Merge branch 'scratch/backtrace-mode'

16 files changed, 2326 insertions, 460 deletions

diff --git a/doc/lispref/debugging.texi b/doc/lispref/debugging.texi
index 1b1f87465db..9b3ba6cf7ee 100644
--- a/doc/lispref/debugging.texi
+++ b/doc/lispref/debugging.texi
@@ -81,7 +81,8 @@ debugger recursively.  @xref{Recursive Editing}.
 * Function Debugging::    Entering it when a certain function is called.
 * Variable Debugging::    Entering it when a variable is modified.
 * Explicit Debug::        Entering it at a certain point in the program.
-* Using Debugger::        What the debugger does; what you see while in it.
+* Using Debugger::        What the debugger does.
+* Backtraces::            What you see while in the debugger.
 * Debugger Commands::     Commands used while in the debugger.
 * Invoking the Debugger:: How to call the function @code{debug}.
 * Internals of Debugger:: Subroutines of the debugger, and global variables.
@@ -392,32 +393,82 @@ this is not what you want, you can either set
 @code{eval-expression-debug-on-error} to @code{nil}, or set
 @code{debug-on-error} to @code{nil} in @code{debugger-mode-hook}.
 
+  The debugger itself must be run byte-compiled, since it makes
+assumptions about the state of the Lisp interpreter.  These
+assumptions are false if the debugger is running interpreted.
+
+@node Backtraces
+@subsection Backtraces
+@cindex backtrace buffer
+
+Debugger mode is derived from Backtrace mode, which is also used to
+show backtraces by Edebug and ERT.  (@pxref{Edebug}, and @ref{Top,the
+ERT manual,, ert, ERT: Emacs Lisp Regression Testing}.)
+
+@cindex stack frame
+The backtrace buffer shows you the functions that are executing and
+their argument values.  When a backtrace buffer is created, it shows
+each stack frame on one, possibly very long, line.  (A stack frame is
+the place where the Lisp interpreter records information about a
+particular invocation of a function.)  The most recently called
+function will be at the top.
+
 @cindex current stack frame
-  The backtrace buffer shows you the functions that are executing and
-their argument values.  It also allows you to specify a stack frame by
-moving point to the line describing that frame.  (A stack frame is the
-place where the Lisp interpreter records information about a particular
-invocation of a function.)  The frame whose line point is on is
-considered the @dfn{current frame}.  Some of the debugger commands
-operate on the current frame.  If a line starts with a star, that means
-that exiting that frame will call the debugger again.  This is useful
-for examining the return value of a function.
-
-  If a function name is underlined, that means the debugger knows
-where its source code is located.  You can click with the mouse on
-that name, or move to it and type @key{RET}, to visit the source code.
+In a backtrace you can specify a stack frame by moving point to a line
+describing that frame.  The frame whose line point is on is considered
+the @dfn{current frame}.
+
+If a function name is underlined, that means Emacs knows where its
+source code is located.  You can click with the mouse on that name, or
+move to it and type @key{RET}, to visit the source code.  You can also
+type @key{RET} while point is on any name of a function or variable
+which is not underlined, to see help information for that symbol in a
+help buffer, if any exists.  The @code{xref-find-definitions} command,
+bound to @key{M-.}, can also be used on any identifier in a backtrace
+(@pxref{Looking Up Identifiers,,,emacs, The GNU Emacs Manual}).
+
+In backtraces, the tails of long lists and the ends of long strings,
+vectors or structures, as well as objects which are deeply nested,
+will be printed as underlined ``...''.  You can click with the mouse
+on a ``...'', or type @key{RET} while point is on it, to show the part
+of the object that was hidden.  To control how much abbreviation is
+done, customize @code{backtrace-line-length}.
+
+Here is a list of commands for navigating and viewing backtraces:
 
-  The debugger itself must be run byte-compiled, since it makes
-assumptions about how many stack frames are used for the debugger
-itself.  These assumptions are false if the debugger is running
-interpreted.
+@table @kbd
+@item v
+Toggle the display of local variables of the current stack frame.
+
+@item p
+Move to the beginning of the frame, or to the beginning
+of the previous frame.
+
+@item n
+Move to the beginning of the next frame.
+
+@item +
+Add line breaks and indentation to the top-level Lisp form at point to
+make it more readable.
+
+@item -
+Collapse the top-level Lisp form at point back to a single line.
+
+@item #
+Toggle @code{print-circle} for the frame at point.
+
+@item .
+Expand all the forms abbreviated with ``...'' in the frame at point.
+
+@end table
 
 @node Debugger Commands
 @subsection Debugger Commands
 @cindex debugger command list
 
   The debugger buffer (in Debugger mode) provides special commands in
-addition to the usual Emacs commands.  The most important use of
+addition to the usual Emacs commands and to the Backtrace mode commands
+described in the previous section.  The most important use of
 debugger commands is for stepping through code, so that you can see
 how control flows.  The debugger can step through the control
 structures of an interpreted function, but cannot do so in a
@@ -427,6 +478,11 @@ the same function.  (To do this, visit the source for the function and
 type @kbd{C-M-x} on its definition.)  You cannot use the Lisp debugger
 to step through a primitive function.
 
+Some of the debugger commands operate on the current frame.  If a
+frame starts with a star, that means that exiting that frame will call the
+debugger again.  This is useful for examining the return value of a
+function.
+
 @c FIXME: Add @findex for the following commands?  --xfq
   Here is a list of Debugger mode commands:
 
@@ -502,8 +558,6 @@ Display a list of functions that will invoke the debugger when called.
 This is a list of functions that are set to break on entry by means of
 @code{debug-on-entry}.
 
-@item v
-Toggle the display of local variables of the current stack frame.
 @end table
 
 @node Invoking the Debugger
@@ -624,20 +678,19 @@ of @code{debug} (@pxref{Invoking the Debugger}).
 @cindex run time stack
 @cindex call stack
 This function prints a trace of Lisp function calls currently active.
-This is the function used by @code{debug} to fill up the
-@file{*Backtrace*} buffer.  It is written in C, since it must have access
-to the stack to determine which function calls are active.  The return
-value is always @code{nil}.
+The trace is identical to the one that @code{debug} would show in the
+@file{*Backtrace*} buffer.  The return value is always nil.
 
 In the following example, a Lisp expression calls @code{backtrace}
 explicitly.  This prints the backtrace to the stream
 @code{standard-output}, which, in this case, is the buffer
 @samp{backtrace-output}.
 
-Each line of the backtrace represents one function call.  The line shows
-the values of the function's arguments if they are all known; if they
-are still being computed, the line says so.  The arguments of special
-forms are elided.
+Each line of the backtrace represents one function call.  The line
+shows the function followed by a list of the values of the function's
+arguments if they are all known; if they are still being computed, the
+line consists of a list containing the function and its unevaluated
+arguments.  Long lists or deeply nested structures may be elided.
 
 @smallexample
 @group
@@ -654,7 +707,7 @@ forms are elided.
 @group
 ----------- Buffer: backtrace-output ------------
   backtrace()
-  (list ...computing arguments...)
+  (list 'testing (backtrace))
 @end group
   (progn ...)
   eval((progn (1+ var) (list 'testing (backtrace))))
@@ -685,7 +738,7 @@ example would look as follows:
 @group
 ----------- Buffer: backtrace-output ------------
   (backtrace)
-  (list ...computing arguments...)
+  (list 'testing (backtrace))
 @end group
   (progn ...)
   (eval (progn (1+ var) (list 'testing (backtrace))))
diff --git a/doc/lispref/edebug.texi b/doc/lispref/edebug.texi
index b9cc1d5afc2..54200b99903 100644
--- a/doc/lispref/edebug.texi
+++ b/doc/lispref/edebug.texi
@@ -442,8 +442,16 @@ Redisplay the most recently known expression result in the echo area
 Display a backtrace, excluding Edebug's own functions for clarity
 (@code{edebug-backtrace}).
 
-You cannot use debugger commands in the backtrace buffer in Edebug as
-you would in the standard debugger.
+@xref{Backtraces}, for a description of backtraces
+and the commands which work on them.
+
+If you would like to see Edebug's functions in the backtrace,
+use @kbd{M-x edebug-backtrace-show-instrumentation}.  To hide them
+again use @kbd{M-x edebug-backtrace-hide-instrumentation}.
+
+If a backtrace frame starts with @samp{>} that means that Edebug knows
+where the source code for the frame is located.  Use @kbd{s} to jump
+to the source code for the current frame.
 
 The backtrace buffer is killed automatically when you continue
 execution.
diff --git a/doc/misc/ert.texi b/doc/misc/ert.texi
index 82e0e27ed1c..6a34f5c5722 100644
--- a/doc/misc/ert.texi
+++ b/doc/misc/ert.texi
@@ -273,9 +273,11 @@ moving point to it and typing @kbd{@key{RET}} jumps to its definition.
 @cindex backtrace of a failed test
 Pressing @kbd{r} re-runs the test near point on its own.  Pressing
 @kbd{d} re-runs it with the debugger enabled.  @kbd{.} jumps to the
-definition of the test near point (@kbd{@key{RET}} has the same effect if
-point is on the name of the test).  On a failed test, @kbd{b} shows
-the backtrace of the failure.
+definition of the test near point (@kbd{@key{RET}} has the same effect
+if point is on the name of the test).  On a failed test, @kbd{b} shows
+the backtrace of the failure.  @xref{Debugging,, Backtraces, elisp,
+GNU Emacs Lisp Reference Manual}, for more information about
+backtraces.
 
 @kindex l@r{, in ert results buffer}
 @kbd{l} shows the list of @code{should} forms executed in the test.
diff --git a/etc/NEWS b/etc/NEWS
index 6ccf6fc0890..9c78bd5a05d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -466,6 +466,14 @@ the shift key.
 *** Isearch now remembers the regexp-based search mode for words/symbols
 and case-sensitivity together with search strings in the search ring.
 
+** Debugger
+
++++
+*** The Lisp Debugger is now based on 'backtrace-mode'.
+Backtrace mode adds fontification and commands for changing the
+appearance of backtrace frames.  See the node "Backtraces" in the Elisp
+manual for documentation of the new mode and its commands.
+
 ** Edebug
 
 +++
@@ -475,14 +483,27 @@ using the new variables 'edebug-behavior-alist',
 'edebug-new-definition-function'.  Edebug's behavior can be changed
 globally or for individual definitions.
 
