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.

3 files changed, 86 insertions, 10 deletions

diff --git a/doc/lispref/debugging.texi b/doc/lispref/debugging.texi
index 12caeaf1289..71e767d0a66 100644
--- a/doc/lispref/debugging.texi
+++ b/doc/lispref/debugging.texi
@@ -457,6 +457,9 @@ Collapse the top-level Lisp form at point back to a single line.
 @item #
 Toggle @code{print-circle} for the frame at point.
 
+@item :
+Toggle @code{print-gensym} for the frame at point.
+
 @item .
 Expand all the forms abbreviated with ``...'' in the frame at point.
 
diff --git a/lisp/emacs-lisp/backtrace.el b/lisp/emacs-lisp/backtrace.el
index 60d146e24a8..0c4c7987c3c 100644
--- a/lisp/emacs-lisp/backtrace.el
+++ b/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.
diff --git a/test/lisp/emacs-lisp/backtrace-tests.el b/test/lisp/emacs-lisp/backtrace-tests.el
index ce827e0166f..be154953423 100644
--- a/test/lisp/emacs-lisp/backtrace-tests.el
+++ b/test/lisp/emacs-lisp/backtrace-tests.el
@@ -335,6 +335,55 @@ line contains the strings \"lambda\" and \"number\"."
       (should (string-match-p results
                               (backtrace-tests--get-substring (point-min) (point-max)))))))
 
+(ert-deftest backtrace-tests--print-gensym ()
+  "Backtrace buffers can toggle `print-gensym' syntax."
+  (ert-with-test-buffer (:name "print-gensym")
+    (let* ((print-gensym nil)
+           (arg (list (gensym "first") (gensym) (gensym "last")))
+           (results (backtrace-tests--make-regexp
+                     (backtrace-tests--result arg)))
+           (results-gensym (regexp-quote (let ((print-gensym t))
+                                           (backtrace-tests--result arg))))
+           (last-frame (backtrace-tests--make-regexp
+                        (format (nth (1- backtrace-tests--line-count)
+                                     (backtrace-tests--backtrace-lines))
+                                arg)))
+           (last-frame-gensym (regexp-quote
+                               (let ((print-gensym t))
+                                 (format (nth (1- backtrace-tests--line-count)
+                                              (backtrace-tests--backtrace-lines))
+                                         arg)))))
+      (backtrace-tests--make-backtrace arg)
+      (backtrace-print)
+      (should (string-match-p results
+                              (backtrace-tests--get-substring (point-min) (point-max))))
+      ;; Go to the last frame.
+      (goto-char (point-max))
+      (forward-line -1)
+      ;; Turn on print-gensym for that frame.
+      (backtrace-toggle-print-gensym)
+      (should (string-match-p last-frame-gensym
+                              (backtrace-tests--get-substring (point) (point-max))))
+      ;; Turn off print-gensym for the frame.
+      (backtrace-toggle-print-gensym)
+      (should (string-match-p last-frame
+                              (backtrace-tests--get-substring (point) (point-max))))
+      (should (string-match-p results
+                              (backtrace-tests--get-substring (point-min) (point-max))))
+      ;; Turn print-gensym on for the buffer.
+      (backtrace-toggle-print-gensym '(4))
+      (should (string-match-p last-frame-gensym
+                              (backtrace-tests--get-substring (point) (point-max))))
+      (should (string-match-p results-gensym
+                              (backtrace-tests--get-substring (point-min) (point-max))))
+      ;; Turn print-gensym off.
+      (backtrace-toggle-print-gensym '(4))
+      (should (string-match-p last-frame
+                              (backtrace-tests--get-substring
+                               (point) (+ (point) (length last-frame)))))
+      (should (string-match-p results
+                              (backtrace-tests--get-substring (point-min) (point-max)))))))
+
 (defun backtrace-tests--make-regexp (str)
   "Make regexp from STR for `backtrace-tests--print-circle'.
 Used for results of printing circular objects without