I'm taking the advice Stafan gave in [1] and squashing lexical-scoping
bugs identified by the byte compiler. I've attached a patch which starts
making a dent in the warnings. There were some things which were
relatively easy, such as adding a leading underscore to `&rest _ignore',
but there were a lot what seem to be false positives. I've taken some
notes on what I've found.

 - `condition-case': The byte-compiler doesn't recognize the handler
   forms and so gives warnings about unused lexical arguments. Consider
   the following code (adapted from `widget-sexp-validate'):

   #+BEGIN_SRC emacs-lisp
     ;; -*- lexical-binding: t; -*-

     (defun foo ()
       (let (err)
         (condition-case data
             (skip-syntax-forward "\\s-")
           (end-of-file (setq err "Unbalanced sexp"))      ;; 1
           (error (setq err (error-message-string data)))) ;; 2
         err))
   #+END_SRC

   Byte-compiling that with `byte-compile-force-lexical-warnings' set
   to `t' warns of "Unused lexical argument `data'". Switching lines 1
   and 2 gives the warning "the function `end-of-file' is not known to
   be defined." Removing line 1 makes it compile without errors. It
   would seem that the byte-compiler is not treating `condition-case'
   as specially as it should.

 - I'm seeing a lot of "Argument foo is not a lexical variable", such as
   in vc/ediff-diff.el:532:56. What does this mean and is this something
   I should "fix"? In some cases, this is because `foo' is defined with
   `defvar' but also used as a function argument. What should be done in
   this case? For example, here is the definition of
   `emerge-remote-exit':

   #+BEGIN_SRC emacs-lisp
     (defun emerge-remote-exit (file-out emerge-exit-func)
       (emerge-write-and-delete file-out)
       (kill-buffer emerge-merge-buffer)
       (funcall emerge-exit-func (if emerge-prefix-argument 1 0)))
   #+END_SRC

   Note that `emerge-exit-func' is a `defvar'ed variable. Should it be
   replaced with something like this:

   #+BEGIN_SRC emacs-lisp
     (defun emerge-remote-exit (file-out exit-func)
       (let ((emerge-exit-func exit-func))
         (emerge-write-and-delete file-out)
         (kill-buffer emerge-merge-buffer)
         (funcall emerge-exit-func (if emerge-prefix-argument 1 0))))
   #+END_SRC

 - In `emerge-revisions-with-ancestor', the variable `cmd' is let-bound,
   but does not appear to be used. Could it be safely removed?

 - Similarly, `cvs-fileinfo<' binds variables `subtypea' and `subtypeb'
   but never uses them. Can they be removed?

 - Superfluously-bound variables: In, for example,
   `ange-ftp-file-attributes', the variables `host', `user', and `name'
   are bound but never used. Should these be prefixed with an
   underscore (to make the byte compiler shut up) or removed altogether
   (since they aren't actually used)? It seems like they should just be
   removed, but I don't want to trample on something needed.

[1] http://article.gmane.org/gmane.emacs.devel/162466

--
Daniel Hackney