Why is ido-mode trying to use this variable?

Question

I have emacs auto-save and backup to a single directory off in my home directory. For some reason, when I try to exit the variable name where I set the save directory is getting passed to ido-mode and it won't let me exit out of emacs. I have tried deleting old versions of my ido.last file (and the symbolic link .#ido.last), but this doesn't seem to consistently take care of the problem. I have full permissions and ownership of the directory the files are stored in and the files themselves. This happens on several systems that I work on covering emacs major versions 21, 22 and 24.

Here are the relevant parts of my emacs configuration:

(defvar home (concat (getenv "HOME") "/"))
(defvar emacs-dir (concat home ".emacs.d/"))
(defvar savedir (concat home ".saves/"))

(setq backup-directory-alist `((".*" . savedir)))
(setq auto-save-file-name-transforms `((".*" ,savedir t)))
(setq backup-by-copying t)
(setq delete-old-versions t
      kept-new-versions 10
      kept-old-versions 6
      version-control t)

(setq ido-save-directory-list-file (concat emacs-dir "cache/ido.last"))
(ido-mode t)
(setq ido-enable-flex-matching t
      ido-everywhere t)

This is what the debugger output looks like when the problem happens.

Debugger entered--Lisp error: (wrong-type-argument stringp savedir)
  expand-file-name(savedir "/home/pinyaka/.emacs.d/cache/")
  make-backup-file-name-1("/home/pinyaka/.emacs.d/cache/ido.last")
  make-backup-file-name("/home/pinyaka/.emacs.d/cache/ido.last")
  find-backup-file-name("/home/pinyaka/.emacs.d/cache/ido.last")
  backup-buffer()
  basic-save-buffer-2()
  basic-save-buffer-1()
  basic-save-buffer()
  save-buffer()
  write-file("/home/pinyaka/.emacs.d/cache/ido.last" nil)
  ido-save-history()
  ido-kill-emacs-hook()
  run-hooks(kill-emacs-hook)
  kill-emacs()
  save-buffers-kill-emacs(nil)
  call-interactively(save-buffers-kill-emacs)

You can see that for some reason ido has gotten a hold of savedir even though I have never used that variable in connection with ido-mode (I have included everywhere that that variable is used as well as all the ido calls that I make). Why does ido do anything with savedir?

Answer 1

I think the problem is:

(setq backup-directory-alist `((".*" . savedir)))

Should be

(setq backup-directory-alist `((".*" . ,savedir)))

Explanation: When Emacs exits, Ido is trying ta save its history; the standard backup procedure of Emacs kicks in and try to backup that file. However you forget to unquote savedir in the configuration for backup-directory-alist, so the cons cell is a pair of string and symbol instead of a pair of strings as expected.