A small utility to make developing well documented vars inline a little more trival.

defvar-maybe.el

;;; defvar-maybe.el --- maybe declare variables
;;; AUTHOR: Corwin Brust <corwin@bru.st>
;;; LICENSE: GPL2 or newer GNU Public License
;;; VERSION: 0.2

;;; Commentary:
;;
;;    A drop-in replacement for `defvar' that substities 'setq' during "DEVEL".
;;    Place the following lines into a package you are hacking on:
;;      (require 'defvar-maybe)
;;      (setq defvar-maybe t) ;; nil=setq, t=defvar, 'custom=defcustom
;;      (defvar-maybe 'my-var "val" "is documented" :group 'mine :type 'string)
;;
;;    Then use `setvar-maybe' just as you would `setvar'.  To clean things up
;;    (if you don't want provide `defvar-local' along with your work):
;;      1. remove above `require' and `setq' statments
;;      2. M-% (M-x query-replace RET) defvar-maybe RET defvar RET !
;;
;;    Version 0.2: var is renamed for the pkg and func; 'custom for defcustom

;;; Code:

(defvar-local defvar-maybe nil
  "Control func: `defvar-maybe', which see for detail.")

(defmacro defvar-maybe (var-name &optional i-value d-string &rest customize-options)
  "Create or set a variable per the buffer-local var: `defvar-maybe'.

Use `setq' instead of `defvar' when var: `defvar-maybe' is not truthy.
When var: `defvar-maybe' is `custom use `defcustom'.

VAR-NAME is the symbol to declare or set.  I-VALUE is either INITVALUE for
`defvar' or VAL for `setq'.  D-STRING is DOCSTRING when `defvar' is used.

This is useful when using `eval-buffer' to run test cases hacked directly into
a package's source files."
  (declare (indent 2) (doc-string 3))
;;;  (message "devel:%s (%s)" defvar-maybe (and (boundp #'defvar-maybe) defvar-maybe))
  (if (and (boundp #'defvar-maybe) defvar-maybe)
      (if (eq 'custom defvar-maybe)
	  `(defcustom ,var-name ,i-value ,d-string ,@customize-options)
      `(setq ,var-name ,i-value))
    `(defvar ,var-name ,i-value ,d-string)))

(provide 'defvar-maybe)
;;; defvar-maybe.el ends here