bug#37460: 27.0.50; Imenu bug when imenu-auto-rescan is enabled

[Alex Peitsinis <alexpeitsinis@gmail.com>]

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The relevant commit (here linked from github):
https://github.com/emacs-mirror/emacs/commit/8635147ccba1fc8e1010010da8c3f8e467562f76#diff-10f71b5f4495401d9de9677a0836462b

With this latest change, if `imenu-auto-rescan` is set to `t` then
calling `imenu--index-alist` will return `nil`. The fix is to change
`(unless imenu-auto-rescan (cons ...))` to
`(if imenu-auto-rescan imenu--index-alist (cons ...))` in `imenu.el`.

Reproduction steps:
- Visit a file that supports `imenu` (e.g. an elisp buffer with some
functions defined)
- C-M-; (setq imenu-auto-rescan nil)
- M-x imenu
- There should be items in the list
- C-M-; (setq imenu-auto-rescan t)
- M-x imenu
- The list should be empty


In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.18.9,
cairo version 1.14.6)
 of 2019-09-17 built on alex-Z370M-D3H
Repository revision: bc1c2cf009e30af77523fd87a8910fdbc4284704
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Ubuntu 16.04.6 LTS

Recent messages:
Mark set [3 times]
Auto-saving...
imenu--make-index-alist
Scanning declarations in Cmdline.hs... ( 89%)
Sorting declarations in Cmdline.hs...done
Quit [2 times]
counsel-imenu
Scanning declarations in Cmdline.hs... ( 89%)
Sorting declarations in Cmdline.hs...done
Quit [2 times]

Configured using:
 'configure --with-imagemagick --with-gnutls --with-xwidgets
 --with-cairo --with-mailutils --with-pop
 --prefix=/home/alex/opt/emacs/install'

Configured features:
XPM JPEG TIFF GIF PNG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY INOTIFY
GNUTLS LIBXML2 FREETYPE HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE
XIM THREADS XWIDGETS PDUMPER GMP

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

Major mode: Haskell

Minor modes in effect:
  subword-mode: t
  haskell-decl-scan-mode: t
  intero-mode: t
  hl-line-mode: t
  global-hi-lock-mode: t
  hi-lock-mode: t
  winner-mode: t
  direnv-mode: t
  pdf-occur-global-minor-mode: t
  ivy-mode: t
  projectile-mode: t
  persp-mode: t
  global-flycheck-mode: t
  flycheck-mode: t
  company-quickhelp-mode: t
  company-quickhelp-local-mode: t
  global-company-mode: t
  company-mode: t
  haskell-indentation-mode: t
  hybrid-mode: t
  global-evil-surround-mode: t
  evil-surround-mode: t
  global-evil-visualstar-mode: t
  evil-visualstar-mode: t
  evil-leader-mode: t
  global-diff-hl-mode: t
  diff-hl-mode: t
  whitespace-mode: t
  highlight-numbers-mode: t
  recentf-mode: t
  xah-math-input-mode: t
  global-xah-math-input-mode: t
  global-anzu-mode: t
  anzu-mode: t
  engine-mode: t
  which-key-mode: t
  show-smartparens-global-mode: t
  show-smartparens-mode: t
  smartparens-mode: t
  global-undo-tree-mode: t
  undo-tree-mode: t
  shell-dirtrack-mode: t
  evil-local-mode: t
  save-place-mode: t
  override-global-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-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:
None found.

Features:
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esh-io esh-arg esh-module esh-groups esh-opt esh-util recentf
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evil-command-window evil-types evil-search evil-ex shell evil-macros
evil-repeat evil-states evil-core evil-common windmove thingatpt rect
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easy-mmode org-version ob-emacs-lisp ob ob-tangle org-src ob-ref ob-lob
ob-table ob-keys ob-exp ob-comint comint ansi-color ring ob-core ob-eval
org-compat org-macs org-loaddefs format-spec find-func cal-menu calendar
cal-loaddefs rx cl-extra help-mode proof-site proof-autoloads edmacro
kmacro info package easymenu browse-url url-handlers url-parse
auth-source cl-seq eieio eieio-core cl-macs eieio-loaddefs
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ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
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prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
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help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads dbusbind inotify dynamic-setting system-font-setting
font-render-setting xwidget-internal cairo move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 672434 60435)
 (symbols 48 42799 6)
 (strings 32 200345 8564)
 (string-bytes 1 6847260)
 (vectors 16 90639)
 (vector-slots 8 1773190 53886)
 (floats 8 464 834)
 (intervals 56 5460 2974)
 (buffers 992 23)
 (heap 1024 64379 13426))

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