Auto format nix files

init.el

@@ -528,6 +528,24 @@ be global."
          ("C-M->" . mc/unmark-previous-like-this)
          ("C-c C-<" . mc/mark-all-like-this)))
 
+(use-package nix-mode
+  :config (progn
+            (defun nixfmt-fix ()
+              (interactive)
+              (when (executable-find "nixfmt")
+                (let ((point (point)))
+                  (unwind-protect
+                      (call-process-region (point-min) (point-max) "nixfmt" t t)
+                    (goto-char point)))))
+
+            (define-minor-mode nixfmt-fix-mode
+              "Use nixfmt to automatically format nix files."
+              :lighter " nixfmt"
+              (if nixfmt-fix-mode
+                  (add-hook 'before-save-hook #'nixfmt-fix nil t)
+                (remove-hook 'before-save-hook #'nixfmt-fix t))))
+  :hook (nix-mode . nixfmt-fix-mode))
+
 (use-package no-littering
   :demand t
   :config (progn