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

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

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

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

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

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

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

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

1 files changed, 29 insertions, 2 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 6ccf6fc0890..486e0d4384a 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 +483,18 @@ 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.
+
 ** 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 +511,14 @@ 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 +689,13 @@ 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