Improve print output options commands in backtrace-mode (bug#36566)

* lisp/emacs-lisp/backtrace.el (backtrace-view): Mention
:print-gensym in docstring.
(backtrace-mode-map): Add keyboard binding for
backtrace-toggle-print-gensym.  Add menu entries for
backtrace-toggle-print-circle and backtrace-toggle-print-gensym.
(backtrace--with-output-variables): Bind print-gensym with value
of :print-gensym found in view plist.
(backtrace-toggle-print-circle): Remove description of
implementation details from docstring.
(backtrace-toggle-print-gensym): New command.
(backtrace--toggle-feature): Add echo area message describing result
of command.

* test/lisp/emacs-lisp/backtrace-tests.el
(backtrace-tests--print-circle): New test.

* doc/lispref/debugging.texi (Backtraces): Document keyboard
binding for backtrace-toggle-print-gensym.

lisp/emacs-lisp/backtrace.el

@@ -175,7 +175,8 @@ This should be a list of `backtrace-frame' objects.")
 
 (defvar-local backtrace-view nil
   "A plist describing how to render backtrace frames.
-Possible entries are :show-flags, :show-locals and :print-circle.")
+Possible entries are :show-flags, :show-locals, :print-circle
+and :print-gensym.")
 
 (defvar-local backtrace-insert-header-function nil
   "Function for inserting a header for the current Backtrace buffer.
@@ -205,6 +206,7 @@ frames where the source code location is known.")
     (define-key map "p" 'backtrace-backward-frame)
     (define-key map "v" 'backtrace-toggle-locals)
     (define-key map "#" 'backtrace-toggle-print-circle)
+    (define-key map ":" 'backtrace-toggle-print-gensym)
     (define-key map "s" 'backtrace-goto-source)
     (define-key map "\C-m" 'backtrace-help-follow-symbol)
     (define-key map "+" 'backtrace-multi-line)
@@ -224,6 +226,18 @@ frames where the source code location is known.")
          :active (backtrace-get-index)
          :selected (plist-get (backtrace-get-view) :show-locals)
          :help "Show or hide the local variables for the frame at point"]
+        ["Show Circular Structures" backtrace-toggle-print-circle
+         :style toggle
+         :active (backtrace-get-index)
+         :selected (plist-get (backtrace-get-view) :print-circle)
+         :help
+         "Condense or expand shared or circular structures in the frame at point"]
+        ["Show Uninterned Symbols" backtrace-toggle-print-gensym
+         :style toggle
+         :active (backtrace-get-index)
+         :selected (plist-get (backtrace-get-view) :print-gensym)
+         :help
+         "Toggle unique printing of uninterned symbols in the frame at point"]
         ["Expand \"...\"s" backtrace-expand-ellipses
          :help "Expand all the abbreviated forms in the current frame"]
         ["Show on Multiple Lines" backtrace-multi-line
@@ -339,6 +353,7 @@ It runs `backtrace-revert-hook', then calls `backtrace-print'."
   `(let ((print-escape-control-characters t)
          (print-escape-newlines t)
          (print-circle (plist-get ,view :print-circle))
+         (print-gensym (plist-get ,view :print-gensym))
          (standard-output (current-buffer)))
      ,@body))
 
@@ -420,12 +435,18 @@ Set it to VALUE unless the button is a `backtrace-ellipsis' button."
 
 (defun backtrace-toggle-print-circle (&optional all)
   "Toggle `print-circle' for the backtrace frame at point.
-With prefix argument ALL, toggle the value of :print-circle in
-`backtrace-view', which affects all of the backtrace frames in
-the buffer."
+With prefix argument ALL, toggle the default value bound to
+`print-circle' for all the frames in the buffer."
   (interactive "P")
   (backtrace--toggle-feature :print-circle all))
 
+(defun backtrace-toggle-print-gensym (&optional all)
+  "Toggle `print-gensym' for the backtrace frame at point.
+With prefix argument ALL, toggle the default value bound to
+`print-gensym' for all the frames in the buffer."
+  (interactive "P")
+  (backtrace--toggle-feature :print-gensym all))
+
 (defun backtrace--toggle-feature (feature all)
   "Toggle FEATURE for the current backtrace frame or for the buffer.
 FEATURE should be one of the options in `backtrace-view'.  If ALL
@@ -450,12 +471,15 @@ position point at the start of the frame it was in before."
           (goto-char (point-min))
           (while (and (not (eql index (backtrace-get-index)))
                       (< (point) (point-max)))
-            (goto-char (backtrace-get-frame-end)))))
-    (let ((index (backtrace-get-index)))
-      (unless index
-        (user-error "Not in a stack frame"))
-      (backtrace--set-feature feature
-                              (not (plist-get (backtrace-get-view) feature))))))
+            (goto-char (backtrace-get-frame-end))))
+        (message "%s is now %s for all frames"
+                 (substring (symbol-name feature) 1) value))
+    (unless (backtrace-get-index)
+      (user-error "Not in a stack frame"))
+    (let ((value (not (plist-get (backtrace-get-view) feature))))
+      (backtrace--set-feature feature value)
+      (message "%s is now %s for this frame"
+               (substring (symbol-name feature) 1) value))))
 
 (defun backtrace--set-feature (feature value)
   "Set FEATURE in the view plist of the frame at point to VALUE.