make flycheck an optional dependency

[Icy-Thought]

NOTE: I am still very new to elisp world and this is the second PR I write for an elisp pacakge. If there is something missing, please ping me and I'll make sure to fix it! :)

Solves: #85

[Icy-Thought]

One thing I am still puzzled by is the autoload of rustic-flycheck-setup. Should we change that, or retain it as is?

[CeleritasCelery]

I reran the latest commit to make sure that was false failure.

https://github.com/emacs-rustic/rustic/actions/runs/13517402444/job/38352881470

It looks like markdown mode now depends on Emacs 28

[Icy-Thought]

It's up to you, because upcoming PR's will have the same complaint. Maybe we should drop the tests for Emacs version > 28? A separate PR would be required for that though, since this is completely unrelated.

[CeleritasCelery]

Thanks! Sorry this took so long.

[Icy-Thought]

No worries, and glad to help out! :)

[jian-lin]

This PR causes compilation error.

Reproducing steps:

1. cd $(mktemp -d)
2. HOME=$PWD emacs -Q
3. eval this

(progn
  (require 'package)
  (add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/") t)
  (package-install 'rustic))

4. see the following error in *Compile-Log* buffer

Compiling file /tmp/tmp.C2ifkR6TQo/.emacs.d/elpa/rustic-20250310.1415/rustic-flycheck.el at Fri Mar 14 17:18:24 2025
rustic-flycheck.el:7:11: Error: Cannot open load file: No such file or directory, flycheck

---

Only non-flycheck users will see this error. This error is not great but harmless for them.

One way to avoid this error is to move the optional file rustic-flycheck.el to a separated MELPA package, as is recommended here. This is a good way to deal with optional dependencies in new MELPA packages. However, for existing MELPA packages, it can be seen as a breaking change because flycheck users will have to install an extra rustic-flycheck MELPA package.

[CeleritasCelery]

Thanks for reporting this. Splitting into a separate package is going to be more of a headache than it's worth. I think we can solve this by removing this line and adding with-eval-after-load 'flycheck to this line.

[jian-lin]

Splitting into a separate package is going to be more of a headache than it's worth.

To be clear, splitting only changes the distribution of rustic and causes no change to the development and repo structure of rustic.

[jian-lin]

FWIW, (info "(elisp)Hooks for Loading") says

Normally, well-designed Lisp programs should not use ‘with-eval-after-load’.

[Icy-Thought]

I should've tested in emacs -Q because that error was not triggered even after I removed the package locally. Very odd.
But here is a question, isn't the whole rustic-flycheck not supposed to be loaded in the first place if flycheck has not been loaded?

rustic/rustic.el

Lines 229 to 230 in 22a5ef8

	(with-eval-after-load 'flycheck
	(require 'rustic-flycheck)))

[jian-lin]

All files get compiled during installation. That's why MELPA suggests distributing optional files/packages (rustic-flycheck.el here) separately.

[CeleritasCelery]

To be clear, splitting only changes the distribution of rustic and causes no change to the development and repo structure of rustic

It doesn't change anything for the developers, but it does mean that users now need to be aware of and import that package. Since I would guess 90% of users are using flycheck and it previously worked out of the box that would cause a big support headache. Not worth it for such a trivial change.