Improve errors & warnings due to fancy quoted vars (Bug#32939)

Add some hints to the message for byte compiler free & unused variable
warnings, and 'void-variable' errors where the variable has confusable
quote characters in it.
* lisp/help.el (uni-confusables), uni-confusables-regexp): New
constants.
(help-command-error-confusable-suggestions): New function, added to
`command-error-function'.
(help-uni-confusable-suggestions): New function.
* lisp/emacs-lisp/bytecomp.el (byte-compile-variable-ref):
* lisp/emacs-lisp/cconv.el (cconv--analyze-use): Use it.

* lisp/emacs-lisp/lisp-mode.el
(lisp--match-confusable-symbol-character): New function.
(lisp-fdefs): Use it to fontify confusable characters with
font-lock-warning-face when they occur in symbol names.
* doc/lispref/modes.texi (Faces for Font Lock):
* doc/lispref/objects.texi (Basic Char Syntax): Recommend backslash
escaping of confusable characters, and mention new fontification.
* etc/NEWS: Announce the new fontification behavior.
* test/lisp/emacs-lisp/lisp-mode-tests.el (lisp-fontify-confusables):
New test.

1 files changed, 4 insertions, 2 deletions

diff --git a/lisp/emacs-lisp/cconv.el b/lisp/emacs-lisp/cconv.el
index 58ca9d5f57e..09af7cb9104 100644
--- a/lisp/emacs-lisp/cconv.el
+++ b/lisp/emacs-lisp/cconv.el
@@ -591,8 +591,10 @@ FORM is the parent form that binds this var."
               (eq ?_ (aref (symbol-name var) 0))
 	      ;; As a special exception, ignore "ignore".
 	      (eq var 'ignored))
-       (byte-compile-warn "Unused lexical %s `%S'"
-                          varkind var)))
+       (let ((suggestions (help-uni-confusable-suggestions (symbol-name var))))
+         (byte-compile-warn "Unused lexical %s `%S'%s"
+                            varkind var
+                            (if suggestions (concat "\n  " suggestions) "")))))
     ;; If it's unused, there's no point converting it into a cons-cell, even if
     ;; it's captured and mutated.
     (`(,binder ,_ t t ,_)