I noticed that author names in my mastodon.el timeline were no longer getting highlighted. Since it behaved correctly when I used emacs -Q, I knew the problem was somewhere in my configuration. Problem: my configuration is very long. (Uh, my .el seems to have grown to be about 460KB…) I started writing some code to help me bisect it, but fortunately I remembered using a package to do that a long time ago. After a little digging around, I found bug-hunter again. I patched it to allow me to specify a header and footer:

(defvar bug-hunter-header nil "Emacs Lisp sexp to add at the beginning of the file.")
(defvar bug-hunter-footer nil "Emacs Lisp sexp to add at the end of the file.")

(defun bug-hunter--print-to-temp (sexp)
  "Print SEXP to a temp file and return the file name."
  (let ((print-length nil)
        (print-level nil)
        (file (make-temp-file "bug-hunter")))
    (with-temp-file file
      (when bug-hunter-header (print bug-hunter-header (current-buffer)))
      (print sexp (current-buffer))
      (when bug-hunter-footer (print bug-hunter-footer (current-buffer))))
    file))

That allowed me to set up this header and footer:

(setq bug-hunter-header
      '(progn
         (package-initialize)
         (use-package quelpa)
         (use-package quelpa-use-package)
         (use-package hydra :commands defhydra)
         (use-package use-package-hydra)))
(setq bug-hunter-footer '(load-file "/tmp/mastodon-test.el"))

where mastodon-test.el was this file:

(package-initialize)
(add-to-list 'load-path "~/vendor/mastodon.el/lisp")
(require 'mastodon)
(setq mastodon-active-user "sachac"
      mastodon-instance-url "https://emacs.ch")
(mastodon)

I wasn't sure how to use bug-hunter's support for assertions because of the asynchronous retrieval of the Mastodon timeline, but at least I could use that code to set things up.

Then I used bug-hunter-file to split up the Emacs Lisp file tangled out of my literate configuration. All I had to do was wait for the Mastodon timeline to show up, quit Emacs, and answer y if I saw the bug or n if I didn't.

Here's what the result was:

You have asked to do an interactive hunt, here's how it goes.
1) I will start a new Emacs instance, which opens a new frame.
2) You will try to reproduce your problem on the new frame.
3) When you’re done, close that frame.
4) I will ask you if you managed to reproduce the problem.
5) We will repeat steps up to 13 times, so hang tight!
Doing some initial tests...
Initial tests done. Hunting for the cause...
"~/sync/emacs/Sacha.el", line 6893 pos 0:
  The assertion returned the following value here:
    t
  Caused by the following expression:
    (use-package highlight-defined :ensure t :custom
      (highlight-defined-face-use-itself t)
      :hook
      (help-mode . highlight-defined-mode)
      (emacs-lisp-mode . highlight-defined-mode))

That's weird, since highlight-defined shouldn't have affected mastodon, but okay. I'm going to skip using highlight-defined for now.

If you're tracking down a weird change in your Emacs configuration file, you might find bug-hunter useful too. It's available on ELPA, or you can get it from Github.