Backport VC-aware bug-reference-mode from 28.x

Fixes gf (+lookup/file) on issue/PR references for Emacs 27.x users,
for example:

  #1234
  doom-emacs#1234
  hlissner/doom-emacs#1234

modules/emacs/vc/autoload/bug-reference-backport.el

@@ -0,0 +1,100 @@
+;;; emacs/vc/autoload/bug-reference-backport.el -*- lexical-binding: t; -*-
+;;;###if (not EMACS28+)
+;; DEPRECATED Remove when Emacs 27.x support is dropped
+
+;; In Emacs 28, the built-in bug-reference package started consulting vc for
+;; repo information (to inform its bug reference URLs). This incredibly useful
+;; feature is not available in 27.x yet, so I've backported it:
+
+(defvar bug-reference-setup-from-vc-alist
+  `(;;
+    ;; GNU projects on savannah.
+    ;;
+    ;; Not all of them use debbugs but that doesn't really matter
+    ;; because the auto-setup is only performed if
+    ;; `bug-reference-url-format' and `bug-reference-bug-regexp'
+    ;; aren't set already.
+    ("git\\.\\(?:sv\\|savannah\\)\\.gnu\\.org:"
+     "\\<\\([Bb]ug ?#?\\)\\([0-9]+\\(?:#[0-9]+\\)?\\)\\>"
+     ,(lambda (_) "https://debbugs.gnu.org/%s"))
+    ;;
+    ;; GitHub projects.
+    ;;
+    ;; Here #17 may refer to either an issue or a pull request but
+    ;; visiting the issue/17 web page will automatically redirect to
+    ;; the pull/17 page if 17 is a PR.  Explicit user/project#17 links
+    ;; to possibly different projects are also supported.
+    ("[/@]github.com[/:]\\([.A-Za-z0-9_/-]+\\)\\.git"
+     "\\([.A-Za-z0-9_/-]+\\)?\\(?:#\\)\\([0-9]+\\)\\>"
+     ,(lambda (groups)
+        (let ((ns-project (nth 1 groups)))
+          (lambda ()
+            (concat "https://github.com/"
+                    (or
+                     ;; Explicit user/proj#18 link.
+                     (match-string 1)
+                     ns-project)
+                    "/issues/"
+                    (match-string 2))))))
+    ;;
+    ;; GitLab projects.
+    ;;
+    ;; Here #18 is an issue and !17 is a merge request.  Explicit
+    ;; namespace/project#18 or namespace/project!17 references to
+    ;; possibly different projects are also supported.
+    ("[/@]gitlab.com[/:]\\([.A-Za-z0-9_/-]+\\)\\.git"
+     "\\(?1:[.A-Za-z0-9_/-]+\\)?\\(?3:[#!]\\)\\(?2:[0-9]+\\)\\>"
+     ,(lambda (groups)
+        (let ((ns-project (nth 1 groups)))
+          (lambda ()
+            (concat "https://gitlab.com/"
+                    (or (match-string 1)
+                        ns-project)
+                    "/-/"
+                    (if (string= (match-string 3) "#")
+                        "issues/"
+                      "merge_requests/")
+                    (match-string 2)))))))
+  "An alist for setting up `bug-reference-mode' based on VC URL.
+
+Each element has the form (URL-REGEXP BUG-REGEXP URL-FORMAT-FN).
+
+URL-REGEXP is matched against the version control URL of the
+current buffer's file.  If it matches, BUG-REGEXP is set as
+`bug-reference-bug-regexp'.  URL-FORMAT-FN is a function of one
+argument that receives a list of the groups 0 to N of matching
+URL-REGEXP against the VCS URL and returns the value to be set as
+`bug-reference-url-format'.")
+
+(defun bug-reference--maybe-setup-from-vc (url url-rx bug-rx bug-url-fmt)
+  (when (string-match url-rx url)
+    (setq-local bug-reference-bug-regexp bug-rx)
+    (setq-local bug-reference-url-format
+                (let (groups)
+                  (dotimes (i (/ (length (match-data)) 2))
+                    (push (match-string i url) groups))
+                  (funcall bug-url-fmt (nreverse groups))))))
+
+;;;###autoload
+(defun bug-reference-try-setup-from-vc ()
+  "Try setting up `bug-reference-mode' based on VC information.
+Test each configuration in `bug-reference-setup-from-vc-alist'
+and apply it if applicable."
+  (when (require 'browse-at-remote nil t)
+    (when-let* ((remote (car (browse-at-remote--get-remotes)))
+                (url (browse-at-remote--get-remote-url remote)))
+      (catch 'found
+        (dolist (config bug-reference-setup-from-vc-alist)
+          (and (apply #'bug-reference--maybe-setup-from-vc
+                      url config)
+               (throw 'found t)))))))
+
+;;;###autoload
+(add-hook! '(bug-reference-mode-hook
+             bug-reference-prog-mode-hook)
+  (defun +vc-init-bug-reference-from-vc-h ()
+    (when (or bug-reference-mode
+              bug-reference-prog-mode)
+      (unless (and bug-reference-bug-regexp
+                   bug-reference-url-format)
+        (bug-reference-try-setup-from-vc)))))

modules/tools/lookup/autoload/lookup.el

@@ -284,20 +284,28 @@ otherwise falling back to ffap.el (find-file-at-point)."
   "Searches for a bug reference in user/repo#123 or #123 format and opens it in
 the browser."
   (require 'bug-reference)
-  (let ((bug-reference-url-format bug-reference-url-format)
+  (when (fboundp 'bug-reference-try-setup-from-vc)
-        (bug-reference-bug-regexp bug-reference-bug-regexp)
+    (let ((old-bug-reference-mode bug-reference-mode)
-        (bug-reference-mode (derived-mode-p 'text-mode 'conf-mode))
+          (old-bug-reference-prog-mode bug-reference-prog-mode)
-        (bug-reference-prog-mode (derived-mode-p 'prog-mode)))
+          (bug-reference-url-format bug-reference-url-format)
-    (bug-reference--run-auto-setup)
+          (bug-reference-bug-regexp bug-reference-bug-regexp))
+      (bug-reference-try-setup-from-vc)
       (unwind-protect
+          (let ((bug-reference-mode t)
+                (bug-reference-prog-mode nil))
             (catch 'found
               (bug-reference-fontify (line-beginning-position) (line-end-position))
               (dolist (o (overlays-at (point)))
                 ;; It should only be possible to have one URL overlay.
                 (when-let (url (overlay-get o 'bug-reference-url))
                   (browse-url url)
-              (throw 'found t))))
+
-      (bug-reference-unfontify (line-beginning-position) (line-end-position)))))
+                  (throw 'found t)))))
+        ;; Restore any messed up fontification as a result of this.
+        (bug-reference-unfontify (line-beginning-position) (line-end-position))
+        (if (or old-bug-reference-mode
+                old-bug-reference-prog-mode)
+            (bug-reference-fontify (line-beginning-position) (line-end-position)))))))
 
 
 ;;