Add backtrace-mode and use it in the debugger, ERT and Edebug

* doc/lispref/debugging.texi (Using Debugger): Remove explanation of
backtrace buffer.  Refer to new node.
(Backtraces): New node.
(Debugger Commands): Refer to new node.  Remove 'v'.
* doc/lispref/edebug.texi (Edebug Misc): Refer to new node.
* doc/misc/ert.texi (Running Tests Interactively): Refer to new node.

* lisp/emacs-lisp-backtrace.el: New file.
* test/lisp/emacs-lisp/backtrace-tests.el: New file.

* lisp/emacs-lisp/debug.el: (debugger-buffer-state): New cl-defstruct.
(debugger--restore-buffer-state): New function.
(debug): Use a debugger-buffer-state object to save and restore buffer
state.  Fix bug#15749 by leaving an unused buffer in debugger-mode,
empty, instead of in fundamental-mode, and then when reusing a buffer,
not calling debugger-mode if the buffer is already in debugger-mode.
(debugger-insert-backtrace): Remove.
(debugger-setup-buffer): Use backtrace-mode.
(debugger--insert-header): New function.
(debugger-continue, debugger-return-value): Change check for flags to
use backtrace-frames.
(debugger-frame-number): Determine backtrace frame number from
backtrace-frames.
(debugger--locals-visible-p, debugger--insert-locals)
(debugger--show-locals, debugger--hide-locals)
(debugger-toggle-locals): Remove.
(debugger-mode-map): Make a child of backtrace-mode-map.  Move
navigation commands to backtrace-mode-map.  Bind 'q' to debugger-quit
instead of top-level.  Make Help Follow menu item call
backtrace-help-follow-symbol.
(debugger-mode): Derive from backtrace-mode.
(debug-help-follow): Remove.  Move body of this function to
'backtrace-help-follow-symbol' in backtrace.el.
(debugger-quit): New function.

* lisp/emacs-lisp/edebug.el (edebug-unwrap-results): Remove warning
in docstring about circular results.
(edebug-unwrap): Use pcase.
(edebug-unwrap1): New function to unwrap circular objects.
(edebug-unwrap*): Use it.
(edebug--frame): New cl-defstruct.
(edebug-backtrace): Call the buffer *Edebug Backtrace* and use
backtrace-mode.  Get the frames from edebug--backtrace-frames.
(edebug--backtrace-frames, edebug--unwrap-and-add-info)
(edebug--symbol-not-prefixed-p): New functions.

* lisp/emacs-lisp/lisp-mode.el
(lisp-el-font-lock-keywords-for-backtraces)
(lisp-el-font-lock-keywords-for-backtraces-1)
(lisp-el-font-lock-keywords-for-backtraces-2): New constants.

* lisp/emacs-lisp/ert.el (ert--print-backtrace): Remove.
(ert--run-test-debugger): Use backtrace-get-frames.
(ert-run-tests-batch): Use backtrace-to-string.
(ert-results-pop-to-backtrace-for-test-at-point): Use backtrace-mode.
(ert--insert-backtrace-header): New function.

* tests/lisp/emacs-lisp/ert-tests.el (ert-test--which-file):
Use backtrace-frame slot accessor.

