There’s not a day in which I don’t find a new gem for Emacs. Today it’s the built-in bug-reference-mode. This fancy little thing turns issue references in text into clickable buttons that browse the corresponding issue in a bug tracker. I discovered it via the bug-reference-github package, which automatically configures the mode for files in Github repositories.

I don’t use that package, though, because I don’t feel that it’s necessary: Directory Variables are easy enough. Just use M-x add-dir-local-variable to set bug-reference-url-format to the URL of your bug tracker.

For instance, I have the following .dir-locals.el for Flycheck:

  (bug-reference-url-format . ""))
  (bug-reference-bug-regexp . "\\[GH-\\(?2:[[:digit:]]+\\)]")))

This sets bug-reference-url-format for all files in Flycheck to the URL of the Github issue tracker. The URL string is passed through format with the issue ID as argument, so be sure to have an %s escape in the URL at the proper place.

In source code, I tend to use pretty standard references such as bug #111, which are caught by the standard bug-reference-bug-regexp. The changelog of Flycheck however uses a more structured format for bug references, to make it easier to parse issue references out of the changelog. A reference to the bug 111 will look like [GH-111].

Hence, I set bug-reference-bug-regexp for rst-mode (the changelog is written in in ReST) to match this format. Note the peculiar (?2:…) grouping: For whatever mysterious reason, bug-reference-mode expects that the second(!) group in the regexp matches the issue ID. Hence my regexp uses an explicitly numbered group for the ID—one of the lesser known features of Emacs.

Now all I am left to do is turn on bug-reference-mode in my init.el:

(add-hook 'text-mode-hook #'bug-reference-mode)
(add-hook 'prog-mode-hook #'bug-reference-prog-mode)

bug-reference-prog-mode is like bug-reference-mode, but only for comments and strings, which is generally what you want in source code. Just like flyspell-prog-mode basically.

Next time I visit the changelog, all bug references are emphasized and turned into buttons. I can either use the mouse to click on them, or press C-c Ret while the point is on a bug reference, to open the corresponding bug on the Github webpage.