add-to-list with lexical variables

[Hongxu Chen <leftcopy.chx@gmail.com>]

Hi list,

  I am writing a snippet to add element into environment variables, and
  it is written as below:

#+BEGIN_SRC elisp  
(defun no-dup-add-env-ele (env env-ele-string)
   (let* ((env-separator (if (string-equal system-type "windows-nt") ";" ":"))
     (env-list (split-string (getenv env) env-separator)))
     (if (string-match-p env-separator env-ele-string)
        (dolist (env-ele (split-string env-ele-string env-separator))
                (add-to-list 'env-list env-ele)) 
     (add-to-list 'env-list env-ele-string))
     (setenv env (mapconcat 'identity env-list ":"))))
#+END_SRC

1. when I set `lexical-binding' to t and byte-compile the file, it
would report this error:

    add-to-list cannot use lexical var `env-list'

2. And when I using `lexical-let*' instead, there would be an warning:

    Warning: assignment to free variable `env-list'

3. However after resetting `lexical-binding' to nil, byte-compiles well.

So what are the differences?

Thanks,
Hongxu Chen