bug#21252: 25.0.50; `describe-variable': use current buffer, not minibuffer, in `completing-read' PREDICATE

[Drew Adams <drew.adams@oracle.com>]

This bug is at least as old as Emacs 20!  Never noticed it before.

1. emacs -Q
2. Type this in *scratch* and evaluate it using `C-x C-e':

   (set (make-local-variable 'toto) 42)

3. C-h v tot TAB

There is no completion for variable `toto'.

The reason is that the PREDICATE arg passed to `completing-read' gets
evaluated with the minibuffer as the current buffer, and `toto' is not
bound in the minibuffer.

The predicate should use `with-current-buffer' to make the originally
current buffer current for the test.

For example, instead of this:

(interactive
 (let ((v (variable-at-point))
       (enable-recursive-minibuffers t)
       val)
   (setq val (completing-read (if (symbolp v)
                                  (format
                                   "Describe variable (default %s): " v)
                                "Describe variable: ")
                              obarray
                              (lambda (vv)
                                (or (get vv 'variable-documentation)
                                    (and (boundp vv) (not (keywordp vv)))))
                              t nil nil
                              (if (symbolp v) (symbol-name v))))
   (list (if (equal val "")
             v (intern val)))))

Use this (or similar - since the file now binds `lexical-binding to t,
you need not resort to backquote+comma for CURBUF):

(interactive
 (let ((v (variable-at-point))
       (enable-recursive-minibuffers t)
       (curbuf (current-buffer))
       val)
   (setq val (completing-read (if (symbolp v)
                                  (format
                                   "Describe variable (default %s): " v)
                                "Describe variable: ")
                              obarray
                              `(lambda (vv)
                                (with-current-buffer ',curbuf
                                  (or (get vv 'variable-documentation)
                                      (and (boundp vv) (not (keywordp vv))))))
                              t nil nil
                              (if (symbolp v) (symbol-name v))))
   (list (if (equal val "")
             v (intern val)))))


In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-20 on LEG570
Bzr revision: 118168 rgm@gnu.org-20141020195941-icp42t8ttcnud09g
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs CPPFLAGS=-DGLYPH_DEBUG=1'