++++
+*** Edebug's backtrace buffer now uses 'backtrace-mode'.
+Backtrace mode adds fontification, links and commands for changing the
+appearance of backtrace frames.  See the node "Backtraces" in the Elisp
+manual for documentation of the new mode and its commands.
+
+The binding of 'd' in Edebug's keymap is now 'edebug-pop-to-backtrace'
+which replaces 'edebug-backtrace'.  Consequently Edebug's backtrace
+windows now behave like those of the Lisp Debugger and of ERT, in that
+when they appear they will be the selected window.
+
+The new 'backtrace-goto-source' command, bound to 's', works in
+Edebug's backtraces on backtrace frames whose source code has
+been instrumented by Edebug.
+
 ** Enhanced xterm support
 
 *** New variable 'xterm-set-window-title' controls whether Emacs sets
 the XTerm window title.  This feature is experimental and is disabled
 by default.
 
-** Gamegrid
-
 ** grep
 
 +++
@@ -499,6 +520,14 @@ The abbreviation can be disabled by the new option
 *** New variable 'ert-quiet' allows to make ERT output in batch mode
 less verbose by removing non-essential information.
 
++++
+*** ERT's backtrace buffer now uses 'backtrace-mode'.
+Backtrace mode adds fontification and commands for changing the
+appearance of backtrace frames.  See the node "Backtraces" in the Elisp
+manual for documentation of the new mode and its commands.
+
+** Gamegrid
+
 ---
 *** Gamegrid now determines its default glyph size based on display
 dimensions, instead of always using 16 pixels.  As a result, Tetris,
@@ -669,6 +698,13 @@ transport strategies as well as a separate API to use them.  A
 transport implementation for process-based communication, such as is
 used by the Language Server Protocol (LSP), is readily available.
 
++++
+** Backtrace mode improves viewing of Elisp backtraces.
+Backtrace mode adds pretty printing, fontification and ellipsis
+expansion to backtrace buffers produced by the Lisp debugger, Edebug
+and ERT.  See the node "Backtraces" in the Elisp manual for
+documentation of the new mode and its commands.
+
 
 * Incompatible Lisp Changes in Emacs 27.1
 
diff --git a/lisp/emacs-lisp/backtrace.el b/lisp/emacs-lisp/backtrace.el
new file mode 100644
index 00000000000..f13b43b465c
--- /dev/null
+++ b/lisp/emacs-lisp/backtrace.el
@@ -0,0 +1,916 @@
+;;; 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.  If set to nil or zero, Backtrace mode will not
+abbreviate the forms it prints."
+  :type 'integer
+  :group 'backtrace
+  :version "27.1")
+
+;;; Backtrace frame data structure
+
+(cl-defstruct
+    (backtrace-frame
+     (:constructor backtrace-make-frame))
+  evald  ; Non-nil if argument evaluation is complete.
+  fun    ; The function called/to call in this frame.
+  args   ; Either evaluated or unevaluated arguments to the function.
+  flags  ; A plist, possible properties are :debug-on-exit and :source-available.
+  locals ; An alist containing variable names and values.
+  buffer ; If non-nil, the buffer in use by eval-buffer or eval-region.
+  pos    ; The position in the buffer.
+  )
+
+(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-buffer frame) (current-buffer))
+            (setf (backtrace-frame-pos frame) (point))))))
+    frames))
+
+;; Button definition for jumping to a buffer position.
+
+(define-button-type 'backtrace-buffer-pos
+  'action #'backtrace--pop-to-buffer-pos
+  'help-echo "mouse-2, RET: Show reading position")
+
+(defun backtrace--pop-to-buffer-pos (button)
+  "Pop to the buffer and position for the BUTTON at point."
+  (let* ((buffer (button-get button 'backtrace-buffer))
+         (pos (button-get button 'backtrace-pos)))
+    (if (buffer-live-p buffer)
+        (progn
+          (pop-to-buffer buffer)
+          (goto-char (max (point-min) (min (point-max) pos))))
+      (message "Buffer has been killed"))))
+
+;; 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.
+  ;; This is necessary because ellipses are text property buttons
+  ;; instead of overlay buttons, which is done because there could
+  ;; be a large number of them.
+  (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, :show-locals 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-local backtrace-goto-source-functions nil
+  "Abnormal hook used to jump to the source code for the current frame.
+Each hook function is called with no argument, and should return
+non-nil if it is able to switch to the buffer containing the
+source code.  Execution of the hook will stop if one of the
+functions returns non-nil.  When adding a function to this hook,
+you should also set the :source-available flag for the backtrace
+frames where the source code location is known.")
+
+(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 "s" 'backtrace-goto-source)
+    (define-key map "\C-m" 'backtrace-help-follow-symbol)
+    (define-key map "+" 'backtrace-multi-line)
+    (define-key map "-" 'backtrace-single-line)
+    (define-key map "." 'backtrace-expand-ellipses)
+    (define-key map [follow-link] 'mouse-face)
+    (define-key map [mouse-2] 'mouse-select-window)
+    (easy-menu-define nil map ""
+      '("Backtrace"
+        ["Next Frame" backtrace-forward-frame
+         :help "Move cursor forwards to the start of a backtrace frame"]
+        ["Previous Frame" backtrace-backward-frame
+         :help "Move cursor backwards to the start of a backtrace frame"]
+        "--"
+        ["Show Variables" backtrace-toggle-locals
+         :style toggle
+         :active (backtrace-get-index)
+         :selected (plist-get (backtrace-get-view) :show-locals)
+         :help "Show or hide the local variables for 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
+         :help "Use line breaks and indentation to make a form more readable"]
+        ["Show on Single Line" backtrace-single-line]
+        "--"
+        ["Go to Source" backtrace-goto-source
+         :active (and (backtrace-get-index)
+                      (plist-get (backtrace-frame-flags
+                                  (nth (backtrace-get-index) backtrace-frames))
+                                 :source-available))
+         :help "Show the source code for the current frame"]
+        ["Help for Symbol" backtrace-help-follow-symbol
+         :help "Show help for symbol at point"]
+        ["Describe Backtrace Mode" describe-mode
+         :help "Display documentation for backtrace-mode"]))
+    map)
+  "Local keymap for `backtrace-mode' buffers.")
+
+(defconst backtrace--flags-width 2
+  "Width in characters of the flags for a backtrace frame.")
+
+;;; 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.
+
+(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-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))
+
+(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-toggle-locals (&optional all)
+  "Toggle the display of local variables for the backtrace frame at point.
+With prefix argument ALL, toggle the value of :show-locals in
+`backtrace-view', which affects all of the backtrace frames in
+the buffer."
+  (interactive "P")
+  (if all
+      (let ((pos (make-marker))
+            (visible (not (plist-get backtrace-view :show-locals))))
+        (setq backtrace-view (plist-put backtrace-view :show-locals visible))
+        (set-marker-insertion-type pos t)
+        (set-marker pos (point))
+        (goto-char (point-min))
+        ;; Skip the header.
+        (unless (backtrace-get-index)
+          (goto-char (backtrace-get-frame-end)))
+        (while (< (point) (point-max))
+          (backtrace--set-frame-locals-visible visible)
+          (goto-char (backtrace-get-frame-end)))
+        (goto-char pos)
+        (when (invisible-p pos)
+          (goto-char (backtrace-get-frame-start))))
+    (let ((index (backtrace-get-index)))
+      (unless index
+        (user-error "Not in a stack frame"))
+      (backtrace--set-frame-locals-visible
+       (not (plist-get (backtrace-get-view) :show-locals))))))
+
+(defun backtrace--set-frame-locals-visible (visible)
+  "Set the visibility of the local vars for the frame at point to VISIBLE."
+  (let ((pos (point))
+        (index (backtrace-get-index))
+        (start (backtrace-get-frame-start))
+        (end (backtrace-get-frame-end))
+        (view (copy-sequence (backtrace-get-view)))
+        (inhibit-read-only t))
+    (setq view (plist-put view :show-locals visible))
+    (goto-char (backtrace-get-frame-start))
+    (while (not (or (= (point) end)
+                    (eq (backtrace-get-section) 'locals)))
+      (goto-char (next-single-property-change (point)
+                                              'backtrace-section nil end)))
+    (cond
+     ((and (= (point) end) visible)
+      ;; The locals section doesn't exist so create it.
+      (let ((standard-output (current-buffer)))
+        (backtrace--with-output-variables view
+          (backtrace--print-locals
+           (nth index backtrace-frames) view))
+        (add-text-properties end (point) `(backtrace-index ,index))
+        (goto-char pos)))
+     ((/= (point) end)
+      ;; The locals section does exist, so add or remove the overlay.
+      (backtrace--set-locals-visible-overlay (point) end visible)
+      (goto-char (if (invisible-p pos) start pos))))
+    (add-text-properties start (backtrace-get-frame-end)
+                         `(backtrace-view ,view))))
+
+(defun backtrace--set-locals-visible-overlay (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 (&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."
+  (interactive "P")
+  (backtrace--toggle-feature :print-circle 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
+is non-nil, toggle FEATURE for all frames in the buffer.  After
+toggling the feature, reprint the affected frame(s).  Afterwards
+position point at the start of the frame it was in before."
+  (if all
+      (let ((index (backtrace-get-index))
+            (pos (point))
+            (at-end (= (point) (point-max)))
+            (value (not (plist-get backtrace-view feature))))
+        (setq backtrace-view (plist-put backtrace-view feature value))
+        (goto-char (point-min))
+        ;; Skip the header.
+        (unless (backtrace-get-index)
+          (goto-char (backtrace-get-frame-end)))
+        (while (< (point) (point-max))
+          (backtrace--set-feature feature value)
+          (goto-char (backtrace-get-frame-end)))
+        (if (not index)
+            (goto-char (if at-end (point-max) pos))
+          (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))))))
+
+(defun backtrace--set-feature (feature value)
+  "Set FEATURE in the view plist of the frame at point to VALUE.
+Reprint the frame with the new view plist."
+  (let ((inhibit-read-only t)
+        (view (copy-sequence (backtrace-get-view)))
+        (index (backtrace-get-index))
+        (min (backtrace-get-frame-start))
+        (max (backtrace-get-frame-end)))
+    (setq view (plist-put view feature value))
+    (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)))
+
+(defun backtrace-expand-ellipsis (button)
+  "Expand display of the elided form at BUTTON."
+  (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))
+         (inhibit-read-only t))
+    (backtrace--with-output-variables (backtrace-get-view)
+      (delete-region begin end)
+      (insert (cl-print-to-string-with-limit #'cl-print-expand-ellipsis value
+                                          backtrace-line-length))
+      (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-expand-ellipses (&optional no-limit)
+  "Expand display of all \"...\"s in the backtrace frame at point.
+\\<backtrace-mode-map>
+Each ellipsis will be limited to `backtrace-line-length'
+characters in its expansion.  With optional prefix argument
+NO-LIMIT, do not limit the number of characters.  Note that with
+or without the argument, using this command can result in very
+long lines and very poor display performance.  If this happens
+and is a problem, use `\\[revert-buffer]' to return to the
+initial state of the Backtrace buffer."
+  (interactive "P")
+  (save-excursion
+    (let ((start (backtrace-get-frame-start))
+          (end (backtrace-get-frame-end))
+          (backtrace-line-length (unless no-limit backtrace-line-length)))
+      (goto-char end)
+      (while (> (point) start)
+        (let ((next (previous-single-property-change (point) 'cl-print-ellipsis
+                                                     nil start)))
+          (when (get-text-property (point) 'cl-print-ellipsis)
+            (push-button (point)))
+          (goto-char next))))))
+
+(defun backtrace-multi-line ()
+  "Show the top level s-expression at point on multiple lines with indentation."
+  (interactive)
+  (backtrace--reformat-sexp #'backtrace--multi-line))
+
+(defun backtrace--multi-line ()
+  "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-single-line ()
+  "Show the top level s-expression at point on one line."
+  (interactive)
+  (backtrace--reformat-sexp #'backtrace--single-line))
+
+(defun backtrace--single-line ()
+  "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)
+  "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, signal an error.
+
+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 "No form here to reformat"))
+      (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-goto-source ()
+  "If its location is known, jump to the source code for the frame at point."
+  (interactive)
+  (let* ((index (or (backtrace-get-index) (user-error "Not in a stack frame")))
+         (frame (nth index backtrace-frames))
+         (source-available (plist-get (backtrace-frame-flags frame)
+                                      :source-available)))
+    (unless (and source-available
+                 (catch 'done
+                   (dolist (func backtrace-goto-source-functions)
+                     (when (funcall func)
+                       (throw 'done t)))))
+      (user-error "Source code location not known"))))
+
+(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.'  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))
+  (with-temp-buffer
+    (insert (cl-print-to-string-with-limit #'backtrace--print sexp limit))
+    ;; Add a unique backtrace-form property.
+    (put-text-property (point-min) (point) 'backtrace-form (gensym))
+    ;; Make buttons from all the "..."s.  Since there might be many of
+    ;; them, use text property buttons.
+    (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))
+        (source (plist-get (backtrace-frame-flags frame) :source-available)))
+    (when (plist-get view :show-flags)
+      (when source (insert ">"))
+      (when flag (insert "*")))
+    (insert (make-string (- backtrace--flags-width (- (point) beg)) ?\s))
+    (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))
+         (def   (and (symbolp fun) (fboundp fun) (symbol-function fun)))
+         (fun-file (or (symbol-file fun 'defun)
+                            (and (subrp def)
+                                 (not (eq 'unevalled (cdr (subr-arity def))))
+                                 (find-lisp-object-file-name fun def))))
+         (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-multi-line
+        ;; will find it.  backtrace-multi-line 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 "  ; Reading at ")
+      (let ((pos (point)))
+        (insert (format "buffer position %d" (backtrace-frame-pos frame)))
+        (make-button pos (point) :type 'backtrace-buffer-pos
+                     'backtrace-buffer (backtrace-frame-buffer frame)
+                     'backtrace-pos (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 according to VIEW.
+Print them only if :show-locals is non-nil in the VIEW plist."
+  (when (plist-get view :show-locals)
+    (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))))
+
+(defun backtrace--print (obj &optional stream)
+  "Attempt to print OBJ to STREAM using `backtrace-print-function'.
+Fall back to `prin1' if there is an error."
+  (condition-case err
+      (funcall backtrace-print-function obj stream)
+    (error
+     (message "Error in backtrace printer: %S" err)
+     (prin1 obj stream))))
+
+(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 backtrace--flags-width)
+          (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).
+ - Maybe modify `backtrace-view' (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 may optionally modify `backtrace-view', which is a
+plist describing the appearance of the backtrace.  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
+            (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)
+  (setq-local indent-line-function 'lisp-indent-line)
+  (setq-local indent-region-function 'lisp-indent-region)
+  (add-hook 'xref-backend-functions #'backtrace--xref-backend nil t))
+
+(put 'backtrace-mode 'mode-class 'special)
+
+;;; Backtrace printing
+
+;;;###autoload
+(defun 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)))
+  nil)
+
+(defun backtrace-to-string(&optional frames)
+  "Format FRAMES, a list of `backtrace-frame' objects, for output.
+Return the result as a string.  If FRAMES is nil, use all
+function calls currently active."
+  (unless frames (setq frames (backtrace-get-frames 'backtrace-to-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/cl-print.el b/lisp/emacs-lisp/cl-print.el
index bf5b1e878d5..c63f5ac005c 100644
--- a/lisp/emacs-lisp/cl-print.el
+++ b/lisp/emacs-lisp/cl-print.el
@@ -55,10 +55,19 @@ call other entry points instead, such as `cl-prin1'."
   ;; we should only use it for objects which don't have nesting.
   (prin1 object stream))
 
