.dir-locals.el: Prevent errors if .dir-locals.el isn't found.

While this repo should presumably always have a .dir-locals.el
by the nature of this file itself, it seems that this behavior "leaks".
See added comment for more details on the fix to this strange bug,
which is likely an upstream emacs or vc-mode issue.

Thanks to Miguel Ángel Arruga Vivas and Maxim Cournoyer for helping
investigate this problem.

* .dir-locals.el: Don't error out if .dir-locals.el isn't found.

1 files changed, 21 insertions, 11 deletions

diff --git a/.dir-locals.el b/.dir-locals.el
index 8e5d3902e3..545c1c3e8b 100644
--- a/.dir-locals.el
+++ b/.dir-locals.el
@@ -17,17 +17,27 @@
      ;; Geiser
      ;; This allows automatically setting the `geiser-guile-load-path'
      ;; variable when using various Guix checkouts (e.g., via git worktrees).
-     (eval . (let* ((root-dir (expand-file-name
-                               (locate-dominating-file
-                                default-directory ".dir-locals.el")))
-                    ;; Workaround for bug https://issues.guix.gnu.org/43818.
-                    (root-dir* (directory-file-name root-dir)))
-               (unless (boundp 'geiser-guile-load-path)
-                 (defvar geiser-guile-load-path '()))
-               (make-local-variable 'geiser-guile-load-path)
-               (require 'cl-lib)
-               (cl-pushnew root-dir* geiser-guile-load-path
-                           :test #'string-equal)))))
+     (eval . (let ((root-dir-unexpanded (locate-dominating-file
+                                         default-directory ".dir-locals.el")))
+               ;; While Guix should in theory always have a .dir-locals.el
+               ;; (we are reading this file, after all) there seems to be a
+               ;; strange problem where this code "escapes" to some other buffers,
+               ;; at least vc-mode.  See:
+               ;;   https://lists.gnu.org/archive/html/guix-devel/2020-11/msg00296.html
+               ;; (TODO: add/replace with upstream emacs bug link when reported)
+               ;; Hence the following "when", which might otherwise be unnecessary;
+               ;; it prevents causing an error when root-dir-unexpanded is nil.
+               (when root-dir-unexpanded
+                 (let* ((root-dir (expand-file-name root-dir-unexpanded))
+                        ;; Workaround for bug https://issues.guix.gnu.org/43818.
+                        (root-dir* (directory-file-name root-dir)))
+
+                   (unless (boundp 'geiser-guile-load-path)
+                     (defvar geiser-guile-load-path '()))
+                   (make-local-variable 'geiser-guile-load-path)
+                   (require 'cl-lib)
+                   (cl-pushnew root-dir* geiser-guile-load-path
+                               :test #'string-equal)))))))
 
  (c-mode          . ((c-file-style . "gnu")))
  (scheme-mode