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.

2 files changed, 7 insertions, 2 deletions

diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index 7283930507f..c554ccdf4a0 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -3287,7 +3287,8 @@ assigned using the ordering as a guide.
 @table @code
 @item font-lock-warning-face
 @vindex font-lock-warning-face
-for a construct that is peculiar, or that greatly changes the meaning of
+for a construct that is peculiar (e.g., an unescaped confusable quote
+in an Emacs Lisp symbol like @samp{‘foo}), or that greatly changes the meaning of
 other text, like @samp{;;;###autoload} in Emacs Lisp and @samp{#error}
 in C.
 
diff --git a/doc/lispref/objects.texi b/doc/lispref/objects.texi
index d9971f6839e..e948814d1f7 100644
--- a/doc/lispref/objects.texi
+++ b/doc/lispref/objects.texi
@@ -424,7 +424,11 @@ without a special escape meaning; thus, @samp{?\+} is equivalent to
 characters.  However, you must add a backslash before any of the
 characters @samp{()[]\;"}, and you should add a backslash before any
 of the characters @samp{|'`#.,} to avoid confusing the Emacs commands
-for editing Lisp code.  You can also add a backslash before whitespace
+for editing Lisp code.  You should also add a backslash before Unicode
+characters which resemble the previously mentioned @acronym{ASCII}
+ones, to avoid confusing people reading your code.  Emacs will
+highlight some non-escaped commonly confused characters such as
+@samp{‘} to encourage this.  You can also add a backslash before whitespace
 characters such as space, tab, newline and formfeed.  However, it is
 cleaner to use one of the easily readable escape sequences, such as
 @samp{\t} or @samp{\s}, instead of an actual whitespace character such