5 files changed, 1064 insertions, 323 deletions

diff --git a/lisp/emacs-lisp/backtrace.el b/lisp/emacs-lisp/backtrace.el
new file mode 100644
index 00000000000..d16edb6a6cf
--- /dev/null
+++ b/lisp/emacs-lisp/backtrace.el
@@ -0,0 +1,767 @@
+;;; backtrace.el --- generic major mode for Elisp backtraces -*- lexical-binding: t -*-
+
+;; Copyright (C) 2018 Free Software Foundation, Inc.
+
+;; Author: Gemini Lasswell
+;; Keywords: lisp, tools, maint
+;; Version: 1.0
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This file defines Backtrace mode, a generic major mode for displaying
+;; Elisp stack backtraces, which can be used as is or inherited from
+;; by another mode.
+
+;; For usage information, see the documentation of `backtrace-mode'.
+
+;;; Code:
+
+(eval-when-compile (require 'cl-lib))
+(eval-when-compile (require 'pcase))
+(eval-when-compile (require 'subr-x))        ; if-let
+(require 'help-mode)     ; Define `help-function-def' button type.
+(require 'lisp-mode)
+
+;;; Options
+
+(defgroup backtrace nil
+  "Viewing of Elisp backtraces."
+  :group 'lisp)
+
+(defcustom backtrace-fontify t
+  "If non-nil, fontify Backtrace buffers.
+Set to nil to disable fontification, which may be necessary in
+order to debug the code that does fontification."
+  :type 'boolean
+  :group 'backtrace
+  :version "27.1")
+
+(defcustom backtrace-line-length 5000
+  "Target length for lines in Backtrace buffers.
+Backtrace mode will attempt to abbreviate printing of backtrace
+frames to make them shorter than this, but success is not
+guaranteed."
+  :type 'integer
+  :group 'backtrace
+  :version "27.1")
+
+;;; Backtrace frame data structure
+
+(cl-defstruct
+    (backtrace-frame
+     (:constructor backtrace-make-frame))
+  evald fun args flags locals pos)
+
+(cl-defun backtrace-get-frames
+    (&optional base &key (constructor #'backtrace-make-frame))
+  "Collect all frames of current backtrace into a list.
+The list will contain objects made by CONSTRUCTOR, which
+defaults to `backtrace-make-frame' and which, if provided, should
+be the constructor of a structure which includes
+`backtrace-frame'.  If non-nil, BASE should be a function, and
+frames before its nearest activation frame are discarded."
+  (let ((frames nil)
+        (eval-buffers eval-buffer-list))
+    (mapbacktrace (lambda (evald fun args flags)
+                    (push (funcall constructor
+                                   :evald evald :fun fun
+                                   :args args :flags flags)
+                          frames))
+                  (or base 'backtrace-get-frames))
+    (setq frames (nreverse frames))
+    ;; Add local variables to each frame, and the buffer position
+    ;; to frames containing eval-buffer or eval-region.
+    (dotimes (idx (length frames))
+      (let ((frame (nth idx frames)))
+        ;; `backtrace--locals' gives an error when idx is 0.  But the
+        ;; locals for frame 0 are not needed, because when we get here
+        ;; from debug-on-entry, the locals aren't bound yet, and when
+        ;; coming from Edebug or ERT there is an Edebug or ERT
+        ;; function at frame 0.
+        (when (> idx 0)
+          (setf (backtrace-frame-locals frame)
+                (backtrace--locals idx (or base 'backtrace-get-frames))))
+        (when (and eval-buffers (memq (backtrace-frame-fun frame)
+                                      '(eval-buffer eval-region)))
+          ;; This will get the wrong result if there are two nested
+          ;; eval-region calls for the same buffer.  That's not a very
+          ;; useful case.
+          (with-current-buffer (pop eval-buffers)
+            (setf (backtrace-frame-pos frame) (point))))))
+    frames))
+
+;; Font Locking support
+
+(defconst backtrace--font-lock-keywords
+  '((backtrace--match-ellipsis-in-string
+     (1 'button prepend)))
+  "Expressions to fontify in Backtrace mode.
+Fontify these in addition to the expressions Emacs Lisp mode
+fontifies.")
+
+(defconst backtrace-font-lock-keywords
+  (append lisp-el-font-lock-keywords-for-backtraces
+          backtrace--font-lock-keywords)
+  "Default expressions to highlight in Backtrace mode.")
+(defconst backtrace-font-lock-keywords-1
+  (append lisp-el-font-lock-keywords-for-backtraces-1
+          backtrace--font-lock-keywords)
+  "Subdued level highlighting for Backtrace mode.")
+(defconst backtrace-font-lock-keywords-2
+  (append lisp-el-font-lock-keywords-for-backtraces-2
+          backtrace--font-lock-keywords)
+  "Gaudy level highlighting for Backtrace mode.")
+
+(defun backtrace--match-ellipsis-in-string (bound)
+  ;; Fontify ellipses within strings as buttons.
+  (when (re-search-forward "\\(\\.\\.\\.\\)\"" bound t)
+    (and (get-text-property (- (point) 2) 'cl-print-ellipsis)
+         (get-text-property (- (point) 3) 'cl-print-ellipsis)
+         (get-text-property (- (point) 4) 'cl-print-ellipsis))))
+
+;;; Xref support
+
+(defun backtrace--xref-backend () 'elisp)
+
+;;; Backtrace mode variables
+
+(defvar-local backtrace-frames nil
+  "Stack frames displayed in the current Backtrace buffer.
+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, :do-xrefs and :print-circle.")
+
+(defvar-local backtrace-insert-header-function nil
+  "Function for inserting a header for the current Backtrace buffer.
+If nil, no header will be created.  Note that Backtrace buffers
+are fontified as in Emacs Lisp Mode, the header text included.")
+
+(defvar backtrace-revert-hook nil
+  "Hook run before reverting a Backtrace buffer.
+This is commonly used to recompute `backtrace-frames'.")
+
+(defvar-local backtrace-print-function #'cl-prin1
+  "Function used to print values in the current Backtrace buffer.")
+
+(defvar backtrace-mode-map
+  (let ((map (copy-keymap special-mode-map)))
+    (set-keymap-parent map button-buffer-map)
+    (define-key map "n" 'backtrace-forward-frame)
+    (define-key map "p" 'backtrace-backward-frame)
+    (define-key map "v" 'backtrace-toggle-locals)
+    (define-key map "#" 'backtrace-toggle-print-circle)
+    (define-key map "\C-m" 'backtrace-help-follow-symbol)
+    (define-key map "+" 'backtrace-pretty-print)
+    (define-key map "=" 'backtrace-collapse)
+    (define-key map [follow-link] 'mouse-face)
+    (define-key map [mouse-2] 'mouse-select-window)
+    map)
+  "Local keymap for `backtrace-mode' buffers.")
+
+;;; Navigation and Text Properties
+
+;; This mode uses the following text properties:
+;; backtrace-index: The index into the buffer-local variable
+;;   `backtrace-frames' for the frame at point, or nil if outside of a
+;;   frame (in the buffer header).
+;; backtrace-view: A plist describing how the frame is printed.  See
+;;   the docstring for the buffer-local variable `backtrace-view.
+;; backtrace-section: The part of a frame which point is in.  Either
+;;   `func' or `locals'.  At the moment just used to show and hide the
+;;   local variables.  Derived modes which do additional printing
+;;   could define their own frame sections.
+;; backtrace-form: A value applied to each printed representation of a
+;;   top-level s-expression, which needs to be different for sexps
+;;   printed adjacent to each other, so the limits can be quickly
+;;   found for pretty-printing.  The value chosen is a list contining
+;;   the values of print-level and print-length used to print the
+;;   sexp, and those values are used when expanding ellipses.
+
+(defsubst backtrace-get-index (&optional pos)
+  "Return the index of the backtrace frame at POS.
+The value is an index into `backtrace-frames', or nil.
+POS, if omitted or nil, defaults to point."
+  (get-text-property (or pos (point)) 'backtrace-index))
+
+(defsubst backtrace-get-section (&optional pos)
+  "Return the section of a backtrace frame at POS.
+POS, if omitted or nil, defaults to point."
+  (get-text-property (or pos (point)) 'backtrace-section))
+
+(defsubst backtrace-get-view (&optional pos)
+  "Return the view plist of the backtrace frame at POS.
+POS, if omitted or nil, defaults to point."
+  (get-text-property (or pos (point)) 'backtrace-view))
+
+(defsubst backtrace-get-form (&optional pos)
+  "Return the backtrace form data for the form printed at POS.
+POS, if omitted or nil, defaults to point."
+  (get-text-property (or pos (point)) 'backtrace-form))
+
+(defun backtrace-get-frame-start (&optional pos)
+  "Return the beginning position of the frame at POS in the buffer.
+POS, if omitted or nil, defaults to point."
+  (let ((posn (or pos (point))))
+    (if (or (= (point-min) posn)
+            (not (eq (backtrace-get-index posn)
+                     (backtrace-get-index (1- posn)))))
+        posn
+      (previous-single-property-change posn 'backtrace-index nil (point-min)))))
+
+(defun backtrace-get-frame-end (&optional pos)
+  "Return the position of the end of the frame at POS in the buffer.
+POS, if omitted or nil, defaults to point."
+  (next-single-property-change (or pos (point))
+                                    'backtrace-index nil (point-max)))
+
+(defun backtrace-get-section-end (&optional pos)
+  "Return the position of the end of the frame section at POS.
+POS, if omitted or nil, defaults to point."
+  (let* ((frame-end (backtrace-get-frame-end pos))
+         (section-end (next-single-property-change
+                       (or pos (point)) 'backtrace-section nil frame-end)))
+    (min frame-end section-end)))
+
+(defun backtrace-forward-frame ()
+  "Move forward to the beginning of the next frame."
+  (interactive)
+  (let ((max (backtrace-get-frame-end)))
+    (when (= max (point-max))
+      (user-error "No next stack frame"))
+    (goto-char max)))
+
+(defun backtrace-backward-frame ()
+  "Move backward to the start of a stack frame."
+  (interactive)
+  (let ((current-index (backtrace-get-index))
+        (min (backtrace-get-frame-start)))
+    (if (or (and (/= (point) (point-max)) (null current-index))
+            (= min (point-min))
+            (and (= min (point))
+                 (null (backtrace-get-index (1- min)))))
+        (user-error "No previous stack frame"))
+    (if (= min (point))
+        (goto-char (backtrace-get-frame-start (1- min)))
+      (goto-char min))))
+
+;; Other Backtrace mode commands
+
+(defun backtrace-revert (&rest _ignored)
+  "The `revert-buffer-function' for `backtrace-mode'.
+It runs `backtrace-revert-hook', then calls `backtrace-print'."
+  (interactive)
+  (unless (derived-mode-p 'backtrace-mode)
+    (error "The current buffer is not in Backtrace mode"))
+  (run-hooks 'backtrace-revert-hook)
+  (backtrace-print t))