+(cl-defgeneric cl-print-object-contents (_object _start _stream)
+  "Dispatcher to print the contents of OBJECT on STREAM.
+Print the contents starting with the item at START, without
+delimiters."
+  ;; Every cl-print-object method which can print an ellipsis should
+  ;; have a matching cl-print-object-contents method to expand an
+  ;; ellipsis.
+  (error "Missing cl-print-object-contents method"))
+
 (cl-defmethod cl-print-object ((object cons) stream)
   (if (and cl-print--depth (natnump print-level)
            (> cl-print--depth print-level))
-      (princ "..." stream)
+      (cl-print-insert-ellipsis object 0 stream)
     (let ((car (pop object))
           (count 1))
       (if (and print-quoted
@@ -84,23 +93,60 @@ call other entry points instead, such as `cl-prin1'."
           (princ " " stream)
           (if (or (not (natnump print-length)) (> print-length count))
               (cl-print-object (pop object) stream)
-            (princ "..." stream)
+            (cl-print-insert-ellipsis object print-length stream)
             (setq object nil))
           (cl-incf count))
         (when object
           (princ " . " stream) (cl-print-object object stream))
         (princ ")" stream)))))
 
+(cl-defmethod cl-print-object-contents ((object cons) _start stream)
+  (let ((count 0))
+    (while (and (consp object)
+                (not (cond
+                      (cl-print--number-table
+                       (numberp (gethash object cl-print--number-table)))
+                      ((memq object cl-print--currently-printing))
+                      (t (push object cl-print--currently-printing)
+                         nil))))
+      (unless (zerop count)
+        (princ " " stream))
+      (if (or (not (natnump print-length)) (> print-length count))
+          (cl-print-object (pop object) stream)
+        (cl-print-insert-ellipsis object print-length stream)
+        (setq object nil))
+      (cl-incf count))
+    (when object
+      (princ " . " stream) (cl-print-object object stream))))
+
 (cl-defmethod cl-print-object ((object vector) stream)
-  (princ "[" stream)
-  (let ((count (length object)))
-    (dotimes (i (if (natnump print-length)
-                    (min print-length count) count))
-      (unless (zerop i) (princ " " stream))
-      (cl-print-object (aref object i) stream))
-    (when (and (natnump print-length) (< print-length count))
-      (princ " ..." stream)))
-  (princ "]" stream))
+  (if (and cl-print--depth (natnump print-level)
+           (> cl-print--depth print-level))
+      (cl-print-insert-ellipsis object 0 stream)
+    (princ "[" stream)
+    (let* ((len (length object))
+           (limit (if (natnump print-length)
+                      (min print-length len) len)))
+      (dotimes (i limit)
+        (unless (zerop i) (princ " " stream))
+        (cl-print-object (aref object i) stream))
+      (when (< limit len)
+        (princ " " stream)
+        (cl-print-insert-ellipsis object limit stream)))
+    (princ "]" stream)))
+
+(cl-defmethod cl-print-object-contents ((object vector) start stream)
+  (let* ((len (length object))
+         (limit (if (natnump print-length)
+                    (min (+ start print-length) len) len))
+         (i start))
+    (while (< i limit)
+      (unless (= i start) (princ " " stream))
+      (cl-print-object (aref object i) stream)
+      (cl-incf i))
+    (when (< limit len)
+      (princ " " stream)
+      (cl-print-insert-ellipsis object limit stream))))
 
 (cl-defmethod cl-print-object ((object hash-table) stream)
   (princ "#<hash-table " stream)
@@ -199,21 +245,135 @@ into a button whose action shows the function's disassembly.")
     (princ ")" stream)))
 
