Fixing leaf-keywords "unrecognized keyword" error in Flycheck

After setting up leaf-keywords and Flycheck properly:

(straight-use-package 'leaf)
(straight-use-package 'leaf-keywords)

(require 'leaf)
(require 'leaf-keywords)
(leaf-keywords-init)

(leaf flycheck
  :straight t
  :hook prog-mode-hook)

Despite leaf-keywords being loaded, Flycheck will now return an error from leaf.el complaining that :straight is an unrecognized keyword.

I tried putting leaf-keywords-init in a eval-when-compile or a (cl-eval-when (compile load eval)) block, to no avail.

For the longest time I just turned Flycheck off in my init file (the only place where leaf.el is used), but yesterday I decided to take another look at it.

This is how Flycheck checks Emacs Lisp code (I only included the command part):

(flycheck-define-checker emacs-lisp
  "An Emacs Lisp syntax checker using the Emacs Lisp Byte compiler.

See Info Node `(elisp)Byte Compilation'."
  :command ("emacs" (eval flycheck-emacs-args)
            (eval
             (let ((path (pcase flycheck-emacs-lisp-load-path
                           (`inherit load-path)
                           (p (seq-map #'expand-file-name p)))))
               (flycheck-prepend-with-option "--directory" path)))
            (option "--eval" flycheck-emacs-lisp-package-user-dir nil
                    flycheck-option-emacs-lisp-package-user-dir)
            (option "--eval" flycheck-emacs-lisp-initialize-packages nil
                    flycheck-option-emacs-lisp-package-initialize)
            (option "--eval" flycheck-emacs-lisp-check-declare nil
                    flycheck-option-emacs-lisp-check-declare)
            "--eval" (eval (flycheck-emacs-lisp-bytecomp-config-form))
            "--eval" (eval flycheck-emacs-lisp-check-form)
            "--"
            source-inplace)
  ;; ...
  )

This calls Emacs in a subprocess with:

flycheck-emacs-args (default is "-Q" and "--batch")
flycheck-emacs-lisp-load-path (or the host's load-path if its value is inherit) passed into load-path of the subprocess so that require's work
package.el packages initialized
the flag for whether declare-function statememts should be checked passed in
the byte compiler set up to run in the right directory

Then finally it evaluates flycheck-emacs-lisp-check-form, which has code that essentially byte-compiles the current buffer and reports the warnings and errors.

I suspected that leaf-keywords does not have a chance to be initialized in the subprocess, and… yeah. I'm not sure why eval-when-compile didn't work though. Perhaps the “compile” in that context is different from what the byte compiler does?

Either way, this is the workaround I ended up with:

;; HACK: without this, the keywords added by leaf-keywords don't
;; take effect in the checker process, so they all trigger an
;; "unrecognized keyword" error.
(setq flycheck-emacs-lisp-check-form
      (if (string-match-p "leaf-keywords"
                          flycheck-emacs-lisp-check-form)
          ;; Don't do anything on subsequent evals
          flycheck-emacs-lisp-check-form
        (format
         "(progn %s %s)"
         '(progn
            (require 'leaf-keywords nil t)
            (leaf-keywords-init))
         flycheck-emacs-lisp-check-form)))

Which just require's leaf-keywords and initializes it right before the actual check happens.

It seems to me that changes to Emacs Lisp behavior — in this case the presence of features of a macro — should take place on load. It is analogous to a hypothetical package providing pcase patterns; it would be similarly surprising if such a package doesn't activate those extensions on load.

But, as it stands right now, the workaround is good enough for me.