Re: [FR] Allow flattened imenu index

[Juri Linkov <juri@linkov.net>]

>>   menu1
>>     sub-item1
>>     sub-item2
>>
>> could be flattened to
>>
>>   menu1 -> sub-item1
>>   menu1 -> sub-item2
>
> By the way, this seems to be exactly what the breadcrumb-jump command
> in my breadcrumb.el package does.  Goes reasonably well with a flex/fuzzy
> completion style.

It would be great to have some form of breadcrumb-jump in imenu.el
since it's useful on its own even for someone who doesn't use breadcrumbs.

> You can look at it for an implementation idea.  Just be sure to do this
> flattening at the presentation level (i.e. M-x imenu), not at the
> internal representation level.

Here are some observations while testing on
emacs/test/manual/etags/ruby-src/test.rb.

Both ruby-mode and ruby-ts-mode provide a list that is already flat:

ruby-mode:
  ModuleExample
  ModuleExample#ModuleExample.module_class_method
  ModuleExample#module_instance_method
  ModuleExample::ClassExample
  ModuleExample::ClassExample#+
  ModuleExample::ClassExample#ClassExample.class_method
  ModuleExample::ClassExample#instance_method

ruby-ts-mode:
  ModuleExample
  ModuleExample#module_instance_method
  ModuleExample.module_class_method
  ModuleExample::ClassExample
  ModuleExample::ClassExample#+
  ModuleExample::ClassExample#instance_method
  ModuleExample::ClassExample.class_method

When eglot is enabled then imenu-create-index-function returns a tree.

What is interesting is that breadcrumb-jump already correctly handles both
a flat list and a tree:

a flat list in breadcrumb-jump completions is exactly the same as in 'imenu':
  ModuleExample
  ModuleExample#module_instance_method
  ModuleExample.module_class_method
  ModuleExample::ClassExample
  ModuleExample::ClassExample#+
  ModuleExample::ClassExample#instance_method
  ModuleExample::ClassExample.class_method

a tree from eglot in breadcrumb-jump completions:
  Class > ModuleExample > ClassExample
  Method > ClassExample > class_method
  Method > ModuleExample > module_class_method
  Method > ModuleExample > module_instance_method
  Method > ModuleExample::ClassExample > +
  Method > ModuleExample::ClassExample > instance_method
  Module >  > ModuleExample

is still usable even without special characters like "#".

> It shouldn't be needed here at all, but in case anyone's thinking
> about it, please avoid messing with imenu's internal representation of
> hierarchies as that structure is relied upon by many extensions (not just
> mine, but several others).  Even certain things supported by certain
> imenu-presenting frontends (like "special elements") are not supported by
> other frontends.  It's a bit of a mess.  The symbols holding/describing
> this representation (imenu--index-alist, maybe others) are incorrectly
> named '--' but they are most definitely externally visible and used
> customization points.

Indeed, it's unfortunate that imenu--make-index-alist and imenu--index-alist
are named as internal.