 (cl-defmethod cl-print-object ((object cl-structure-object) stream)
-  (princ "#s(" stream)
+  (if (and cl-print--depth (natnump print-level)
+           (> cl-print--depth print-level))
+      (cl-print-insert-ellipsis object 0 stream)
+    (princ "#s(" stream)
+    (let* ((class (cl-find-class (type-of object)))
+           (slots (cl--struct-class-slots class))
+           (len (length slots))
+           (limit (if (natnump print-length)
+                      (min print-length len) len)))
+      (princ (cl--struct-class-name class) stream)
+      (dotimes (i limit)
+        (let ((slot (aref slots i)))
+          (princ " :" stream)
+          (princ (cl--slot-descriptor-name slot) stream)
+          (princ " " stream)
+          (cl-print-object (aref object (1+ i)) stream)))
+      (when (< limit len)
+        (princ " " stream)
+        (cl-print-insert-ellipsis object limit stream)))
+    (princ ")" stream)))
+
+(cl-defmethod cl-print-object-contents ((object cl-structure-object) start stream)
   (let* ((class (cl-find-class (type-of object)))
          (slots (cl--struct-class-slots class))
-         (count (length slots)))
-    (princ (cl--struct-class-name class) stream)
-    (dotimes (i (if (natnump print-length)
-                    (min print-length count) count))
+         (len (length slots))
+         (limit (if (natnump print-length)
+                    (min (+ start print-length) len) len))
+         (i start))
+    (while (< i limit)
       (let ((slot (aref slots i)))
-        (princ " :" stream)
+        (unless (= i start) (princ " " stream))
+        (princ ":" stream)
         (princ (cl--slot-descriptor-name slot) stream)
         (princ " " stream)
-        (cl-print-object (aref object (1+ i)) stream)))
-    (when (and (natnump print-length) (< print-length count))
-      (princ " ..." stream)))
-  (princ ")" stream))
+        (cl-print-object (aref object (1+ i)) stream))
+      (cl-incf i))
+    (when (< limit len)
+      (princ " " stream)
+      (cl-print-insert-ellipsis object limit stream))))
+
+(cl-defmethod cl-print-object ((object string) stream)
+  (unless stream (setq stream standard-output))
+  (let* ((has-properties (or (text-properties-at 0 object)
+                             (next-property-change 0 object)))
+         (len (length object))
+         (limit (if (natnump print-length) (min print-length len) len)))
+    (if (and has-properties
+             cl-print--depth
+             (natnump print-level)
+             (> cl-print--depth print-level))
+        (cl-print-insert-ellipsis object 0 stream)
+      ;; Print all or part of the string
+      (when has-properties
+        (princ "#(" stream))
+      (if (= limit len)
+          (prin1 (if has-properties (substring-no-properties object) object)
+                 stream)
+        (let ((part (concat (substring-no-properties object 0 limit) "...")))
+          (prin1 part stream)
+          (when (bufferp stream)
+            (with-current-buffer stream
+              (cl-print-propertize-ellipsis object limit
+                                            (- (point) 4)
+                                            (- (point) 1) stream)))))
+      ;; Print the property list.
+      (when has-properties
+        (let* ((interval-limit (and (natnump print-length)
+                                    (max 1 (/ print-length 3))))
+               (interval-count 0)
+               (start-pos (if (text-properties-at 0 object)
+                              0 (next-property-change 0 object)))
+               (end-pos (next-property-change start-pos object len)))
+          (while (and (or (null interval-limit)
+                          (< interval-count interval-limit))
+                      (< start-pos len))
+            (let ((props (text-properties-at start-pos object)))
+              (when props
+                (princ " " stream) (princ start-pos stream)
+                (princ " " stream) (princ end-pos stream)
+                (princ " " stream) (cl-print-object props stream)
+                (cl-incf interval-count))
+              (setq start-pos end-pos
+                    end-pos (next-property-change start-pos object len))))
+          (when (< start-pos len)
+            (princ " " stream)
+            (cl-print-insert-ellipsis object (list start-pos) stream)))
+        (princ ")" stream)))))
+
+(cl-defmethod cl-print-object-contents ((object string) start stream)
+  ;; If START is an integer, it is an index into the string, and the
+  ;; ellipsis that needs to be expanded is part of the string.  If
+  ;; START is a cons, its car is an index into the string, and the
+  ;; ellipsis that needs to be expanded is in the property list.
+  (let* ((len (length object)))
+    (if (atom start)
+        ;; Print part of the string.
+        (let* ((limit (if (natnump print-length)
+                          (min (+ start print-length) len) len))
+               (substr (substring-no-properties object start limit))
+               (printed (prin1-to-string substr))
+               (trimmed (substring printed 1 (1- (length printed)))))
+          (princ trimmed)
+          (when (< limit len)
+            (cl-print-insert-ellipsis object limit stream)))
+
+      ;; Print part of the property list.
+      (let* ((first t)
+             (interval-limit (and (natnump print-length)
+                                  (max 1 (/ print-length 3))))
+             (interval-count 0)
+             (start-pos (car start))
+             (end-pos (next-property-change start-pos object len)))
+        (while (and (or (null interval-limit)
+                        (< interval-count interval-limit))
+                    (< start-pos len))
+          (let ((props (text-properties-at start-pos object)))
+            (when props
+              (if first
+                  (setq first nil)
+                (princ " " stream))
+              (princ start-pos stream)
+              (princ " " stream) (princ end-pos stream)
+              (princ " " stream) (cl-print-object props stream)
+              (cl-incf interval-count))
+            (setq start-pos end-pos
+                  end-pos (next-property-change start-pos object len))))
+        (when (< start-pos len)
+          (princ " " stream)
+          (cl-print-insert-ellipsis object (list start-pos) stream))))))
 
 ;;; Circularity and sharing.
 
@@ -275,8 +435,17 @@ into a button whose action shows the function's disassembly.")
                  (push cdr stack)
                  (push car stack))
                 ((pred stringp)
-                 ;; We presumably won't print its text-properties.
-                 nil)
+                 (let* ((len (length object))
+                        (start (if (text-properties-at 0 object)
+                                   0 (next-property-change 0 object)))
+                        (end (and start
+                                  (next-property-change start object len))))
+                   (while (and start (< start len))
+                     (let ((props (text-properties-at start object)))
+                       (when props
+                         (push props stack))
+                       (setq start end
+                             end (next-property-change start object len))))))
                 ((or (pred arrayp) (pred byte-code-function-p))
                  ;; FIXME: Inefficient for char-tables!
                  (dotimes (i (length object))
@@ -291,6 +460,48 @@ into a button whose action shows the function's disassembly.")
         (cl-print--find-sharing object print-number-table)))
     print-number-table))
 
+(defun cl-print-insert-ellipsis (object start stream)
+  "Print \"...\" to STREAM with the `cl-print-ellipsis' text property.
+Save state in the text property in order to print the elided part
+of OBJECT later.  START should be 0 if the whole OBJECT is being
+elided, otherwise it should be an index or other pointer into the
+internals of OBJECT which can be passed to
+`cl-print-object-contents' at a future time."
+  (unless stream (setq stream standard-output))
+  (let ((ellipsis-start (and (bufferp stream)
+                             (with-current-buffer stream (point)))))
+    (princ "..." stream)
+    (when ellipsis-start
+      (with-current-buffer stream
+        (cl-print-propertize-ellipsis object start ellipsis-start (point)
+                                      stream)))))
+
+(defun cl-print-propertize-ellipsis (object start beg end stream)
+  "Add the `cl-print-ellipsis' property between BEG and END.
+STREAM should be a buffer.  OBJECT and START are as described in
+`cl-print-insert-ellipsis'."
+  (let ((value (list object start cl-print--number-table
+                     cl-print--currently-printing)))
+    (with-current-buffer stream
+      (put-text-property beg end 'cl-print-ellipsis value stream))))
+
+;;;###autoload
+(defun cl-print-expand-ellipsis (value stream)
+  "Print the expansion of an ellipsis to STREAM.
+VALUE should be the value of the `cl-print-ellipsis' text property
+which was attached to the ellipsis by `cl-prin1'."
+  (let ((cl-print--depth 1)
+        (object (nth 0 value))
+        (start (nth 1 value))
+        (cl-print--number-table (nth 2 value))
+        (print-number-table (nth 2 value))
+        (cl-print--currently-printing (nth 3 value)))
+    (when (eq object (car cl-print--currently-printing))
+      (pop cl-print--currently-printing))
+    (if (equal start 0)
+        (cl-print-object object stream)
+      (cl-print-object-contents object start stream))))
+
 ;;;###autoload
 (defun cl-prin1 (object &optional stream)
   "Print OBJECT on STREAM according to its type.
@@ -313,5 +524,45 @@ node `(elisp)Output Variables'."
     (cl-prin1 object (current-buffer))
     (buffer-string)))
 