+
+(defun backtrace-toggle-locals ()
+  "Toggle the display of local variables for the backtrace frame at point.
+TODO with argument, toggle all frames."
+  (interactive)
+  (let ((index (backtrace-get-index)))
+    (unless index
+      (user-error "Not in a stack frame"))
+    (let ((pos (point)))
+      (goto-char (backtrace-get-frame-start))
+      (while (and (eq index (backtrace-get-index))
+                  (not (eq (backtrace-get-section) 'locals)))
+        (goto-char (next-single-property-change (point) 'backtrace-section)))
+      (let ((end (backtrace-get-section-end)))
+        (backtrace--set-locals-visible (point) end (invisible-p (point)))
+
+        (goto-char (if (invisible-p pos) end pos))))))
+
+(defun backtrace--set-locals-visible (beg end visible)
+  (backtrace--change-button-skip beg end (not visible))
+  (if visible
+      (remove-overlays beg end 'invisible t)
+    (let ((o (make-overlay beg end)))
+      (overlay-put o 'invisible t)
+      (overlay-put o 'evaporate t))))
+
+(defun backtrace--change-button-skip (beg end value)
+  "Change the skip property on all buttons between BEG and END.
+Set it to VALUE unless the button is a `backtrace-ellipsis' button."
+  (let ((inhibit-read-only t))
+    (setq beg (next-button beg))
+    (while (and beg (< beg end))
+      (unless (eq (button-type beg) 'backtrace-ellipsis)
+          (button-put beg 'skip value))
+      (setq beg (next-button beg)))))
+
+(defun backtrace-toggle-print-circle ()
+  "Toggle `print-circle' for the backtrace frame at point."
+  ;; TODO with argument, toggle the whole buffer.
+  (interactive)
+  (backtrace--toggle-feature :print-circle))
+
+(defun backtrace--toggle-feature (feature)
+  "Toggle FEATURE for the backtrace frame at point.
+FEATURE should be one of the options in `backtrace-view'.
+After toggling the feature, reprint the frame and position
+point at the start of the section of the frame it was in
+before."
+  ;; TODO preserve (in)visibility of locals
+  (let ((index (backtrace-get-index))
+        (view (copy-sequence (backtrace-get-view))))
+    (unless index
+      (user-error "Not in a stack frame"))
+    (setq view (plist-put view feature (not (plist-get view feature))))
+    (let ((inhibit-read-only t)
+          (index (backtrace-get-index))
+          (section (backtrace-get-section))
+          (min (backtrace-get-frame-start))
+          (max (backtrace-get-frame-end)))
+      (delete-region min max)
+      (goto-char min)
+      (backtrace-print-frame (nth index backtrace-frames) view)
+      (add-text-properties min (point)
+                           `(backtrace-index ,index backtrace-view ,view))
+      (goto-char min)
+      (when (not (eq section (backtrace-get-section)))
+        (if-let ((pos (text-property-any (backtrace-get-frame-start)
+                                         (backtrace-get-frame-end)
+                                         'backtrace-section section)))
+            (goto-char pos))))))
+
+(defmacro backtrace--with-output-variables (view &rest body)
+  "Bind output variables according to VIEW and execute BODY."
+  (declare (indent 1))
+  `(let ((print-escape-control-characters t)
+         (print-escape-newlines t)
+         (print-circle (plist-get ,view :print-circle))
+         (standard-output (current-buffer)))
+     ,@body))
+
+(defun backtrace-expand-ellipsis (button)
+  "Expand display of the elided form at BUTTON."
+  ;; TODO a command to expand all ... in form at point
+  ;; with argument, don't bind print-level, length??
+  ;; Enable undo so there's a way to go back?
+  (interactive)
+  (goto-char (button-start button))
+  (unless (get-text-property (point) 'cl-print-ellipsis)
+    (if (and (> (point) (point-min))
+             (get-text-property (1- (point)) 'cl-print-ellipsis))
+        (backward-char)
+      (user-error "No ellipsis to expand here")))
+  (let* ((end (next-single-property-change (point) 'cl-print-ellipsis))
+         (begin (previous-single-property-change end 'cl-print-ellipsis))
+         (value (get-text-property begin 'cl-print-ellipsis))
+         (props (backtrace-get-text-properties begin))
+         (tag (backtrace-get-form begin))
+         (length (nth 0 tag))  ; TODO should this work with a target char count
+         (level (nth 1 tag))   ; like backtrace-print-to-string?
+         (inhibit-read-only t))
+    (backtrace--with-output-variables (backtrace-get-view)
+      (let ((print-level level)
+            (print-length length))
+        (delete-region begin end)
+        (cl-print-expand-ellipsis value (current-buffer))
+        (setq end (point))
+        (goto-char begin)
+        (while (< (point) end)
+          (let ((next (next-single-property-change (point) 'cl-print-ellipsis
+                                                   nil end)))
+            (when (get-text-property (point) 'cl-print-ellipsis)
+              (make-text-button (point) next :type 'backtrace-ellipsis))
+            (goto-char next)))
+        (goto-char begin)
+        (add-text-properties begin end props)))))
+
+(defun backtrace-pretty-print ()
+  "Pretty-print the top level s-expression at point."
+  (interactive)
+  (backtrace--reformat-sexp #'backtrace--pretty-print
+                            "No form here to pretty-print"))
+
+(defun backtrace--pretty-print ()
+  "Pretty print the current buffer, then remove the trailing newline."
+  (set-syntax-table emacs-lisp-mode-syntax-table)
+  (pp-buffer)
+  (goto-char (1- (point-max)))
+  (delete-char 1))
+
+(defun backtrace-collapse ()
+  "Collapse the top level s-expression at point onto one line."
+  (interactive)
+  (backtrace--reformat-sexp #'backtrace--collapse "No form here to collapse"))
+
+(defun backtrace--collapse ()
+  "Replace line breaks and following indentation with spaces.
+Works on the current buffer."
+  (goto-char (point-min))
+  (while (re-search-forward "\n[[:blank:]]*" nil t)
+    (replace-match " ")))
+
+(defun backtrace--reformat-sexp (format-function error-message)
+  "Reformat the top level sexp at point.
+Locate the top level sexp at or following point on the same line,
+and reformat it with FORMAT-FUNCTION, preserving the location of
+point within the sexp.  If no sexp is found before the end of
+the line or buffer, show ERROR-MESSAGE instead.
+
+FORMAT-FUNCTION will be called without arguments, with the
+current buffer set to a temporary buffer containing only the
+content of the sexp."
+  (let* ((orig-pos (point))
+         (pos (point))
+         (tag (backtrace-get-form pos))
+         (end (next-single-property-change pos 'backtrace-form))
+         (begin (previous-single-property-change end 'backtrace-form
+                                                 nil (point-min))))
+    (unless tag
+      (when (or (= end (point-max)) (> end (point-at-eol)))
+        (user-error error-message))
+      (goto-char end)
+      (setq pos end
+            end (next-single-property-change pos 'backtrace-form)
+            begin (previous-single-property-change end 'backtrace-form
+                                                   nil (point-min))))
+    (let* ((offset (when (>= orig-pos begin) (- orig-pos begin)))
+           (offset-marker (when offset (make-marker)))
+           (content (buffer-substring begin end))
+           (props (backtrace-get-text-properties begin))
+           (inhibit-read-only t))
+      (delete-region begin end)
+      (insert (with-temp-buffer
+                (insert content)
+                (when offset
+                  (set-marker-insertion-type offset-marker t)
+                  (set-marker offset-marker (+ (point-min) offset)))
+                (funcall format-function)
+                (when offset
+                  (setq offset (- (marker-position offset-marker) (point-min))))
+                (buffer-string)))
+      (when offset
+        (set-marker offset-marker (+ begin offset)))
+      (save-excursion
+        (goto-char begin)
+        (indent-sexp))
+      (add-text-properties begin (point) props)
+      (if offset
+          (goto-char (marker-position offset-marker))
+        (goto-char orig-pos)))))
+
+(defun backtrace-get-text-properties (pos)
+  "Return a plist of backtrace-mode's text properties at POS."
+  (apply #'append
+         (mapcar (lambda (prop)
+                   (list prop (get-text-property pos prop)))
+                 '(backtrace-section backtrace-index backtrace-view
+                                     backtrace-form))))
+
+(defun backtrace-help-follow-symbol (&optional pos)
+  "Follow cross-reference at POS, defaulting to point.
+For the cross-reference format, see `help-make-xrefs'."
+  (interactive "d")
+  (unless pos
+    (setq pos (point)))
+  (unless (push-button pos)
+    ;; Check if the symbol under point is a function or variable.
+    (let ((sym
+           (intern
+            (save-excursion
+              (goto-char pos) (skip-syntax-backward "w_")
+              (buffer-substring (point)
+                                (progn (skip-syntax-forward "w_")
+                                       (point)))))))
+      (when (or (boundp sym) (fboundp sym) (facep sym))
+        (describe-symbol sym)))))
+
+;; Print backtrace frames
+
+(defun backtrace-print (&optional remember-pos)
+  "Populate the current Backtrace mode buffer.
+This erases the buffer and inserts printed representations of the
+frames.  Optional argument REMEMBER-POS, if non-nil, means to
+move point to the entry with the same ID element as the current
+line and recenter window line accordingly."
+  (let ((inhibit-read-only t)
+        entry-index saved-pt window-line)
+    (and remember-pos
+         (setq entry-index (backtrace-get-index))
+         (when (eq (window-buffer) (current-buffer))
+           (setq window-line
+                 (count-screen-lines (window-start) (point)))))
+    (erase-buffer)
+    (when backtrace-insert-header-function
+      (funcall backtrace-insert-header-function))
+    (dotimes (idx (length backtrace-frames))
+      (let ((beg (point))
+            (elt (nth idx backtrace-frames)))
+        (and entry-index
+             (equal entry-index idx)
+             (setq entry-index nil
+                   saved-pt (point)))
+        (backtrace-print-frame elt backtrace-view)
+        (add-text-properties
+         beg (point)
+         `(backtrace-index ,idx backtrace-view ,backtrace-view))))
+    (set-buffer-modified-p nil)
+    ;; If REMEMBER-POS was specified, move to the "old" location.
+    (if saved-pt
+        (progn (goto-char saved-pt)
