Don't always escape "." and "?" in `prin1'

* src/print.c (print_object): Only escape "." and "?" when
appearing as the first character in a symbol (bug#23130).

etc/NEWS

@@ -1076,6 +1076,22 @@ Emacs buffers, like indentation and the like.  The new ert function
 
 * Incompatible Lisp Changes in Emacs 29.1
 
+---
+** 'prin1' doesn't always escape "." and "?" in symbols any more.
+Previously, symbols like 'foo.bar' would be printed by 'prin1' as
+"foo\.bar".  This now prints as "foo.bar" instead.  The Emacs Lisp
+reader interprets these strings as referring to the same symbol, so
+this is virtually always backwards-compatible, but there may
+theoretically be code out there that expects a specific printed
+representation.
+
+The same is the case with the "?" character: The 'foo?' symbol is now
+printed as "foo?" instead of "foo\?".
+
+If the "." and "?" characters are the first character in the symbol,
+they will still be escaped, so the '.foo' symbol is still printed as
+"\.foo" and the '?bar' symbol is still printed as "\?bar".
+
 +++
 ** Remapping 'mode-line' no longer works as expected.
 'mode-line' is now the parent face of the new 'mode-line-active' face,

src/print.c

@@ -2171,14 +2171,19 @@ print_object (Lisp_Object obj, Lisp_Object printcharfun, bool escapeflag)
 	Lisp_Object name = SYMBOL_NAME (obj);
 	ptrdiff_t size_byte = SBYTES (name);
 
-	/* Set CONFUSING if NAME looks like a number, calling
-	   string_to_number for non-obvious cases.  */
 	char *p = SSDATA (name);
 	bool signedp = *p == '-' || *p == '+';
 	ptrdiff_t len;
-	bool confusing = ((c_isdigit (p[signedp]) || p[signedp] == '.')
-			  && !NILP (string_to_number (p, 10, &len))
-			  && len == size_byte);
+	bool confusing =
+	  /* Set CONFUSING if NAME looks like a number, calling
+	     string_to_number for non-obvious cases.  */
+	  ((c_isdigit (p[signedp]) || p[signedp] == '.')
+	   && !NILP (string_to_number (p, 10, &len))
+	   && len == size_byte)
+	  /* We don't escape "." or "?" (unless they're the first
+	     character in the symbol name).  */
+	  || *p == '?'
+	  || *p == '.';
 
 	if (! NILP (Vprint_gensym)
 	    && !SYMBOL_INTERNED_IN_INITIAL_OBARRAY_P (obj))
@@ -2201,8 +2206,8 @@ print_object (Lisp_Object obj, Lisp_Object printcharfun, bool escapeflag)
 	      {
 		if (c == '\"' || c == '\\' || c == '\''
 		    || c == ';' || c == '#' || c == '(' || c == ')'
-		    || c == ',' || c == '.' || c == '`'
-		    || c == '[' || c == ']' || c == '?' || c <= 040
+		    || c == ',' || c == '`'
+		    || c == '[' || c == ']' || c <= 040
 		    || c == NO_BREAK_SPACE
 		    || confusing)
 		  {

test/src/print-tests.el

@@ -417,5 +417,13 @@ otherwise, use a different charset."
            t)))
     (should (equal (prin1-to-string (make-marker)) ""))))
 
+(ert-deftest test-dots ()
+  (should (equal (prin1-to-string 'foo.bar) "foo.bar"))
+  (should (equal (prin1-to-string '.foo) "\\.foo"))
+  (should (equal (prin1-to-string '.foo.) "\\.foo."))
+  (should (equal (prin1-to-string 'bar?bar) "bar?bar"))
+  (should (equal (prin1-to-string '\?bar) "\\?bar"))
+  (should (equal (prin1-to-string '\?bar?) "\\?bar?")))
+
 (provide 'print-tests)
 ;;; print-tests.el ends here