* Completion for cross-references
To integrate AUCTeX with reftex and get completion for ~\cite~
(bibliography) and ~\ref~ (intra-document labels):
#+begin_src elisp
;; In init file:
(put 'TeX-auto-save 'safe-local-variable 'booleanp)
(put 'TeX-parse-self 'safe-local-variable 'booleanp)
(add-hook 'LaTeX-mode-hook 'turn-on-reftex)
(setq reftex-plug-into-AUCTeX t)
(setq reftex-label-illegal-re "[^-a-zA-Z0-9_+=:;,./]")  ; Allow slashes.
(setq TeX-auto-local ".auctex")

;; Top-level .dir-locals.el:
((latex-mode
  . ((TeX-auto-save . t)
     (TeX-parse-self . t)))
 (bibtex-mode
  . ((TeX-auto-save . t)
     (TeX-parse-self . t))))

;; .dir-locals.el files in subfolders:
((latex-mode
  . ((TeX-master . "../path/to/main.tex"))))
#+end_src