+               (when window-line
+                 (recenter window-line)))
+      (goto-char (point-min)))))
+
+;; Define button type used for ...'s.
+;; Set skip property so you don't have to TAB through 100 of them to
+;; get to the next function name.
+(define-button-type 'backtrace-ellipsis
+  'skip t 'action #'backtrace-expand-ellipsis
+  'help-echo "mouse-2, RET: expand this ellipsis")
+
+(defun backtrace-print-to-string (obj &optional limit)
+  "Return a printed representation of OBJ formatted for backtraces.
+Attempt to get the length of the returned string under LIMIT
+charcters with appropriate settings of `print-level' and
+`print-length.'  Attach the settings used with the text property
+`backtrace-form'.  LIMIT defaults to `backtrace-line-length'."
+  (backtrace--with-output-variables backtrace-view
+    (backtrace--print-to-string obj limit)))
+
+(defun backtrace--print-to-string (sexp &optional limit)
+  ;; This is for use by callers who wrap the call with
+  ;; backtrace--with-output-variables.
+  (setq limit (or limit backtrace-line-length))
+  (let* ((length 50)  ; (/ backtrace-line-length 100) ??
+         (level (truncate (log limit)))
+         (delta (truncate (/ length level))))
+    (with-temp-buffer
+       (catch 'done
+         (while t
+           (erase-buffer)
+           (let ((standard-output (current-buffer))
+                 (print-length length)
+                 (print-level level))
+             (backtrace--print sexp))
+           ;; Stop when either the level is too low or the sexp is
+           ;; successfully printed in the space allowed.
+           (when (or (< (- (point-max) (point-min)) limit) (= level 2))
+             (throw 'done nil))
+           (cl-decf level)
+           (cl-decf length delta)))
+       (put-text-property (point-min) (point)
+                          'backtrace-form (list length level))
+       ;; Make buttons from all the "..."s.
+       ;; TODO should this be under control of :do-ellipses in the view
+       ;; plist?
+       (goto-char (point-min))
+       (while (< (point) (point-max))
+         (let ((end (next-single-property-change (point) 'cl-print-ellipsis
+                                                 nil (point-max))))
+           (when (get-text-property (point) 'cl-print-ellipsis)
+             (make-text-button (point) end :type 'backtrace-ellipsis))
+           (goto-char end)))
+       (buffer-string))))
+
+(defun backtrace-print-frame (frame view)
+  "Insert a backtrace FRAME at point formatted according to VIEW.
+Tag the sections of the frame with the `backtrace-section' text
+property for use by navigation."
+  (backtrace--with-output-variables view
+   (backtrace--print-flags frame view)
+   (backtrace--print-func-and-args frame view)
+   (backtrace--print-locals frame view)))
+
+(defun backtrace--print-flags (frame view)
+  "Print the flags of a backtrace FRAME if enabled in VIEW."
+  (let ((beg (point))
+        (flag (plist-get (backtrace-frame-flags frame) :debug-on-exit)))
+    (insert (if (and (plist-get view :show-flags) flag) "* " "  "))
+    (put-text-property beg (point) 'backtrace-section 'func)))
+
+(defun backtrace--print-func-and-args (frame view)
+  "Print the function, arguments and buffer position of a backtrace FRAME.
+Format it according to VIEW."
+  (let* ((beg (point))
+         (evald (backtrace-frame-evald frame))
+         (fun   (backtrace-frame-fun frame))
+         (args  (backtrace-frame-args frame))
+         (fun-file (and (plist-get view :do-xrefs) (symbol-file fun 'defun)))
+         (fun-pt (point)))
+    (cond
+     ((and evald (not debugger-stack-frame-as-list))
+      (if (atom fun)
+          (funcall backtrace-print-function fun)
+        (insert
+         (backtrace--print-to-string fun (when args (/ backtrace-line-length 2)))))
+      (if args
+          (insert (backtrace--print-to-string
+                   args (max (truncate (/ backtrace-line-length 5))
+                             (- backtrace-line-length (- (point) beg)))))
+        ;; The backtrace-form property is so that
+        ;; backtrace-pretty-print will find it.
+        ;; backtrace-pretty-print doesn't do anything useful with it,
+        ;; just being consistent.
+        (let ((start (point)))
+          (insert "()")
+          (put-text-property start (point) 'backtrace-form t))))
+     (t
+      (let ((fun-and-args (cons fun args)))
+        (insert (backtrace--print-to-string fun-and-args)))
+      (cl-incf fun-pt)))
+    (when fun-file
+      (make-text-button fun-pt (+ fun-pt (length (symbol-name fun)))
+                        :type 'help-function-def
+                        'help-args (list fun fun-file)))
+    ;; After any frame that uses eval-buffer, insert a comment that
+    ;; states the buffer position it's reading at.
+    (when (backtrace-frame-pos frame)
+      (insert (format "  ; Reading at buffer position %d"
+                      (backtrace-frame-pos frame))))
+    (insert "\n")
+    (put-text-property beg (point) 'backtrace-section 'func)))
+
+(defun backtrace--print-locals (frame _view)
+  "Print a backtrace FRAME's local variables.
+Make them invisible initially."
+  (let* ((beg (point))
+         (locals (backtrace-frame-locals frame)))
+    (if (null locals)
+        (insert "    [no locals]\n")
+      (pcase-dolist (`(,symbol . ,value) locals)
+        (insert "    ")
+        (backtrace--print symbol)
+        (insert " = ")
+        (insert (backtrace--print-to-string value))
+        (insert "\n")))
+    (put-text-property beg (point) 'backtrace-section 'locals)
+    (backtrace--set-locals-visible beg (point) nil)))
+
+(defun backtrace--print (obj)
+  "Attempt to print OBJ using `backtrace-print-function'.
+Fall back to `prin1' if there is an error."
+  (condition-case err
+      (funcall backtrace-print-function obj)
+    (error
+     (message "Error in backtrace printer: %S" err)
+     (prin1 obj))))
+
+(defun backtrace-update-flags ()
+  "Update the display of the flags in the backtrace frame at point."
+  (let ((view (backtrace-get-view))
+        (begin (backtrace-get-frame-start)))
+    (when (plist-get view :show-flags)
+      (save-excursion
+        (goto-char begin)
+        (let ((props (backtrace-get-text-properties begin))
+              (inhibit-read-only t)
+              (standard-output (current-buffer)))
+          (delete-char 2)
+          (backtrace--print-flags (nth (backtrace-get-index) backtrace-frames)
+                                  view)
+          (add-text-properties begin (point) props))))))
+
+(defun backtrace--filter-visible (beg end &optional _delete)
+  "Return the visible text between BEG and END."
+  (let ((result ""))
+    (while (< beg end)
+      (let ((next (next-single-char-property-change beg 'invisible)))
+        (unless (get-char-property beg 'invisible)
+          (setq result (concat result (buffer-substring beg (min end next)))))
+        (setq beg next)))
+    result))
+
+;;; The mode definition
+
+(define-derived-mode backtrace-mode special-mode "Backtrace"
+  "Generic major mode for examining an Elisp stack backtrace.
+This mode can be used directly, or other major modes can be
+derived from it, using `define-derived-mode'.
+
+In this major mode, the buffer contains some optional lines of
+header text followed by backtrace frames, each consisting of one
+or more whole lines.
+
+Letters in this mode do not insert themselves; instead they are
+commands.
+\\<backtrace-mode-map>
+\\{backtrace-mode-map}
+
+A mode which inherits from Backtrace mode, or a command which
+creates a backtrace-mode buffer, should usually do the following:
+
+ - Set `backtrace-revert-hook', if the buffer contents need
+   to be specially recomputed prior to `revert-buffer'.
+ - Maybe set `backtrace-insert-header-function' to a function to create
+   header text for the buffer.
+ - Set `backtrace-frames' (see below).
+ - Set `backtrace-view' if desired (see below).
+ - Maybe set `backtrace-print-function'.
+
+A command which creates or switches to a Backtrace mode buffer,
+such as `ert-results-pop-to-backtrace-for-test-at-point', should
+initialize `backtrace-frames' to a list of `backtrace-frame'
+objects (`backtrace-get-frames' is provided for that purpose, if
+desired), and `backtrace-view' to a plist describing how it wants
+the backtrace to appear.  Finally, it should call `backtrace-print'.
+
+`backtrace-print' calls `backtrace-insert-header-function'
+followed by `backtrace-print-frame', once for each stack frame."
+  :syntax-table emacs-lisp-mode-syntax-table
+  (when backtrace-fontify
+    (setq font-lock-defaults
+          '((backtrace-font-lock-keywords
+             backtrace-font-lock-keywords-1
+             backtrace-font-lock-keywords-2)
+            nil nil nil nil
+            ;; TODO This one doesn't look necessary:
+            ;; (font-lock-mark-block-function . mark-defun)
+            (font-lock-syntactic-face-function
+             . lisp-font-lock-syntactic-face-function))))
+  (setq truncate-lines t)
+  (buffer-disable-undo)
+  ;; In debug.el, from 1998 to 2009 this was set to nil, reason stated
+  ;; was because of bytecode. Since 2009 it's been set to t, but the
+  ;; default is t so I think this isn't necessary.
+  ;; (set-buffer-multibyte t)
+  (setq-local revert-buffer-function #'backtrace-revert)
+  (setq-local filter-buffer-substring-function #'backtrace--filter-visible)
+  (add-hook 'xref-backend-functions #'backtrace--xref-backend nil t))
+
+(put 'backtrace-mode 'mode-class 'special)
+
+;;; Backtrace printing
+
+(defun backtrace-backtrace ()
+  "Print a trace of Lisp function calls currently active.
+Output stream used is value of `standard-output'."
+  (princ (backtrace-to-string (backtrace-get-frames 'backtrace-backtrace))))
+
+(defun backtrace-to-string(frames)
+  "Format FRAMES, a list of `backtrace-frame' objects, for output.
+Return the result as a string."
+  (let ((backtrace-fontify nil))
+    (with-temp-buffer
+      (backtrace-mode)
+      (setq backtrace-view '(:show-flags t)
+            backtrace-frames frames
+            backtrace-print-function #'cl-prin1)
+      (backtrace-print)
+      (substring-no-properties (filter-buffer-substring (point-min)
+                                                        (point-max))))))
+
+(provide 'backtrace)
+
+;;; backtrace.el ends here
diff --git a/lisp/emacs-lisp/debug.el b/lisp/emacs-lisp/debug.el
index 0efaa637129..707e0cfa186 100644
--- a/lisp/emacs-lisp/debug.el
+++ b/lisp/emacs-lisp/debug.el
@@ -28,6 +28,7 @@
 ;;; Code:
 
