What happened to the ido-imenu in ruby-mode function in Emacs24?

Question

Emacs 23.2 in emacs-starter-kit v1 has C-x C-i (or ido-imenu) (similar to Sublime Text's Cmd+R). Emacs24 in emacs-starter-kit v2 lacks this function. I found this github issue and a fix, which try to recreate the functionality. While this ido-imenu works in elisp-mode, it stopped working in ruby-mode. I get:

imenu--make-index-alist: No items suitable for an index found in this buffer

1. Has anyone figured out how to get this to work?
2. Why was this taken out of Emacs24?
3. Is there a new replacement for this function?

Answer 1

So I finally figured it out, after reading the Defining an Imenu Menu for a Mode section on emacs-wiki again.

Short answer: you need to add this bit to your customization. Feel free to add more types to the list (I am happy with just methods).

(add-hook 'ruby-mode-hook
          (lambda ()
            (set (make-local-variable imenu-generic-expression)
                 '(("Methods"  "^\\( *\\(def\\) +.+\\)"          1)
                   ))))

Longer answer: I first tried to define a ruby-imenu-generic-expression function and set that to imenu-generic-expression by using the ruby-mode-hook:

(defvar ruby-imenu-generic-expression
  '(("Methods"  "^\\( *\\(def\\) +.+\\)"          1))
  "The imenu regex to parse an outline of the ruby file")

(defun ruby-set-imenu-generic-expression ()
  (make-local-variable 'imenu-generic-expression)
  (make-local-variable 'imenu-create-index-function)
  (setq imenu-create-index-function 'imenu-default-create-index-function)
  (setq imenu-generic-expression ruby-imenu-generic-expression))

(add-hook 'ruby-mode-hook 'ruby-set-imenu-generic-expression)

This however did not work (I would get the same error as before). More reading of the Defining an Imenu Menu for a Mode section showed me the way. Now, I'm not an elisp expert, so here's my hypothesis: basically, the above method works for modes where the

major mode supports a buffer local copy of the “real” variable, ‘imenu-generic-expression’. If your mode doesn’t do it, you will have to rely on a hook.

The example for foo-mode made it clear how to do it for ruby-mode. So it appears that ruby-mode does not have a buffer-local copy of the real imenu-generic-expression variable. I still can't explain why it worked in Emacs 23.2 (with ESK v1) but does not on Emacs24, but hey at least I found a working solution.

Answer 2

Since the function is part of ESK (as opposed to something budled with Emacs) you'd probably do best to report the bug upstream. On a related note ESK main competitor Emacs Prelude offers the same functionality (bound to C-c i by default) and it seems to be working fine with ruby-mode in Emacs 24. Here you can find more on ido-imenu.