode: t
  eglot--managed-mode: t
  flymake-mode: t
  paredit-mode: t
  whitespace-mode: t
  idle-highlight-mode: t
  hl-line-mode: t
  company-mode: t
  shell-dirtrack-mode: t
  winner-mode: t
  display-battery-mode: t
  display-time-mode: t
  save-place-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  prettify-symbols-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
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/usr/share/emacs/site-lisp/elpa/mu4e-1.8.14/mu4e-server hides 
/usr/share/emacs/site-lisp/elpa-src/mu4e-1.8.14/mu4e-server
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/usr/share/emacs/site-lisp/elpa-src/mu4e-1.8.14/mu4e-pkg
/usr/share/emacs/site-lisp/elpa/mu4e-1.8.14/mu4e-autoloads hides 
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/usr/share/emacs/site-lisp/elpa/mu4e-1.8.14/mu4e-icalendar hides 
/usr/share/emacs/site-lisp/elpa-src/mu4e-1.8.14/mu4e-icalendar
/usr/share/emacs/site-lisp/elpa/tuareg-2.2.0/tuareg-pkg hides 
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/usr/share/emacs/site-lisp/elpa/tuareg-2.2.0/tuareg-autoloads hides 
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/usr/share/emacs/site-lisp/elpa/tuareg-2.2.0/tuareg-site-file hides 
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/usr/share/emacs/site-lisp/utop hides 
/usr/share/emacs/site-lisp/elpa-src/utop-1.11/utop
~/.emacs.d//phil/../custom hides /usr/share/emacs/28.2/lisp/custom

Features:
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Memory information:
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