 (require 'cl-lib)
+(require 'backtrace)
 (require 'button)
 
 (defgroup debugger nil
@@ -133,6 +134,25 @@ where CAUSE can be:
 - exit: called because of exit of a flagged function.
 - error: called because of `debug-on-error'.")
 
+(cl-defstruct (debugger--buffer-state
+            (:constructor debugger--save-buffer-state
+                          (&aux (mode     major-mode)
+                                (header   backtrace-insert-header-function)
+                                (frames   backtrace-frames)
+                                (content  (buffer-string))
+                                (pos      (point)))))
+  mode header frames content pos)
+
+(defun debugger--restore-buffer-state (state)
+  (unless (derived-mode-p (debugger--buffer-state-mode state))
+    (funcall (debugger--buffer-state-mode state)))
+  (setq backtrace-insert-header-function (debugger--buffer-state-header state)
+        backtrace-frames (debugger--buffer-state-frames state))
+  (let ((inhibit-read-only t))
+    (erase-buffer)
+    (insert (debugger--buffer-state-content state)))
+  (goto-char (debugger--buffer-state-pos state)))
+
 ;;;###autoload
 (setq debugger 'debug)
 ;;;###autoload
@@ -174,7 +194,7 @@ first will be printed into the backtrace buffer."
           (debugger-previous-state
            (if (get-buffer "*Backtrace*")
                (with-current-buffer (get-buffer "*Backtrace*")
-                 (list major-mode (buffer-string)))))
+                 (debugger--save-buffer-state))))
           (debugger-args args)
           (debugger-buffer (get-buffer-create "*Backtrace*"))
           (debugger-old-buffer (current-buffer))
@@ -236,7 +256,8 @@ first will be printed into the backtrace buffer."
                             (window-total-height debugger-window)))
                       (error nil)))
                 (setq debugger-previous-window debugger-window))
-              (debugger-mode)
+              (unless (derived-mode-p 'debugger-mode)
+                (debugger-mode))
               (debugger-setup-buffer debugger-args)
               (when noninteractive
                 ;; If the backtrace is long, save the beginning
@@ -280,15 +301,14 @@ first will be printed into the backtrace buffer."
             (setq debugger-previous-window nil))
           ;; Restore previous state of debugger-buffer in case we were
           ;; in a recursive invocation of the debugger, otherwise just
-          ;; erase the buffer and put it into fundamental mode.
+          ;; erase the buffer.
           (when (buffer-live-p debugger-buffer)
             (with-current-buffer debugger-buffer
-              (let ((inhibit-read-only t))
-                (erase-buffer)
-                (if (null debugger-previous-state)
-                    (fundamental-mode)
-                  (insert (nth 1 debugger-previous-state))
-                  (funcall (nth 0 debugger-previous-state))))))
+              (if debugger-previous-state
+                  (debugger--restore-buffer-state debugger-previous-state)
+                (setq backtrace-insert-header-function nil)
+                (setq backtrace-frames nil)
+                (backtrace-print))))
           (with-timeout-unsuspend debugger-with-timeout-suspend)
           (set-match-data debugger-outer-match-data)))
       (setq debug-on-next-call debugger-step-after-exit)
@@ -301,112 +321,80 @@ first will be printed into the backtrace buffer."
      (message "Error in debug printer: %S" err)
      (prin1 obj stream))))
 
-(defun debugger-insert-backtrace (frames do-xrefs)
-  "Format and insert the backtrace FRAMES at point.
-Make functions into cross-reference buttons if DO-XREFS is non-nil."
-  (let ((standard-output (current-buffer))
-        (eval-buffers eval-buffer-list))
-    (require 'help-mode)     ; Define `help-function-def' button type.
-    (pcase-dolist (`(,evald ,fun ,args ,flags) frames)
-      (insert (if (plist-get flags :debug-on-exit)
-                  "* " "  "))
-      (let ((fun-file (and do-xrefs (symbol-file fun 'defun)))
-            (fun-pt (point)))
-        (cond
-         ((and evald (not debugger-stack-frame-as-list))
-          (debugger--print fun)
-          (if args (debugger--print args) (princ "()")))
-         (t
-          (debugger--print (cons fun args))
-          (cl-incf fun-pt)))
-        (when fun-file
-          (make-text-button fun-pt (+ fun-pt (length (symbol-name fun)))
-                            :type 'help-function-def
-                            'help-args (list fun fun-file))))
-      ;; After any frame that uses eval-buffer, insert a line that
-      ;; states the buffer position it's reading at.
-      (when (and eval-buffers (memq fun '(eval-buffer eval-region)))
-        (insert (format "  ; Reading at buffer position %d"
-                        ;; This will get the wrong result if there are
-                        ;; two nested eval-region calls for the same
-                        ;; buffer.  That's not a very useful case.
-                        (with-current-buffer (pop eval-buffers)
-                          (point)))))
-      (insert "\n"))))
-
 (defun debugger-setup-buffer (args)
   "Initialize the `*Backtrace*' buffer for entry to the debugger.
-That buffer should be current already."
-  (setq buffer-read-only nil)
-  (erase-buffer)
-  (set-buffer-multibyte t)              ;Why was it nil ?  -stef
-  (setq buffer-undo-list t)
+That buffer should be current already and in debugger-mode."
+  (setq backtrace-frames (nthcdr
+                          ;; Remove debug--implement-debug-on-entry and the
+                          ;; advice's `apply' frame.
+                          (if (eq (car args) 'debug) 3 1)
+                          (backtrace-get-frames 'debug)))
+  (when (eq (car-safe args) 'exit)
+    (setq debugger-value (nth 1 args))
+    (setf (cl-getf (backtrace-frame-flags (car backtrace-frames))
+                   :debug-on-exit)
+          nil))
+
+  (setq backtrace-view '(:do-xrefs t :show-flags t)
+        backtrace-insert-header-function (lambda ()
+                                           (debugger--insert-header args))
+        backtrace-print-function debugger-print-function)
+  (backtrace-print)
+  ;; Place point on "stack frame 0" (bug#15101).
+  (goto-char (point-min))
+  (search-forward ":" (line-end-position) t)
+  (when (and (< (point) (line-end-position))
+             (= (char-after) ?\s))
+    (forward-char)))
+
+(defun debugger--insert-header (args)
+  "Insert the header for the debugger's Backtrace buffer.
+Include the reason for debugger entry from ARGS."
   (insert "Debugger entered")
-  (let ((frames (nthcdr
-                 ;; Remove debug--implement-debug-on-entry and the
-                 ;; advice's `apply' frame.
-                 (if (eq (car args) 'debug) 3 1)
-                 (backtrace-frames 'debug)))
-        (print-escape-newlines t)
-        (print-escape-control-characters t)
-        ;; If you increase print-level, add more depth in call_debugger.
-        (print-level 8)
-        (print-length 50)
-        (pos (point)))
-    (pcase (car args)
-      ;; lambda is for debug-on-call when a function call is next.
-      ;; debug is for debug-on-entry function called.
-      ((or `lambda `debug)
-       (insert "--entering a function:\n")
-       (setq pos (1- (point))))
-      ;; Exiting a function.
-      (`exit
-       (insert "--returning value: ")
-       (setq pos (point))
-       (setq debugger-value (nth 1 args))
-       (debugger--print debugger-value (current-buffer))
-       (setf (cl-getf (nth 3 (car frames)) :debug-on-exit) nil)
-       (insert ?\n))
-      ;; Watchpoint triggered.
-      ((and `watchpoint (let `(,symbol ,newval . ,details) (cdr args)))
-       (insert
-        "--"
-        (pcase details
-          (`(makunbound nil) (format "making %s void" symbol))
-          (`(makunbound ,buffer) (format "killing local value of %s in buffer %s"
-                                         symbol buffer))
-          (`(defvaralias ,_) (format "aliasing %s to %s" symbol newval))
-          (`(let ,_) (format "let-binding %s to %S" symbol newval))
-          (`(unlet ,_) (format "ending let-binding of %s" symbol))
-          (`(set nil) (format "setting %s to %S" symbol newval))
-          (`(set ,buffer) (format "setting %s in buffer %s to %S"
-                                  symbol buffer newval))
-          (_ (error "unrecognized watchpoint triggered %S" (cdr args))))
-        ": ")
-       (setq pos (point))
-       (insert ?\n))
-      ;; Debugger entered for an error.
-      (`error
-       (insert "--Lisp error: ")
-       (setq pos (point))
-       (debugger--print (nth 1 args) (current-buffer))
-       (insert ?\n))
-      ;; debug-on-call, when the next thing is an eval.
-      (`t
-       (insert "--beginning evaluation of function call form:\n")
-       (setq pos (1- (point))))
-      ;; User calls debug directly.
-      (_
-       (insert ": ")
-       (setq pos (point))
-       (debugger--print
-                (if (eq (car args) 'nil)
-                    (cdr args) args)
-                (current-buffer))
-       (insert ?\n)))
-    (debugger-insert-backtrace frames t)
-    ;; Place point on "stack frame 0" (bug#15101).
-    (goto-char pos)))
+  (pcase (car args)
+    ;; lambda is for debug-on-call when a function call is next.
+    ;; debug is for debug-on-entry function called.
+    ((or `lambda `debug)
+     (insert "--entering a function:\n"))
+    ;; Exiting a function.
+    (`exit
+     (insert "--returning value: ")
+     (insert (backtrace-print-to-string debugger-value))
+     (insert ?\n))
+    ;; Watchpoint triggered.
+    ((and `watchpoint (let `(,symbol ,newval . ,details) (cdr args)))
+     (insert
+      "--"
+      (pcase details
+        (`(makunbound nil) (format "making %s void" symbol))
+        (`(makunbound ,buffer) (format "killing local value of %s in buffer %s"
+                                       symbol buffer))
+        (`(defvaralias ,_) (format "aliasing %s to %s" symbol newval))
+        (`(let ,_) (format "let-binding %s to %s" symbol
+                           (backtrace-print-to-string newval)))
+        (`(unlet ,_) (format "ending let-binding of %s" symbol))
+        (`(set nil) (format "setting %s to %s" symbol
+                            (backtrace-print-to-string newval)))
+        (`(set ,buffer) (format "setting %s in buffer %s to %s"
+                                symbol buffer
+                                (backtrace-print-to-string newval)))
+        (_ (error "unrecognized watchpoint triggered %S" (cdr args))))
+      ": ")
+     (insert ?\n))
+    ;; Debugger entered for an error.
+    (`error
+     (insert "--Lisp error: ")
+     (insert (backtrace-print-to-string (nth 1 args)))
+     (insert ?\n))
+    ;; debug-on-call, when the next thing is an eval.
+    (`t
+     (insert "--beginning evaluation of function call form:\n"))
+    ;; User calls debug directly.
+    (_
+     (insert ": ")
+     (insert (backtrace-print-to-string (if (eq (car args) 'nil)
+                                            (cdr args) args)))
+     (insert ?\n))))
 
