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, 17 insertions, 1 deletions

diff --git a/lisp/emacs-lisp/lisp-mode.el b/lisp/emacs-lisp/lisp-mode.el
index 5df52ebc98f..56f8ef63682 100644
--- a/lisp/emacs-lisp/lisp-mode.el
+++ b/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.")