+;;;###autoload
+(defun cl-print-to-string-with-limit (print-function value limit)
+  "Return a string containing a printed representation of VALUE.
+Attempt to get the length of the returned string under LIMIT
+characters with appropriate settings of `print-level' and
+`print-length.'  Use PRINT-FUNCTION to print, which should take
+the arguments VALUE and STREAM and which should respect
+`print-length' and `print-level'.  LIMIT may be nil or zero in
+which case PRINT-FUNCTION will be called with `print-level' and
+`print-length' bound to nil.
+
+Use this function with `cl-prin1' to print an object,
+abbreviating it with ellipses to fit within a size limit.  Use
+this function with `cl-prin1-expand-ellipsis' to expand an
+ellipsis, abbreviating the expansion to stay within a size
+limit."
+  (setq limit (and (natnump limit)
+                   (not (zerop limit))
+                   limit))
+  ;; Since this is used by the debugger when stack space may be
+  ;; limited, if you increase print-level here, add more depth in
+  ;; call_debugger (bug#31919).
+  (let* ((print-length (when limit (min limit 50)))
+         (print-level (when limit (min 8 (truncate (log limit)))))
+         (delta (when limit
+                  (max 1 (truncate (/ print-length print-level))))))
+    (with-temp-buffer
+      (catch 'done
+        (while t
+          (erase-buffer)
+          (funcall print-function value (current-buffer))
+          ;; Stop when either print-level is too low or the value is
+          ;; successfully printed in the space allowed.
+          (when (or (not limit)
+                    (< (- (point-max) (point-min)) limit)
+                    (= print-level 2))
+            (throw 'done (buffer-string)))
+          (cl-decf print-level)
+          (cl-decf print-length delta))))))
+
 (provide 'cl-print)
 ;;; cl-print.el ends here
diff --git a/lisp/emacs-lisp/debug.el b/lisp/emacs-lisp/debug.el
index 0efaa637129..7fc2b41c70c 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 (plist-put backtrace-view :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 +528,10 @@ 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)
+  (let ((map (make-keymap)))
+    (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)
@@ -634,63 +539,47 @@ 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
-                  :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
-                  :help "Follow cross-reference"))
-    (define-key menu-map [deb-nxt]
-      '(menu-item "Next Line" next-line
-                  :help "Move cursor down"))
-    (define-key menu-map [deb-s1] '("--"))
-    (define-key menu-map [deb-lfunc]
-      '(menu-item "List debug on entry functions" debugger-list-functions
-                  :help "Display a list of all the functions now set to debug on entry"))
-    (define-key menu-map [deb-fclear]
-      '(menu-item "Cancel debug frame" debugger-frame-clear
-                  :help "Do not enter debugger when this frame exits"))
-    (define-key menu-map [deb-frame]
-      '(menu-item "Debug frame" debugger-frame
-                  :help "Request entry to debugger when this frame exits"))
-    (define-key menu-map [deb-s2] '("--"))
-    (define-key menu-map [deb-ret]
-      '(menu-item "Return value..." debugger-return-value
-                  :help "Continue, specifying value to return."))
-    (define-key menu-map [deb-rec]
-      '(menu-item "Display and Record Expression" debugger-record-expression
-                  :help "Display a variable's value and record it in `*Backtrace-record*' buffer"))
-    (define-key menu-map [deb-eval]
-      '(menu-item "Eval Expression..." debugger-eval-expression
-                  :help "Eval an expression, in an environment like that outside the debugger"))
-    (define-key menu-map [deb-jump]
-      '(menu-item "Jump" debugger-jump
-                  :help "Continue to exit from this frame, with all debug-on-entry suspended"))
-    (define-key menu-map [deb-cont]
-      '(menu-item "Continue" debugger-continue
-        :help "Continue, evaluating this expression without stopping"))
-    (define-key menu-map [deb-step]
-      '(menu-item "Step through" debugger-step-through
-        :help "Proceed, stepping through subexpressions of this expression"))
+    (easy-menu-define nil map ""
+      '("Debugger"
+        ["Step through" debugger-step-through
+         :help "Proceed, stepping through subexpressions of this expression"]
+        ["Continue" debugger-continue
+         :help "Continue, evaluating this expression without stopping"]
+        ["Jump" debugger-jump
+         :help "Continue to exit from this frame, with all debug-on-entry suspended"]
+        ["Eval Expression..." debugger-eval-expression
+         :help "Eval an expression, in an environment like that outside the debugger"]
+        ["Display and Record Expression" debugger-record-expression
+         :help "Display a variable's value and record it in `*Backtrace-record*' buffer"]
+        ["Return value..." debugger-return-value
+         :help "Continue, specifying value to return."]
+        "--"
+        ["Debug frame" debugger-frame
+         :help "Request entry to debugger when this frame exits"]
+        ["Cancel debug frame" debugger-frame-clear
+         :help "Do not enter debugger when this frame exits"]
+        ["List debug on entry functions" debugger-list-functions
+         :help "Display a list of all the functions now set to debug on entry"]
+        "--"
+        ["Next Line" next-line
+         :help "Move cursor down"]
+        ["Help for Symbol" backtrace-help-follow-symbol
+         :help "Show help for symbol at point"]
+        ["Describe Debugger Mode" describe-mode
+         :help "Display documentation for debugger-mode"]
+        "--"
+        ["Quit" debugger-quit
+         :help "Quit debugging and return to top level"]))
     map))
 
 (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 +593,6 @@ 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 +619,6 @@ 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 +719,13 @@ 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..fa418c68281 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)))
 
 
@@ -3658,7 +3692,7 @@ be installed in `emacs-lisp-mode-map'.")
 
     ;; misc
     (define-key map "?" 'edebug-help)
-    (define-key map "d" 'edebug-backtrace)
+    (define-key map "d" 'edebug-pop-to-backtrace)
 
     (define-key map "-" 'negative-argument)
 
@@ -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'.
@@ -3947,49 +3983,136 @@ Otherwise call `debug' normally."
     (apply #'debug arg-mode args)
     ))
 
-
-(defun edebug-backtrace ()
-  "Display a non-working backtrace.  Better than nothing..."
+;;; Backtrace buffer
+
+(defvar-local edebug-backtrace-frames nil
+  "Stack frames of the current Edebug Backtrace buffer without instrumentation.
+This should be a list of `edebug---frame' objects.")
+(defvar-local edebug-instrumented-backtrace-frames nil
+  "Stack frames of the current Edebug Backtrace buffer with instrumentation.
+This should be a list of `edebug---frame' objects.")
+
+;; 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-pop-to-backtrace ()
+  "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))
-          )))
-      )))))
+  (pop-to-buffer edebug-backtrace-buffer)
+  (unless (derived-mode-p 'backtrace-mode)
+    (backtrace-mode)
+    (add-hook 'backtrace-goto-source-functions 'edebug--backtrace-goto-source))
+  (setq edebug-instrumented-backtrace-frames
+        (backtrace-get-frames 'edebug-debugger
+                              :constructor #'edebug--make-frame)
+        edebug-backtrace-frames (edebug--strip-instrumentation
+                                 edebug-instrumented-backtrace-frames)
+        backtrace-frames edebug-backtrace-frames)
+  (backtrace-print)
+  (goto-char (point-min)))
+
+(defun edebug--strip-instrumentation (frames)
+  "Return a new list of backtrace frames with instrumentation removed.
+Remove frames for Edebug's functions and the lambdas in
+`edebug-enter' wrappers.  Fill in the def-name, before-index
+and after-index fields in both FRAMES and the returned list
+of deinstrumented frames, for those frames where the source
+code location is known."
+  (let (skip-next-lambda def-name before-index after-index results
+        (index (length frames)))
+    (dolist (frame (reverse frames))
+      (let ((new-frame (copy-edebug--frame frame))
+            (fun (edebug--frame-fun frame))
+            (args (edebug--frame-args frame)))
+        (cl-decf index)
+        (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-frame new-frame)
+           (edebug--add-source-info new-frame def-name before-index after-index)
+           (edebug--add-source-info frame def-name before-index after-index)
+           (push new-frame results)
+           (setq before-index nil
+                 after-index nil))
+          (`(,(or 'lambda 'closure) . ,_)
+           (unless skip-next-lambda
+             (edebug--unwrap-frame new-frame)
+             (edebug--add-source-info frame def-name before-index after-index)
+             (edebug--add-source-info new-frame def-name before-index after-index)
+             (push new-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-frame (frame)
+  "Remove Edebug's instrumentation from FRAME.
+Strip it from the function and any unevaluated arguments."
+  (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)))))
+
+(defun edebug--add-source-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."
+  (when (and before-index def-name)
+    (setf (edebug--frame-flags frame)
+          (plist-put (copy-sequence (edebug--frame-flags frame))
+                     :source-available t)))
+  (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))
+
+(defun edebug--backtrace-goto-source ()
+  (let* ((index (backtrace-get-index))
+         (frame (nth index backtrace-frames)))
+    (when (edebug--frame-def-name frame)
+      (let* ((data (get (edebug--frame-def-name frame) 'edebug))
+             (marker (nth 0 data))
+             (offsets (nth 2 data)))
+        (pop-to-buffer (marker-buffer marker))
+        (goto-char (+ (marker-position marker)
+                      (aref offsets (edebug--frame-before-index frame))))))))
+
+(defun edebug-backtrace-show-instrumentation ()
+  "Show Edebug's instrumentation in an Edebug Backtrace buffer."
+  (interactive)
+  (unless (eq backtrace-frames edebug-instrumented-backtrace-frames)
+    (setq backtrace-frames edebug-instrumented-backtrace-frames)
+    (revert-buffer)))
 
+(defun edebug-backtrace-hide-instrumentation ()
+  "Hide Edebug's instrumentation in an Edebug Backtrace buffer."
+  (interactive)
+  (unless (eq backtrace-frames edebug-backtrace-frames)
+    (setq backtrace-frames edebug-backtrace-frames)
+    (revert-buffer)))
 
 ;;; Trace display
 
@@ -4163,7 +4286,7 @@ It is removed when you hit any char."
      ["Bounce to Current Point" edebug-bounce-point t]
      ["View Outside Windows" edebug-view-outside t]
      ["Previous Result" edebug-previous-result t]
-     ["Show Backtrace" edebug-backtrace t]
+     ["Show Backtrace" edebug-pop-to-backtrace t]
      ["Display Freq Count" edebug-display-freq-count t])
 
     ("Eval"
diff --git a/lisp/emacs-lisp/ert.el b/lisp/emacs-lisp/ert.el
index cad21044f15..eb9695d0c12 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 +2443,20 @@ 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-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
diff --git a/lisp/subr.el b/lisp/subr.el
index f8c19efc379..fbb3e49a35c 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -4687,25 +4687,6 @@ The properties used on SYMBOL are `composefunc', `sendfunc',
   (put symbol 'hookvar (or hookvar 'mail-send-hook)))
 