 
 (defun debugger-step-through ()
@@ -426,12 +414,12 @@ Enter another debugger on next entry to eval, apply or funcall."
   (unless debugger-may-continue
     (error "Cannot continue"))
   (message "Continuing.")
-  (save-excursion
-    ;; Check to see if we've flagged some frame for debug-on-exit, in which
-    ;; case we'll probably come back to the debugger soon.
-    (goto-char (point-min))
-    (if (re-search-forward "^\\* " nil t)
-        (setq debugger-will-be-back t)))
+
+  ;; Check to see if we've flagged some frame for debug-on-exit, in which
+  ;; case we'll probably come back to the debugger soon.
+  (dolist (frame backtrace-frames)
+    (when (plist-get (backtrace-frame-flags frame) :debug-on-exit)
+      (setq debugger-will-be-back t)))
   (exit-recursive-edit))
 
 (defun debugger-return-value (val)
@@ -446,12 +434,11 @@ will be used, such as in a debug on exit from a frame."
   (setq debugger-value val)
   (princ "Returning " t)
   (debugger--print debugger-value)
-  (save-excursion
     ;; Check to see if we've flagged some frame for debug-on-exit, in which
     ;; case we'll probably come back to the debugger soon.
-    (goto-char (point-min))
-    (if (re-search-forward "^\\* " nil t)
-        (setq debugger-will-be-back t)))
+  (dolist (frame backtrace-frames)
+    (when (plist-get (backtrace-frame-flags frame) :debug-on-exit)
+      (setq debugger-will-be-back t)))
   (exit-recursive-edit))
 
 (defun debugger-jump ()
@@ -473,63 +460,40 @@ removes itself from that hook."
 
 (defun debugger-frame-number (&optional skip-base)
   "Return number of frames in backtrace before the one point points at."
-  (save-excursion
-    (beginning-of-line)
-    (if (looking-at " *;;;\\|[a-z]")
-        (error "This line is not a function call"))
-    (let ((opoint (point))
-          (count 0))
-      (unless skip-base
+  (let ((index (backtrace-get-index))
+        (count 0))
+    (unless index
+      (error "This line is not a function call"))
+    (unless skip-base
         (while (not (eq (cadr (backtrace-frame count)) 'debug))
           (setq count (1+ count)))
         ;; Skip debug--implement-debug-on-entry frame.
         (when (eq 'debug--implement-debug-on-entry
                   (cadr (backtrace-frame (1+ count))))
           (setq count (+ 2 count))))
-      (goto-char (point-min))
-      (when (looking-at "Debugger entered--\\(Lisp error\\|returning value\\):")
-        (goto-char (match-end 0))
-        (forward-sexp 1))
-      (forward-line 1)
-      (while (progn
-               (forward-char 2)
-               (cond ((debugger--locals-visible-p)
-                      (goto-char (next-single-char-property-change
-                                  (point) 'locals-visible)))
-                     ((= (following-char) ?\()
-                      (forward-sexp 1))
-                     (t
-                      (forward-sexp 2)))
-               (forward-line 1)
-               (<= (point) opoint))
-        (if (looking-at " *;;;")
-            (forward-line 1))
-        (setq count (1+ count)))
-      count)))
+    (+ count index)))
 
 (defun debugger-frame ()
   "Request entry to debugger when this frame exits.
 Applies to the frame whose line point is on in the backtrace."
   (interactive)
   (backtrace-debug (debugger-frame-number) t)
-  (beginning-of-line)
-  (if (= (following-char) ? )
-      (let ((inhibit-read-only t))
-        (delete-char 1)
-        (insert ?*)))
-  (beginning-of-line))
+  (setf
+   (cl-getf (backtrace-frame-flags (nth (backtrace-get-index) backtrace-frames))
+            :debug-on-exit)
+   t)
+  (backtrace-update-flags))
 
 (defun debugger-frame-clear ()
   "Do not enter debugger when this frame exits.
 Applies to the frame whose line point is on in the backtrace."
   (interactive)
   (backtrace-debug (debugger-frame-number) nil)
-  (beginning-of-line)
-  (if (= (following-char) ?*)
-      (let ((inhibit-read-only t))
-        (delete-char 1)
-        (insert ? )))
-  (beginning-of-line))
+  (setf
+   (cl-getf (backtrace-frame-flags (nth (backtrace-get-index) backtrace-frames))
+            :debug-on-exit)
+   nil)
+  (backtrace-update-flags))
 
 (defmacro debugger-env-macro (&rest body)
   "Run BODY in original environment."
@@ -564,69 +528,11 @@ The environment used is the one when entering the activation frame at point."
           (let ((str (eval-expression-print-format val)))
             (if str (princ str t))))))))
 
-(defun debugger--locals-visible-p ()
-  "Are the local variables of the current stack frame visible?"
-  (save-excursion
-    (move-to-column 2)
-    (get-text-property (point) 'locals-visible)))
-
-(defun debugger--insert-locals (locals)
-  "Insert the local variables LOCALS at point."
-  (cond ((null locals)
-         (insert "\n    [no locals]"))
-        (t
-         (let ((print-escape-newlines t))
-           (dolist (s+v locals)
-             (let ((symbol (car s+v))
-                   (value (cdr s+v)))
-               (insert "\n    ")
-               (prin1 symbol (current-buffer))
-               (insert " = ")
-               (debugger--print value (current-buffer))))))))
-
-(defun debugger--show-locals ()
-  "For the frame at point, insert locals and add text properties."
-  (let* ((nframe (1+ (debugger-frame-number 'skip-base)))
-         (base (debugger--backtrace-base))
-         (locals (backtrace--locals nframe base))
-         (inhibit-read-only t))
-    (save-excursion
-      (let ((start (progn
-                     (move-to-column 2)
-                     (point))))
-        (end-of-line)
-        (debugger--insert-locals locals)
-        (add-text-properties start (point) '(locals-visible t))))))
-
-(defun debugger--hide-locals ()
-  "Delete local variables and remove the text property."
-  (let* ((col (current-column))
-         (end (progn
-                (move-to-column 2)
-                (next-single-char-property-change (point) 'locals-visible)))
-         (start (previous-single-char-property-change end 'locals-visible))
-         (inhibit-read-only t))
-    (remove-text-properties start end '(locals-visible))
-    (goto-char start)
-    (end-of-line)
-    (delete-region (point) end)
-    (move-to-column col)))
-
-(defun debugger-toggle-locals ()
-  "Show or hide local variables of the current stack frame."
-  (interactive)
-  (cond ((debugger--locals-visible-p)
-         (debugger--hide-locals))
-        (t
-         (debugger--show-locals))))
-
 
 (defvar debugger-mode-map
   (let ((map (make-keymap))
         (menu-map (make-sparse-keymap)))
-    (set-keymap-parent map button-buffer-map)
-    (suppress-keymap map)
-    (define-key map "-" 'negative-argument)
+    (set-keymap-parent map backtrace-mode-map)
     (define-key map "b" 'debugger-frame)
     (define-key map "c" 'debugger-continue)
     (define-key map "j" 'debugger-jump)
@@ -634,24 +540,20 @@ The environment used is the one when entering the activation frame at point."
     (define-key map "u" 'debugger-frame-clear)
     (define-key map "d" 'debugger-step-through)
     (define-key map "l" 'debugger-list-functions)
-    (define-key map "h" 'describe-mode)
-    (define-key map "q" 'top-level)
+    (define-key map "q" 'debugger-quit)
     (define-key map "e" 'debugger-eval-expression)
-    (define-key map "v" 'debugger-toggle-locals) ; "v" is for "variables".
-    (define-key map " " 'next-line)
     (define-key map "R" 'debugger-record-expression)
-    (define-key map "\C-m" 'debug-help-follow)
     (define-key map [mouse-2] 'push-button)
     (define-key map [menu-bar debugger] (cons "Debugger" menu-map))
     (define-key menu-map [deb-top]
-      '(menu-item "Quit" top-level
+      '(menu-item "Quit" debugger-quit
                   :help "Quit debugging and return to top level"))
     (define-key menu-map [deb-s0] '("--"))
     (define-key menu-map [deb-descr]
       '(menu-item "Describe Debugger Mode" describe-mode
                   :help "Display documentation for debugger-mode"))
     (define-key menu-map [deb-hfol]
-      '(menu-item "Help Follow" debug-help-follow
+      '(menu-item "Help Follow" backtrace-help-follow-symbol
                   :help "Follow cross-reference"))
     (define-key menu-map [deb-nxt]
       '(menu-item "Next Line" next-line
@@ -689,8 +591,8 @@ The environment used is the one when entering the activation frame at point."
 
