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.

lisp/emacs-lisp/bytecomp.el

@@ -3428,7 +3428,10 @@ for symbols generated by the byte compiler itself."
 		  (boundp var)
 		  (memq var byte-compile-bound-variables)
 		  (memq var byte-compile-free-references))
-	(byte-compile-warn "reference to free variable `%S'" var)
+        (let* ((varname (prin1-to-string var))
+               (suggestions (help-uni-confusable-suggestions varname)))
+          (byte-compile-warn "reference to free variable `%s'%s" varname
+                             (if suggestions (concat "\n  " suggestions) "")))
 	(push var byte-compile-free-references))
       (byte-compile-dynamic-variable-op 'byte-varref var))))
 
@@ -3444,7 +3447,10 @@ for symbols generated by the byte compiler itself."
 		  (boundp var)
 		  (memq var byte-compile-bound-variables)
 		  (memq var byte-compile-free-assignments))
-	(byte-compile-warn "assignment to free variable `%s'" var)
+        (let* ((varname (prin1-to-string var))
+               (suggestions (help-uni-confusable-suggestions varname)))
+          (byte-compile-warn "assignment to free variable `%s'%s" varname
+                             (if suggestions (concat "\n  " suggestions) "")))
 	(push var byte-compile-free-assignments))
       (byte-compile-dynamic-variable-op 'byte-varset var))))
 

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 ,_)

lisp/emacs-lisp/lisp-mode.el

@@ -280,6 +280,19 @@ This will generate compile-time constants from BINDINGS."
          `(face ,font-lock-warning-face
                 help-echo "This \\ has no effect"))))
 
+(defun lisp--match-confusable-symbol-character  (limit)
+  ;; Match a confusable character within a Lisp symbol.
+  (catch 'matched
+    (while t
+      (if (re-search-forward uni-confusables-regexp limit t)
+          ;; Skip confusables which are backslash escaped, or inside
+          ;; strings or comments.
+          (save-match-data
+            (unless (or (eq (char-before (match-beginning 0)) ?\\)
+                        (nth 8 (syntax-ppss)))
+              (throw 'matched t)))
+        (throw 'matched nil)))))
+
 (let-when-compile
     ((lisp-fdefs '("defmacro" "defun"))
      (lisp-vdefs '("defvar"))
@@ -463,7 +476,10 @@ This will generate compile-time constants from BINDINGS."
            (3 'font-lock-regexp-grouping-construct prepend))
          (lisp--match-hidden-arg
           (0 '(face font-lock-warning-face
-               help-echo "Hidden behind deeper element; move to another line?")))
+                    help-echo "Hidden behind deeper element; move to another line?")))
+         (lisp--match-confusable-symbol-character
+          0 '(face font-lock-warning-face
+                    help-echo "Confusable character"))
          ))
       "Gaudy level highlighting for Emacs Lisp mode.")