 
-(defun backtrace--print-frame (evald func args flags)
-  "Print a trace of a single stack frame to `standard-output'.
-EVALD, FUNC, ARGS, FLAGS are as in `mapbacktrace'."
-  (princ (if (plist-get flags :debug-on-exit) "* " "  "))
-  (cond
-   ((and evald (not debugger-stack-frame-as-list))
-    (cl-prin1 func)
-    (if args (cl-prin1 args) (princ "()")))
-   (t
-    (cl-prin1 (cons func args))))
-  (princ "\n"))
-
-(defun backtrace ()
-  "Print a trace of Lisp function calls currently active.
-Output stream used is value of `standard-output'."
-  (let ((print-level (or print-level 8))
-        (print-escape-control-characters t))
-    (mapbacktrace #'backtrace--print-frame 'backtrace)))
-
 (defun backtrace-frames (&optional base)
   "Collect all frames of current backtrace into a list.
 If non-nil, BASE should be a function, and frames before its
diff --git a/test/lisp/emacs-lisp/backtrace-tests.el b/test/lisp/emacs-lisp/backtrace-tests.el
new file mode 100644
index 00000000000..edd45c770c5
--- /dev/null
+++ b/test/lisp/emacs-lisp/backtrace-tests.el
@@ -0,0 +1,436 @@
+;;; backtrace-tests.el --- Tests for backtraces -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2018 Free Software Foundation, Inc.
+
+;; Author: Gemini Lasswell
+
+;; 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/>.
+
+;;; Code:
+
+(require 'backtrace)
+(require 'ert)
+(require 'ert-x)
+(require 'seq)
+
+;; Delay evaluation of the backtrace-creating functions until
+;; load so that the backtraces are the same whether this file
+;; is compiled or not.
+
+(eval-and-compile
+  (defconst backtrace-tests--uncompiled-functions
+    '(progn
+       (defun backtrace-tests--make-backtrace (arg)
+         (backtrace-tests--setup-buffer))
+
+       (defun backtrace-tests--setup-buffer ()
+         "Set up the current buffer in backtrace mode."
+         (backtrace-mode)
+         (setq backtrace-frames (backtrace-get-frames))
+         (let ((this-index))
+           ;; Discard all past `backtrace-tests-make-backtrace'.
+           (dotimes (index (length backtrace-frames))
+             (when (eq (backtrace-frame-fun (nth index backtrace-frames))
+                       'backtrace-tests--make-backtrace)
+               (setq this-index index)))
+           (setq backtrace-frames (seq-subseq backtrace-frames 0 (1+ this-index))))
+         (backtrace-print))))
+
+  (eval backtrace-tests--uncompiled-functions))
+
+(defun backtrace-tests--backtrace-lines ()
+  (if debugger-stack-frame-as-list
+      '("  (backtrace-get-frames)\n"
+        "  (setq backtrace-frames (backtrace-get-frames))\n"
+        "  (backtrace-tests--setup-buffer)\n"
+        "  (backtrace-tests--make-backtrace %s)\n")
+    '("  backtrace-get-frames()\n"
+      "  (setq backtrace-frames (backtrace-get-frames))\n"
+      "  backtrace-tests--setup-buffer()\n"
+      "  backtrace-tests--make-backtrace(%s)\n")))
+
+(defconst backtrace-tests--line-count (length (backtrace-tests--backtrace-lines)))
+
+(defun backtrace-tests--backtrace-lines-with-locals ()
+  (let ((lines (backtrace-tests--backtrace-lines))
+        (locals '("    [no locals]\n"
+                  "    [no locals]\n"
+                  "    [no locals]\n"
+                  "    arg = %s\n")))
+    (apply #'append (cl-mapcar #'list lines locals))))
+
+(defun backtrace-tests--result (value)
+  (format (apply #'concat (backtrace-tests--backtrace-lines))
+          (cl-prin1-to-string value)))
+
+(defun backtrace-tests--result-with-locals (value)
+  (let ((str (cl-prin1-to-string value)))
+    (format (apply #'concat (backtrace-tests--backtrace-lines-with-locals))
+            str str)))
+
+;; TODO check that debugger-batch-max-lines still works
+
+(defconst backtrace-tests--header "Test header\n")
+(defun backtrace-tests--insert-header ()
+  (insert backtrace-tests--header))
+
+;;; Tests
+
+(ert-deftest backtrace-tests--variables ()
+  "Backtrace buffers can show and hide local variables."
+  (ert-with-test-buffer (:name "variables")
+    (let ((results (concat backtrace-tests--header
+                           (backtrace-tests--result 'value)))
+          (last-frame (format (nth (1- backtrace-tests--line-count)
+                                   (backtrace-tests--backtrace-lines)) 'value))
+          (last-frame-with-locals
+           (format (apply #'concat (nthcdr (* 2 (1- backtrace-tests--line-count))
+                                           (backtrace-tests--backtrace-lines-with-locals)))
+                   'value 'value)))
+      (backtrace-tests--make-backtrace 'value)
+      (setq backtrace-insert-header-function #'backtrace-tests--insert-header)
+      (backtrace-print)
+      (should (string= (backtrace-tests--get-substring (point-min) (point-max))
+                       results))
+      ;; Go to the last frame.
+      (goto-char (point-max))
+      (forward-line -1)
+      ;; Turn on locals for that frame.
+      (backtrace-toggle-locals)
+      (should (string= (backtrace-tests--get-substring (point) (point-max))
+                       last-frame-with-locals))
+      (should (string= (backtrace-tests--get-substring (point-min) (point-max))
+                       (concat results
+                               (format (car (last (backtrace-tests--backtrace-lines-with-locals)))
+                                       'value))))
+      ;; Turn off locals for that frame.
+      (backtrace-toggle-locals)
+      (should (string= (backtrace-tests--get-substring (point) (point-max))
+                       last-frame))
+      (should (string= (backtrace-tests--get-substring (point-min) (point-max))
+                       results))
+      ;; Turn all locals on.
+      (backtrace-toggle-locals '(4))
+      (should (string= (backtrace-tests--get-substring (point) (point-max))
+                       last-frame-with-locals))
+      (should (string= (backtrace-tests--get-substring (point-min) (point-max))
+                       (concat backtrace-tests--header
+                               (backtrace-tests--result-with-locals 'value))))
+      ;; Turn all locals off.
+      (backtrace-toggle-locals '(4))
+      (should (string= (backtrace-tests--get-substring
+                        (point) (+ (point) (length last-frame)))
+                       last-frame))
+      (should (string= (backtrace-tests--get-substring (point-min) (point-max))
+                       results)))))
+
+(ert-deftest backtrace-tests--backward-frame ()
+  "`backtrace-backward-frame' moves backward to the start of a frame."
+  (ert-with-test-buffer (:name "backward")
+    (let ((results (concat backtrace-tests--header
+                           (backtrace-tests--result nil))))
+      (backtrace-tests--make-backtrace nil)
+      (setq backtrace-insert-header-function #'backtrace-tests--insert-header)
+      (backtrace-print)
+      (should (string= (backtrace-tests--get-substring (point-min) (point-max))
+                       results))
+
+      ;; Try to move backward from header.
+      (goto-char (+ (point-min) (/ (length backtrace-tests--header) 2)))
+      (let ((pos (point)))
+        (should-error (backtrace-backward-frame))
+        (should (= pos (point))))
+
+      ;; Try to move backward from start of first line.
+      (forward-line)
+      (let ((pos (point)))
+        (should-error (backtrace-backward-frame))
+        (should (= pos (point))))
+
+      ;; Move backward from middle of line.
+      (let ((start (point)))
+        (forward-char (/ (length (nth 0 (backtrace-tests--backtrace-lines))) 2))
+        (backtrace-backward-frame)
+        (should (= start (point))))
+
+      ;; Move backward from end of buffer.
+      (goto-char (point-max))
+      (backtrace-backward-frame)
+      (let* ((last (format (car (last (backtrace-tests--backtrace-lines))) nil))
+             (len (length last)))
+        (should (string= (buffer-substring-no-properties (point) (+ (point) len))
+                         last)))
+
+      ;; Move backward from start of line.
+      (backtrace-backward-frame)
+      (let* ((line (car (last (backtrace-tests--backtrace-lines) 2)))
+             (len (length line)))
+        (should (string= (buffer-substring-no-properties (point) (+ (point) len))
+                         line))))))
+
+(ert-deftest backtrace-tests--forward-frame ()
+  "`backtrace-forward-frame' moves forward to the start of a frame."
+  (ert-with-test-buffer (:name "forward")
+    (let* ((arg '(1 2 3))
+           (results (concat backtrace-tests--header
+                            (backtrace-tests--result arg)))
+           (first-line (nth 0 (backtrace-tests--backtrace-lines))))
+      (backtrace-tests--make-backtrace arg)
+      (setq backtrace-insert-header-function #'backtrace-tests--insert-header)
+      (backtrace-print)
+      (should (string= (backtrace-tests--get-substring (point-min) (point-max))
+                       results))
+      ;; Move forward from header.
+      (goto-char (+ (point-min) (/ (length backtrace-tests--header) 2)))
+      (backtrace-forward-frame)
+      (should (string= (backtrace-tests--get-substring
+                        (point) (+ (point) (length first-line)))
+                       first-line))
+
+      (let ((start (point))
+            (offset (/ (length first-line) 2))
+            (second-line (nth 1 (backtrace-tests--backtrace-lines))))
+        ;; Move forward from start of first frame.
+        (backtrace-forward-frame)
+        (should (string= (backtrace-tests--get-substring
+                          (point) (+ (point) (length second-line)))
+                         second-line))
+        ;; Move forward from middle of first frame.
+        (goto-char (+ start offset))
+        (backtrace-forward-frame)
+        (should (string= (backtrace-tests--get-substring
+                          (point) (+ (point) (length second-line)))
+                         second-line)))
+      ;; Try to move forward from middle of last frame.
+      (goto-char (- (point-max)
+                    (/ 2 (length (car (last (backtrace-tests--backtrace-lines)))))))
+      (should-error (backtrace-forward-frame))
+      ;; Try to move forward from end of buffer.
+      (goto-char (point-max))
+      (should-error (backtrace-forward-frame)))))
+
+(ert-deftest backtrace-tests--single-and-multi-line ()
+  "Forms in backtrace frames can be on a single line or on multiple lines."
+  (ert-with-test-buffer (:name "single-multi-line")
+    (let* ((arg '(lambda (x)  ; Quote this so it isn't made into a closure.
+                   (let ((number (1+ x)))
+                     (+ x number))))
+           (header-string "Test header: ")
+           (header (format "%s%s\n" header-string arg))
+           (insert-header-function (lambda ()
+                                     (insert header-string)
+                                     (insert (backtrace-print-to-string arg))
+                                     (insert "\n")))
+           (results (concat header (backtrace-tests--result arg)))
+           (last-line (format (nth (1- backtrace-tests--line-count)
+                                   (backtrace-tests--backtrace-lines))
+                              arg))
+           (last-line-locals (format (nth (1- (* 2 backtrace-tests--line-count))
+                                          (backtrace-tests--backtrace-lines-with-locals))
+                                     arg)))
+
+      (backtrace-tests--make-backtrace arg)
+      (setq backtrace-insert-header-function insert-header-function)
+      (backtrace-print)
+      (should (string= (backtrace-tests--get-substring (point-min) (point-max))
+                       results))
+      ;; Check pp and collapse for the form in the header.
+      (goto-char (point-min))
+      (backtrace-tests--verify-single-and-multi-line header)
+      ;; Check pp and collapse for the last frame.
+      (goto-char (point-max))
+      (backtrace-backward-frame)
+      (backtrace-tests--verify-single-and-multi-line last-line)
+      ;; Check pp and collapse for local variables in the last line.
+      (goto-char (point-max))
+      (backtrace-backward-frame)
+      (backtrace-toggle-locals)
+      (forward-line)
+      (backtrace-tests--verify-single-and-multi-line last-line-locals))))
+
+(defun backtrace-tests--verify-single-and-multi-line (line)
+  "Verify that `backtrace-single-line' and `backtrace-multi-line' work at point.
+Point should be at the beginning of a line, and LINE should be a
+string containing the text of the line at point.  Assume that the
+line contains the strings \"lambda\" and \"number\"."
+  (let ((pos (point)))
+    (backtrace-multi-line)
+    ;; Verify point is still at the start of the line.
+    (should (= pos (point))))
+
+  ;; Verify the form now spans multiple lines.
+  (let ((pos (point)))
+    (search-forward "number")
+    (should-not (= pos (point-at-bol))))
+  ;; Collapse the form.
+  (backtrace-single-line)
+  ;; Verify that the form is now back on one line,
+  ;; and that point is at the same place.
+  (should (string= (backtrace-tests--get-substring
+                    (- (point) 6) (point)) "number"))
+  (should-not (= (point) (point-at-bol)))
+  (should (string= (backtrace-tests--get-substring
+                    (point-at-bol) (1+ (point-at-eol)))
+                   line)))
+
+(ert-deftest backtrace-tests--print-circle ()
+  "Backtrace buffers can toggle `print-circle' syntax."
+  (ert-with-test-buffer (:name "print-circle")
+    (let* ((print-circle nil)
+           (arg (let ((val (make-list 5 'a))) (nconc val val) val))
+           (results (backtrace-tests--make-regexp
+                     (backtrace-tests--result arg)))
+           (results-circle (regexp-quote (let ((print-circle 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-circle (regexp-quote
+                               (let ((print-circle 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-circle for that frame.
+      (backtrace-toggle-print-circle)
+      (should (string-match-p last-frame-circle
+                              (backtrace-tests--get-substring (point) (point-max))))
+      ;; Turn off print-circle for the frame.
+      (backtrace-toggle-print-circle)
+      (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-circle on for the buffer.
+      (backtrace-toggle-print-circle '(4))
+      (should (string-match-p last-frame-circle
+                              (backtrace-tests--get-substring (point) (point-max))))
+      (should (string-match-p results-circle
+                              (backtrace-tests--get-substring (point-min) (point-max))))
+      ;; Turn print-circle off.
+      (backtrace-toggle-print-circle '(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
+`print-circle' on.  Look for #n in string STR where n is any
+digit and replace with #[0-9]."
+  (let ((regexp (regexp-quote str)))
+    (with-temp-buffer
+      (insert regexp)
+      (goto-char (point-min))
+      (while (re-search-forward "#[0-9]" nil t)
+        (replace-match "#[0-9]")))
+    (buffer-string)))
+
+(ert-deftest backtrace-tests--expand-ellipsis ()
+  "Backtrace buffers ellipsify large forms as buttons which expand the ellipses."
+  ;; make a backtrace with an ellipsis
+  ;; expand the ellipsis
+  (ert-with-test-buffer (:name "variables")
+    (let* ((print-level nil)
+           (print-length nil)
+           (backtrace-line-length 300)
+           (arg (make-list 40 (make-string 10 ?a)))
+           (results (backtrace-tests--result arg)))
+      (backtrace-tests--make-backtrace arg)
+      (backtrace-print)
+
+      ;; There should be an ellipsis. Find and expand it.
+      (goto-char (point-min))
+      (search-forward "...")
+      (backward-char)
+      (push-button)
+
+      (should (string= (backtrace-tests--get-substring (point-min) (point-max))
+                       results)))))
+
+(ert-deftest backtrace-tests--expand-ellipses ()
+  "Backtrace buffers ellipsify large forms and can expand the ellipses."
+  (ert-with-test-buffer (:name "variables")
+    (let* ((print-level nil)
+           (print-length nil)
+           (backtrace-line-length 300)
+           (arg (let ((outer (make-list 40 (make-string 10 ?a)))
+                      (nested (make-list 40 (make-string 10 ?b))))
+                  (setf (nth 39 nested) (make-list 40 (make-string 10 ?c)))
+                  (setf (nth 39 outer) nested)
+                  outer))
+           (results (backtrace-tests--result-with-locals arg)))
+
+      ;; Make a backtrace with local variables visible.
+      (backtrace-tests--make-backtrace arg)
+      (backtrace-print)
+      (backtrace-toggle-locals '(4))
+
+      ;; There should be two ellipses.
+      (goto-char (point-min))
+      (should (search-forward "..."))
+      (should (search-forward "..."))
+      (should-error (search-forward "..."))
+
+      ;; Expanding the last frame without argument should expand both
+      ;; ellipses, but the expansions will contain one ellipsis each.
+      (let ((buffer-len (- (point-max) (point-min))))
+        (goto-char (point-max))
+        (backtrace-backward-frame)
+        (backtrace-expand-ellipses)
+        (should (> (- (point-max) (point-min)) buffer-len))
+        (goto-char (point-min))
+        (should (search-forward "..."))
+        (should (search-forward "..."))
+        (should-error (search-forward "...")))
+
+      ;; Expanding with argument should remove all ellipses.
+      (goto-char (point-max))
+      (backtrace-backward-frame)
+      (backtrace-expand-ellipses '(4))
+      (goto-char (point-min))
+
+      (should-error (search-forward "..."))
+      (should (string= (backtrace-tests--get-substring (point-min) (point-max))
+                       results)))))
+
+
+(ert-deftest backtrace-tests--to-string ()
+  "Backtraces can be produced as strings."
+  (let ((frames (ert-with-test-buffer (:name nil)
+                  (backtrace-tests--make-backtrace "string")
+                  backtrace-frames)))
+    (should (string= (backtrace-to-string frames)
+                     (backtrace-tests--result "string")))))
+
+(defun backtrace-tests--get-substring (beg end)
+  "Return the visible text between BEG and END.
+Strip the string properties because it makes failed test results
+easier to read."
+  (substring-no-properties (filter-buffer-substring beg end)))
+
+(provide 'backtrace-tests)
+
+;;; backtrace-tests.el ends here
diff --git a/test/lisp/emacs-lisp/cl-print-tests.el b/test/lisp/emacs-lisp/cl-print-tests.el
index 404d323d0c1..a469b5526c0 100644
--- a/test/lisp/emacs-lisp/cl-print-tests.el
+++ b/test/lisp/emacs-lisp/cl-print-tests.el
@@ -56,19 +56,30 @@
   (let ((long-list (make-list 5 'a))
         (long-vec (make-vector 5 'b))
         (long-struct (cl-print-tests-con))
+        (long-string (make-string 5 ?a))
         (print-length 4))
     (should (equal "(a a a a ...)" (cl-prin1-to-string long-list)))
     (should (equal "[b b b b ...]" (cl-prin1-to-string long-vec)))
     (should (equal "#s(cl-print-tests-struct :a nil :b nil :c nil :d nil ...)"
-                   (cl-prin1-to-string long-struct)))))
+                   (cl-prin1-to-string long-struct)))
+    (should (equal "\"aaaa...\"" (cl-prin1-to-string long-string)))))
 