 (put 'debugger-mode 'mode-class 'special)
 
-(define-derived-mode debugger-mode fundamental-mode "Debugger"
-  "Mode for backtrace buffers, selected in debugger.
+(define-derived-mode debugger-mode backtrace-mode "Debugger"
+  "Mode for debugging Emacs Lisp using a backtrace.
 \\<debugger-mode-map>
 A line starts with `*' if exiting that frame will call the debugger.
 Type \\[debugger-frame] or \\[debugger-frame-clear] to set or remove the `*'.
@@ -704,8 +606,6 @@ which functions will enter the debugger when called.
 
 Complete list of commands:
 \\{debugger-mode-map}"
-  (setq truncate-lines t)
-  (set-syntax-table emacs-lisp-mode-syntax-table)
   (add-hook 'kill-buffer-hook
             (lambda () (if (> (recursion-depth) 0) (top-level)))
             nil t)
@@ -732,27 +632,6 @@ Complete list of commands:
              (buffer-substring (line-beginning-position 0)
                                (line-end-position 0)))))
 
-(defun debug-help-follow (&optional pos)
-  "Follow cross-reference at POS, defaulting to point.
-
-For the cross-reference format, see `help-make-xrefs'."
-  (interactive "d")
-  ;; Ideally we'd just do (call-interactively 'help-follow) except that this
-  ;; assumes we're already in a *Help* buffer and reuses it, so it ends up
-  ;; incorrectly "reusing" the *Backtrace* buffer to show the help info.
-  (unless pos
-    (setq pos (point)))
-  (unless (push-button pos)
-    ;; check if the symbol under point is a function or variable
-    (let ((sym
-           (intern
-            (save-excursion
-              (goto-char pos) (skip-syntax-backward "w_")
-              (buffer-substring (point)
-                                (progn (skip-syntax-forward "w_")
-                                       (point)))))))
-      (when (or (boundp sym) (fboundp sym) (facep sym))
-        (describe-symbol sym)))))
 
 ;; When you change this, you may also need to change the number of
 ;; frames that the debugger skips.
@@ -853,6 +732,13 @@ To specify a nil argument interactively, exit with an empty minibuffer."
           ;;(princ "be set to debug on entry, even if it is in the list.")
           )))))
 
+(defun debugger-quit ()
+  "Quit debugging and return to the top level."
+  (interactive)
+  (if (= (recursion-depth) 0)
+      (quit-window)
+    (top-level)))
+
 (defun debug--implement-debug-watch (symbol newval op where)
   "Conditionally call the debugger.
 This function is called when SYMBOL's value is modified."
diff --git a/lisp/emacs-lisp/edebug.el b/lisp/emacs-lisp/edebug.el
index f0c0db182ed..b22c8952da0 100644
--- a/lisp/emacs-lisp/edebug.el
+++ b/lisp/emacs-lisp/edebug.el
@@ -52,6 +52,7 @@
 
 ;;; Code:
 
+(require 'backtrace)
 (require 'macroexp)
 (require 'cl-lib)
 (eval-when-compile (require 'pcase))
@@ -206,8 +207,7 @@ Use this with caution since it is not debugged."
   "Non-nil if Edebug should unwrap results of expressions.
 That is, Edebug will try to remove its own instrumentation from the result.
 This is useful when debugging macros where the results of expressions
-are instrumented expressions.  But don't do this when results might be
-circular or an infinite loop will result."
+are instrumented expressions."
   :type 'boolean
   :group 'edebug)
 
@@ -1265,25 +1265,59 @@ purpose by adding an entry to this alist, and setting
 (defun edebug-unwrap (sexp)
   "Return the unwrapped SEXP or return it as is if it is not wrapped.
 The SEXP might be the result of wrapping a body, which is a list of
-expressions; a `progn' form will be returned enclosing these forms."
-  (if (consp sexp)
-      (cond
-       ((eq 'edebug-after (car sexp))
-        (nth 3 sexp))
-       ((eq 'edebug-enter (car sexp))
-        (macroexp-progn (nthcdr 2 (nth 1 (nth 3 sexp)))))
-       (t sexp);; otherwise it is not wrapped, so just return it.
-       )
-    sexp))
+expressions; a `progn' form will be returned enclosing these forms.
+Does not unwrap inside vectors, records, structures, or hash tables."
+  (pcase sexp
+    (`(edebug-after ,_before-form ,_after-index ,form)
+     form)
+    (`(lambda ,args (edebug-enter ',_sym ,_arglist
+                                   (function (lambda nil . ,body))))
+     `(lambda ,args ,@body))
+    (`(closure ,env ,args (edebug-enter ',_sym ,_arglist
+                                         (function (lambda nil . ,body))))
+     `(closure ,env ,args ,@body))
+    (`(edebug-enter ',_sym ,_args (function (lambda nil . ,body)))
+     (macroexp-progn body))
+    (_ sexp)))
 
 (defun edebug-unwrap* (sexp)
   "Return the SEXP recursively unwrapped."
+  (let ((ht (make-hash-table :test 'eq)))
+    (edebug--unwrap1 sexp ht)))
+
+(defun edebug--unwrap1 (sexp hash-table)
+  "Unwrap SEXP using HASH-TABLE of things already unwrapped.
+HASH-TABLE contains the results of unwrapping cons cells within
+SEXP, which are reused to avoid infinite loops when SEXP is or
+contains a circular object."
   (let ((new-sexp (edebug-unwrap sexp)))
     (while (not (eq sexp new-sexp))
       (setq sexp new-sexp
             new-sexp (edebug-unwrap sexp)))
     (if (consp new-sexp)
-        (mapcar #'edebug-unwrap* new-sexp)
+        (let ((result (gethash new-sexp hash-table nil)))
+          (unless result
+            (let ((remainder new-sexp)
+                  current)
+              (setq result (cons nil nil)
+                    current result)
+              (while
+                  (progn
+                    (puthash remainder current hash-table)
+                    (setf (car current)
+                          (edebug--unwrap1 (car remainder) hash-table))
+                    (setq remainder (cdr remainder))
+                    (cond
+                     ((atom remainder)
+                      (setf (cdr current)
+                            (edebug--unwrap1 remainder hash-table))
+                      nil)
+                     ((gethash remainder hash-table nil)
+                      (setf (cdr current) (gethash remainder hash-table nil))
+                      nil)
+                     (t (setq current
+                              (setf (cdr current) (cons nil nil)))))))))
+          result)
       new-sexp)))
 
 
@@ -3916,8 +3950,10 @@ Global commands prefixed by `global-edebug-prefix':
 ;; (setq debugger 'debug)  ; use the standard debugger
 
 ;; Note that debug and its utilities must be byte-compiled to work,
-;; since they depend on the backtrace looking a certain way.  But
-;; edebug is not dependent on this, yet.
+;; since they depend on the backtrace looking a certain way.  Edebug
+;; will work if not byte-compiled, but it will not be able correctly
+;; remove its instrumentation from backtraces unless it is
+;; byte-compiled.
 
 (defun edebug (&optional arg-mode &rest args)
   "Replacement for `debug'.
@@ -3947,48 +3983,96 @@ Otherwise call `debug' normally."
     (apply #'debug arg-mode args)
     ))
 
+;;; Backtrace buffer
+
+;; Data structure for backtrace frames with information
+;; from Edebug instrumentation found in the backtrace.
+(cl-defstruct
+    (edebug--frame
+     (:constructor edebug--make-frame)
+     (:include backtrace-frame))
+  def-name before-index after-index)
 
