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, 5 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 3a2efcfc2e0..688b3f85cbb 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2973,6 +2973,11 @@ and if the new behavior breaks your code please email
 integers, they now support the '+' and space flags.
 
 +++
+** In Emacs Lisp mode, symbols with confusable quotes are highlighted.
+For example, the first character in '‘foo' would be highlighted in
+'font-lock-warning-face'.
+
++++
 ** Omitting variables after '&optional' and '&rest' is now allowed.
 For example '(defun foo (&optional))' is no longer an error.  This is
 sometimes convenient when writing macros.  See the ChangeLog entry