 (ert-deftest cl-print-tests-4 ()
   "CL printing observes `print-level'."
-  (let ((deep-list '(a (b (c (d (e))))))
-        (deep-struct (cl-print-tests-con))
-        (print-level 4))
+  (let* ((deep-list '(a (b (c (d (e))))))
+         (buried-vector '(a (b (c (d [e])))))
+         (deep-struct (cl-print-tests-con))
+         (buried-struct `(a (b (c (d ,deep-struct)))))
+         (buried-string '(a (b (c (d #("hello" 0 5 (cl-print-test t)))))))
+         (buried-simple-string '(a (b (c (d "hello")))))
+         (print-level 4))
     (setf (cl-print-tests-struct-a deep-struct) deep-list)
     (should (equal "(a (b (c (d ...))))" (cl-prin1-to-string deep-list)))
+    (should (equal "(a (b (c (d ...))))" (cl-prin1-to-string buried-vector)))
+    (should (equal "(a (b (c (d ...))))" (cl-prin1-to-string buried-struct)))
+    (should (equal "(a (b (c (d ...))))" (cl-prin1-to-string buried-string)))
+    (should (equal "(a (b (c (d \"hello\"))))"
+                   (cl-prin1-to-string buried-simple-string)))
     (should (equal "#s(cl-print-tests-struct :a (a (b (c ...))) :b nil :c nil :d nil :e nil)"
                    (cl-prin1-to-string deep-struct)))))
 
@@ -82,6 +93,129 @@
       (should (equal "((quote a) (function b) (\\` ((\\, c) (\\,@ d))))"
                      (cl-prin1-to-string quoted-stuff))))))
 
