bug#73812: 30.0.91; ERC 5.6.0.30.1: Customizing erc-modules loads ERC when starting Emacs

["J.P." <jp@neverwas.me>]

"J.P." <jp@neverwas.me> writes:

>>> Eli, can we add this patch to the release branch? It removes a single
>>> line containing an autoload cookie that's new in Emacs 30. (I'm running
>>> the check-expensive suite with the patch now.) Thanks.
>>
>> Are we sure this doesn't break any customization scenarios?
>
> The worst case is probably someone unaware of the implicit loading who
> recently added code referring to symbols defined in erc.el after an
> Emacs-managed `custom-set-variables' form in their `custom-file' (or a
> `:custom' section in a `use-package' declaration). These customizations
> would necessarily have to contain an entry for `erc-modules', but this
> is perhaps the most common ERC customization. However, the user would
> have to be running the pre-release or master.
>
>>  Why was this line added in the first place?
>
> The line was initially added in a misguided attempt to allow new users
> unfamiliar with Emacs to run M-x customize-option RET erc-modules RET
> without first having to run M-: (require 'erc) RET.
>
>>  And why is it urgent to remove it before Emacs 30 is released?
>
> ERC has a great many symbols, which people won't want to see in
> completion tables, etc. Longtime ERC users trying Emacs 30 for the first
> time may find ERC loading whenever they start Emacs, which may not be
> desirable in all Emacs sessions. And since, as mentioned, `erc-modules'
> is likely among the most commonly customized of ERC's options, this may
> also affect non-ERC users who perhaps only tried it once many years ago
> or even folks using a shared config containing such a customization. For
> these reasons, I suspect we'll start noticing ERC-related pollution in
> the automated evidence collection for bug reports filed with M-x
> report-emacs-bug RET once Emacs 30 goes mainstream.

While we await a ruling on this...

Here's an example of how `erc-modules' customizations inadvertently
creep into a person's `custom-file' (for any curious future victims of
this bug).

From emacs -Q (using Emacs 27 as an example):

0. Connect to a server
1. M-x customize-group RET erc RET
2. Twirl open the "Erc Modules" section
3. In an ERC buffer: M-x erc-scrolltobottom-mode RET
4. In the Custom buffer, edit an option, like "Erc Email Userid"
5. C-x C-s
6. Notice that your `custom-file' now contains an entry for `erc-modules'

Among those who've ever done the above or similar, only the lucky few to
have since cleaned up their `custom-file' will be spared ERC invasively
loading when next they fire up Emacs 30.

As may be obvious, ERC is guilty of the familiar anti-pattern of
mutating user options silently behind the scenes. For reasons of
compatibility, we've put off addressing this until a major release.
FWIW, new modules following the recommended practice of declaring
themselves `local' do not exhibit this malignant behavior.