 (defun edebug-backtrace ()
-  "Display a non-working backtrace.  Better than nothing..."
+  "Display the current backtrace in a `backtrace-mode' window."
   (interactive)
   (if (or (not edebug-backtrace-buffer)
           (null (buffer-name edebug-backtrace-buffer)))
       (setq edebug-backtrace-buffer
-            (generate-new-buffer "*Backtrace*"))
+            (generate-new-buffer "*Edebug Backtrace*"))
     ;; Else, could just display edebug-backtrace-buffer.
     )
   (with-output-to-temp-buffer (buffer-name edebug-backtrace-buffer)
     (setq edebug-backtrace-buffer standard-output)
-    (let ((print-escape-newlines t)
-          (print-length 50)     ; FIXME cf edebug-safe-prin1-to-string
-          last-ok-point)
-      (backtrace)
-
-      ;; Clean up the backtrace.
-      ;; Not quite right for current edebug scheme.
-      (set-buffer edebug-backtrace-buffer)
-      (setq truncate-lines t)
-      (goto-char (point-min))
-      (setq last-ok-point (point))
-      (if t (progn
-
-      ;; Delete interspersed edebug internals.
-      (while (re-search-forward "^  (?edebug" nil t)
-        (beginning-of-line)
-        (cond
-         ((looking-at "^  (edebug-after")
-          ;; Previous lines may contain code, so just delete this line.
-          (setq last-ok-point (point))
-          (forward-line 1)
-          (delete-region last-ok-point (point)))
-
-         ((looking-at (if debugger-stack-frame-as-list
-                          "^  (edebug"
-                        "^  edebug"))
-          (forward-line 1)
-          (delete-region last-ok-point (point))
-          )))
-      )))))
+    (with-current-buffer edebug-backtrace-buffer
+      (unless (derived-mode-p 'backtrace-mode)
+        (backtrace-mode))
+      (setq backtrace-frames (edebug--backtrace-frames)
+            backtrace-view '(:do-xrefs t))
+      (backtrace-print)
+      (goto-char (point-min)))))
+
+(defun edebug--backtrace-frames ()
+  "Return backtrace frames with instrumentation removed.
+Remove frames for Edebug's functions and the lambdas in
+`edebug-enter' wrappers."
+  (let* ((frames (backtrace-get-frames 'edebug-debugger
+                                       :constructor #'edebug--make-frame))
+         skip-next-lambda def-name before-index after-index
+         results
+         (index (length frames)))
+    (dolist (frame (reverse frames))
+      (let ((fun (edebug--frame-fun frame))
+            (args (edebug--frame-args frame)))
+        (cl-decf index)
+        (when (edebug--frame-evald frame)
+          (setq before-index nil
+                after-index nil))
+        (pcase fun
+          ('edebug-enter
+           (setq skip-next-lambda t
+                 def-name (nth 0 args)))
+          ('edebug-after
+           (setq before-index (if (consp (nth 0 args))
+                                  (nth 1 (nth 0 args))
+                                (nth 0 args))
+                 after-index (nth 1 args)))
+          ((pred edebug--symbol-not-prefixed-p)
+           (edebug--unwrap-and-add-info frame def-name before-index after-index)
+           (setf (edebug--frame-def-name frame) (and before-index def-name))
+           (setf (edebug--frame-before-index frame) before-index)
+           (setf (edebug--frame-after-index frame) after-index)
+           (push frame results)
+           (setq before-index nil
+                 after-index nil))
+          (`(,(or 'lambda 'closure) . ,_)
+           (unless skip-next-lambda
+             (edebug--unwrap-and-add-info frame def-name before-index after-index)
+             (push frame results))
+           (setq before-index nil
+                 after-index nil
+                 skip-next-lambda nil)))))
+    results))
+
+(defun edebug--symbol-not-prefixed-p (sym)
+  "Return non-nil if SYM is a symbol not prefixed by \"edebug-\"."
+  (and (symbolp sym)
+       (not (string-prefix-p "edebug-" (symbol-name sym)))))
+
+(defun edebug--unwrap-and-add-info (frame def-name before-index after-index)
+  "Update FRAME with the additional info needed by an edebug--frame.
+Save DEF-NAME, BEFORE-INDEX and AFTER-INDEX in FRAME.  Also
+remove Edebug's instrumentation from the function and any
+unevaluated arguments in FRAME."
+  (setf (edebug--frame-def-name frame) (and before-index def-name))
+  (setf (edebug--frame-before-index frame) before-index)
+  (setf (edebug--frame-after-index frame) after-index)
+  (setf (edebug--frame-fun frame) (edebug-unwrap* (edebug--frame-fun frame)))
+  (unless (edebug--frame-evald frame)
+    (let (results)
+      (dolist (arg (edebug--frame-args frame))
+        (push (edebug-unwrap* arg) results))
+      (setf (edebug--frame-args frame) (nreverse results)))))
 
 
 ;;; Trace display
diff --git a/lisp/emacs-lisp/ert.el b/lisp/emacs-lisp/ert.el
index cad21044f15..7178493ebe5 100644
--- a/lisp/emacs-lisp/ert.el
+++ b/lisp/emacs-lisp/ert.el
@@ -60,6 +60,7 @@
 (require 'cl-lib)
 (require 'button)
 (require 'debug)
+(require 'backtrace)
 (require 'easymenu)
 (require 'ewoc)
 (require 'find-func)
@@ -677,13 +678,6 @@ and is displayed in front of the value of MESSAGE-FORM."
 (cl-defstruct (ert-test-aborted-with-non-local-exit
                (:include ert-test-result)))
 
-(defun ert--print-backtrace (backtrace do-xrefs)
-  "Format the backtrace BACKTRACE to the current buffer."
-  (let ((print-escape-newlines t)
-        (print-level 8)
-        (print-length 50))
-    (debugger-insert-backtrace backtrace do-xrefs)))
-
 ;; A container for the state of the execution of a single test and
 ;; environment data needed during its execution.
 (cl-defstruct ert--test-execution-info
@@ -732,7 +726,7 @@ run.  ARGS are the arguments to `debugger'."
               ;; use.
               ;;
               ;; Grab the frames above the debugger.
-              (backtrace (cdr (backtrace-frames debugger)))
+              (backtrace (cdr (backtrace-get-frames debugger)))
               (infos (reverse ert--infos)))
          (setf (ert--test-execution-info-result info)
                (cl-ecase type
@@ -1406,9 +1400,8 @@ Returns the stats object."
               (ert-test-result-with-condition
                (message "Test %S backtrace:" (ert-test-name test))
                (with-temp-buffer
-                 (ert--print-backtrace
-                  (ert-test-result-with-condition-backtrace result)
-                  nil)
+                 (insert (backtrace-to-string
+                          (ert-test-result-with-condition-backtrace result)))
                  (if (not ert-batch-backtrace-right-margin)
                      (message "%s"
                               (buffer-substring-no-properties (point-min)
@@ -2450,20 +2443,21 @@ To be used in the ERT results buffer."
     (cl-etypecase result
       (ert-test-passed (error "Test passed, no backtrace available"))
       (ert-test-result-with-condition
-       (let ((backtrace (ert-test-result-with-condition-backtrace result))
-             (buffer (get-buffer-create "*ERT Backtrace*")))
+       (let ((buffer (get-buffer-create "*ERT Backtrace*")))
          (pop-to-buffer buffer)
-         (let ((inhibit-read-only t))
-           (buffer-disable-undo)
-           (erase-buffer)
-           (ert-simple-view-mode)
-           (set-buffer-multibyte t)     ; mimic debugger-setup-buffer
-           (setq truncate-lines t)
-           (ert--print-backtrace backtrace t)
-           (goto-char (point-min))
-           (insert (substitute-command-keys "Backtrace for test `"))
-           (ert-insert-test-name-button (ert-test-name test))
-           (insert (substitute-command-keys "':\n"))))))))
+         (unless (derived-mode-p 'backtrace-mode)
+           (backtrace-mode))
+         (setq backtrace-insert-header-function
+               (lambda () (ert--insert-backtrace-header (ert-test-name test)))
+               backtrace-frames (ert-test-result-with-condition-backtrace result)
+               backtrace-view '(:do-xrefs t))
+         (backtrace-print)
+         (goto-char (point-min)))))))
+
+(defun ert--insert-backtrace-header (name)
+  (insert (substitute-command-keys "Backtrace for test `"))
+  (ert-insert-test-name-button name)
+  (insert (substitute-command-keys "':\n")))
 
 (defun ert-results-pop-to-messages-for-test-at-point ()
   "Display the part of the *Messages* buffer generated during the test at point.
diff --git a/lisp/emacs-lisp/lisp-mode.el b/lisp/emacs-lisp/lisp-mode.el
index 6313c63ecfe..afb7cbd1dd7 100644
--- a/lisp/emacs-lisp/lisp-mode.el
+++ b/lisp/emacs-lisp/lisp-mode.el
@@ -517,6 +517,16 @@ This will generate compile-time constants from BINDINGS."
 (defvar lisp-cl-font-lock-keywords lisp-cl-font-lock-keywords-1
   "Default expressions to highlight in Lisp modes.")
 
+;; Support backtrace mode.
+(defconst lisp-el-font-lock-keywords-for-backtraces lisp-el-font-lock-keywords
+  "Default highlighting from Emacs Lisp mod used in Backtrace mode.")
+(defconst lisp-el-font-lock-keywords-for-backtraces-1 lisp-el-font-lock-keywords-1
+  "Subdued highlighting from Emacs Lisp mode used in Backtrace mode.")
+(defconst lisp-el-font-lock-keywords-for-backtraces-2
+  (remove (assoc 'lisp--match-hidden-arg lisp-el-font-lock-keywords-2)
+          lisp-el-font-lock-keywords-2)
+  "Gaudy highlighting from Emacs Lisp mode used in Backtrace mode.")
+
 (defun lisp-string-in-doc-position-p (listbeg startpos)
    "Return true if a doc string may occur at STARTPOS inside a list.
 LISTBEG is the position of the start of the innermost list