+(ert-deftest cl-print-tests-strings ()
+  "CL printing prints strings and propertized strings."
+  (let* ((str1 "abcdefghij")
+         (str2 #("abcdefghij" 3 6 (bold t) 7 9 (italic t)))
+         (str3 #("abcdefghij" 0 10 (test t)))
+         (obj '(a b))
+         ;; Since the byte compiler reuses string literals,
+         ;; and the put-text-property call is destructive, use
+         ;; copy-sequence to make a new string.
+         (str4 (copy-sequence "abcdefghij")))
+    (put-text-property 0 5 'test obj str4)
+    (put-text-property 7 10 'test obj str4)
+
+    (should (equal "\"abcdefghij\"" (cl-prin1-to-string str1)))
+    (should (equal "#(\"abcdefghij\" 3 6 (bold t) 7 9 (italic t))"
+                   (cl-prin1-to-string str2)))
+    (should (equal "#(\"abcdefghij\" 0 10 (test t))"
+                   (cl-prin1-to-string str3)))
+    (let ((print-circle nil))
+      (should
+       (equal
+        "#(\"abcdefghij\" 0 5 (test (a b)) 7 10 (test (a b)))"
+        (cl-prin1-to-string str4))))
+    (let ((print-circle t))
+      (should
+       (equal
+        "#(\"abcdefghij\" 0 5 (test #1=(a b)) 7 10 (test #1#))"
+        (cl-prin1-to-string str4))))))
+
+(ert-deftest cl-print-tests-ellipsis-cons ()
+  "Ellipsis expansion works in conses."
+  (let ((print-length 4)
+        (print-level 3))
+    (cl-print-tests-check-ellipsis-expansion
+     '(0 1 2 3 4 5) "(0 1 2 3 ...)" "4 5")
+    (cl-print-tests-check-ellipsis-expansion
+     '(0 1 2 3 4 5 6 7 8 9) "(0 1 2 3 ...)" "4 5 6 7 ...")
+    (cl-print-tests-check-ellipsis-expansion
+     '(a (b (c (d (e))))) "(a (b (c ...)))" "(d (e))")
+    (cl-print-tests-check-ellipsis-expansion
+     (let ((x (make-list 6 'b)))
+       (setf (nthcdr 6 x) 'c)
+       x)
+     "(b b b b ...)" "b b . c")))
+
+(ert-deftest cl-print-tests-ellipsis-vector ()
+  "Ellipsis expansion works in vectors."
+  (let ((print-length 4)
+        (print-level 3))
+    (cl-print-tests-check-ellipsis-expansion
+     [0 1 2 3 4 5] "[0 1 2 3 ...]" "4 5")
+    (cl-print-tests-check-ellipsis-expansion
+     [0 1 2 3 4 5 6 7 8 9] "[0 1 2 3 ...]" "4 5 6 7 ...")
+    (cl-print-tests-check-ellipsis-expansion
+     [a [b [c [d [e]]]]] "[a [b [c ...]]]" "[d [e]]")))
+
+(ert-deftest cl-print-tests-ellipsis-string ()
+  "Ellipsis expansion works in strings."
+  (let ((print-length 4)
+        (print-level 3))
+    (cl-print-tests-check-ellipsis-expansion
+     "abcdefg" "\"abcd...\"" "efg")
+    (cl-print-tests-check-ellipsis-expansion
+     "abcdefghijk" "\"abcd...\"" "efgh...")
+    (cl-print-tests-check-ellipsis-expansion
+     '(1 (2 (3 #("abcde" 0 5 (test t)))))
+     "(1 (2 (3 ...)))" "#(\"abcd...\" 0 5 (test t))")
+    (cl-print-tests-check-ellipsis-expansion
+     #("abcd" 0 1 (bold t) 1 2 (invisible t) 3 4 (italic t))
+     "#(\"abcd\" 0 1 (bold t) ...)" "1 2 (invisible t) ...")))
+
+(ert-deftest cl-print-tests-ellipsis-struct ()
+  "Ellipsis expansion works in structures."
+  (let ((print-length 4)
+        (print-level 3)
+        (struct (cl-print-tests-con)))
+    (cl-print-tests-check-ellipsis-expansion
+     struct "#s(cl-print-tests-struct :a nil :b nil :c nil :d nil ...)" ":e nil")
+    (let ((print-length 2))
+      (cl-print-tests-check-ellipsis-expansion
+       struct "#s(cl-print-tests-struct :a nil :b nil ...)" ":c nil :d nil ..."))
+    (cl-print-tests-check-ellipsis-expansion
+     `(a (b (c ,struct)))
+     "(a (b (c ...)))"
+     "#s(cl-print-tests-struct :a nil :b nil :c nil :d nil ...)")))
+
+(ert-deftest cl-print-tests-ellipsis-circular ()
+  "Ellipsis expansion works with circular objects."
+  (let ((wide-obj (list 0 1 2 3 4))
+        (deep-obj `(0 (1 (2 (3 (4))))))
+        (print-length 4)
+        (print-level 3))
+    (setf (nth 4 wide-obj) wide-obj)
+    (setf (car (cadadr (cadadr deep-obj))) deep-obj)
+    (let ((print-circle nil))
+      (cl-print-tests-check-ellipsis-expansion-rx
+       wide-obj (regexp-quote "(0 1 2 3 ...)") "\\`#[0-9]\\'")
+      (cl-print-tests-check-ellipsis-expansion-rx
+       deep-obj (regexp-quote "(0 (1 (2 ...)))") "\\`(3 (#[0-9]))\\'"))
+    (let ((print-circle t))
+      (cl-print-tests-check-ellipsis-expansion
+       wide-obj "#1=(0 1 2 3 ...)" "#1#")
+      (cl-print-tests-check-ellipsis-expansion
+       deep-obj "#1=(0 (1 (2 ...)))" "(3 (#1#))"))))
+
+(defun cl-print-tests-check-ellipsis-expansion (obj expected expanded)
+  (let* ((result (cl-prin1-to-string obj))
+         (pos (next-single-property-change 0 'cl-print-ellipsis result))
+         value)
+    (should pos)
+    (setq value (get-text-property pos 'cl-print-ellipsis result))
+    (should (equal expected result))
+    (should (equal expanded (with-output-to-string (cl-print-expand-ellipsis
+                                                    value nil))))))
+
+(defun cl-print-tests-check-ellipsis-expansion-rx (obj expected expanded)
+  (let* ((result (cl-prin1-to-string obj))
+         (pos (next-single-property-change 0 'cl-print-ellipsis result))
+         (value (get-text-property pos 'cl-print-ellipsis result)))
+    (should (string-match expected result))
+    (should (string-match expanded (with-output-to-string
+                                     (cl-print-expand-ellipsis value nil))))))
+
 (ert-deftest cl-print-circle ()
   (let ((x '(#1=(a . #1#) #1#)))
     (let ((print-circle nil))
@@ -99,5 +233,41 @@
     (let ((print-circle t))
       (should (equal "(0 . #1=(0 . #1#))" (cl-prin1-to-string x))))))
 
+(ert-deftest cl-print-tests-print-to-string-with-limit ()
+  (let* ((thing10 (make-list 10 'a))
+         (thing100 (make-list 100 'a))
+         (thing10x10 (make-list 10 thing10))
+         (nested-thing (let ((val 'a))
+                         (dotimes (_i 20)
+                           (setq val (list val)))
+                         val))
+         ;; Make a consistent environment for this test.
+         (print-circle nil)
+         (print-level nil)
+         (print-length nil))
+
+    ;; Print something that fits in the space given.
+    (should (string= (cl-prin1-to-string thing10)
+                     (cl-print-to-string-with-limit #'cl-prin1 thing10 100)))
+
+    ;; Print something which needs to be abbreviated and which can be.
+    (should (< (length (cl-print-to-string-with-limit #'cl-prin1 thing100 100))
+               100
+               (length (cl-prin1-to-string thing100))))
+
+    ;; Print something resistant to easy abbreviation.
+    (should (string= (cl-prin1-to-string thing10x10)
+                     (cl-print-to-string-with-limit #'cl-prin1 thing10x10 100)))
+
+    ;; Print something which should be abbreviated even if the limit is large.
+    (should (< (length (cl-print-to-string-with-limit #'cl-prin1 nested-thing 1000))
+               (length (cl-prin1-to-string nested-thing))))
+
+    ;; Print with no limits.
+    (dolist (thing (list thing10 thing100 thing10x10 nested-thing))
+      (let ((rep (cl-prin1-to-string thing)))
+        (should (string= rep (cl-print-to-string-with-limit #'cl-prin1 thing 0)))
+        (should (string= rep (cl-print-to-string-with-limit #'cl-prin1 thing nil)))))))
+
 
 ;;; cl-print-tests.el ends here.
diff --git a/test/lisp/emacs-lisp/edebug-resources/edebug-test-code.el b/test/lisp/emacs-lisp/edebug-resources/edebug-test-code.el
index f3fc78d4e12..97dead057a9 100644
--- a/test/lisp/emacs-lisp/edebug-resources/edebug-test-code.el
+++ b/test/lisp/emacs-lisp/edebug-resources/edebug-test-code.el
@@ -41,7 +41,7 @@
 (defun edebug-test-code-range (num)
   !start!(let ((index 0)
         (result nil))
-    (while (< index num)!test!
+    (while !lt!(< index num)!test!
       (push index result)!loop!
       (cl-incf index))!end-loop!
     (nreverse result)))
diff --git a/test/lisp/emacs-lisp/edebug-tests.el b/test/lisp/emacs-lisp/edebug-tests.el
index 7d780edf285..7880aaf95bc 100644
--- a/test/lisp/emacs-lisp/edebug-tests.el
+++ b/test/lisp/emacs-lisp/edebug-tests.el
@@ -432,9 +432,11 @@ test and possibly others should be updated."
     (verify-keybinding "P" 'edebug-view-outside) ;; same as v
     (verify-keybinding "W" 'edebug-toggle-save-windows)
     (verify-keybinding "?" 'edebug-help)
-    (verify-keybinding "d" 'edebug-backtrace)
+    (verify-keybinding "d" 'edebug-pop-to-backtrace)
     (verify-keybinding "-" 'negative-argument)
-    (verify-keybinding "=" 'edebug-temp-display-freq-count)))
+    (verify-keybinding "=" 'edebug-temp-display-freq-count)
+    (should (eq (lookup-key backtrace-mode-map "n") 'backtrace-forward-frame))
+    (should (eq (lookup-key backtrace-mode-map "s") 'backtrace-goto-source))))
 
 (ert-deftest edebug-tests-stop-point-at-start-of-first-instrumented-function ()
   "Edebug stops at the beginning of an instrumented function."
@@ -924,5 +926,17 @@ test and possibly others should be updated."
     "g"
     (should (equal edebug-tests-@-result "The result of applying + to (1 x) is 11")))))
 
+(ert-deftest edebug-tests-backtrace-goto-source ()
+  "Edebug can jump to instrumented source from its *Edebug-Backtrace* buffer."
+  (edebug-tests-with-normal-env
+   (edebug-tests-setup-@ "range" '(2) t)
+   (edebug-tests-run-kbd-macro
+    "@ SPC SPC"
+    (edebug-tests-should-be-at "range" "lt")
+    "dns"  ; Pop to backtrace, next frame, goto source.
+    (edebug-tests-should-be-at "range" "start")
+    "g"
+    (should (equal edebug-tests-@-result '(0 1))))))
+
 (provide 'edebug-tests)
 ;;; edebug-tests.el ends here
diff --git a/test/lisp/emacs-lisp/ert-tests.el b/test/lisp/emacs-lisp/ert-tests.el
index cb957bd9fd6..1fe5b79ef36 100644
--- a/test/lisp/emacs-lisp/ert-tests.el
+++ b/test/lisp/emacs-lisp/ert-tests.el
@@ -376,7 +376,7 @@ This macro is used to test if macroexpansion in `should' works."
          (test (make-ert-test :body test-body))
          (result (ert-run-test test)))
     (should (ert-test-failed-p result))
-    (should (eq (nth 1 (car (ert-test-failed-backtrace result)))
+    (should (eq (backtrace-frame-fun (car (ert-test-failed-backtrace result)))
                 'signal))))
 
 (ert-deftest ert-test-messages ()