New user option 'debugger-stack-frame-as-list'

* src/eval.c (syms_of_eval) <debugger-stack-frame-as-list>: New
variable.
* lisp/cus-start.el (standard): Add debugger-stack-frame-as-list.
* lisp/emacs-lisp/debug.el (debugger-setup-buffer): Adjust
backtrace processing for the value of debugger-stack-frame-as-list.
* lisp/emacs-lisp/edebug.el (edebug-backtrace): Adjust backtrace
processing for the value of debugger-stack-frame-as-list.
* doc/lispref/debugging.texi (Internals of Debugger): Document
debugger-stack-frame-as-list.
* etc/NEWS: Mention 'debugger-stack-frame-as-list'.

doc/lispref/debugging.texi

@@ -623,6 +623,37 @@ forms are elided.
 @end smallexample
 @end deffn
 
+@defvar debugger-stack-frame-as-list
+If this variable is non-@code{nil}, every stack frame of the backtrace
+is displayed as a list.  This aims at improving the backtrace
+readability at the cost of special forms no longer being visually
+different from regular function calls.
+
+With @code{debugger-stack-frame-as-list} non-@code{nil}, the above
+example would look as follows:
+
+@smallexample
+@group
+----------- Buffer: backtrace-output ------------
+  (backtrace)
+  (list ...computing arguments...)
+@end group
+  (progn ...)
+  (eval (progn (1+ var) (list (quote testing) (backtrace))))
+  (setq ...)
+  (save-excursion ...)
+  (let ...)
+  (with-output-to-temp-buffer ...)
+  (eval (with-output-to-temp-buffer ...))
+  (eval-last-sexp-1 nil)
+@group
+  (eval-last-sexp nil)
+  (call-interactively eval-last-sexp)
+----------- Buffer: backtrace-output ------------
+@end group
+@end smallexample
+@end defvar
+
 @defvar debug-on-next-call
 @cindex @code{eval}, and debugging
 @cindex @code{apply}, and debugging

etc/NEWS

@@ -205,6 +205,11 @@ questions, with a handy way to display help texts.
 +++
 ** 'switch-to-buffer-preserve-window-point' now defaults to t.
 
++++
+** The new variable 'debugger-stack-frame-as-list' allows displaying
+all call stack frames in a Lisp backtrace buffer as lists.  Both
+debug.el and edebug.el have been updated to heed to this variable.
+
 
 * Editing Changes in Emacs 25.3
 

lisp/cus-start.el

@@ -246,6 +246,7 @@ Leaving \"Default\" unchecked is equivalent with specifying a default of
 	     (debug-ignored-errors debug (repeat (choice symbol regexp)))
 	     (debug-on-quit debug boolean)
 	     (debug-on-signal debug boolean)
+             (debugger-stack-frame-as-list debugger boolean)
 	     ;; fileio.c
 	     (delete-by-moving-to-trash auto-save boolean "23.1")
 	     (auto-save-visited-file-name auto-save boolean)

lisp/emacs-lisp/debug.el

@@ -279,7 +279,9 @@ That buffer should be current already."
   (goto-char (point-min))
   (delete-region (point)
 		 (progn
-		   (search-forward "\n  debug(")
+		   (search-forward (if debugger-stack-frame-as-list
+                                       "\n  (debug "
+                                     "\n  debug("))
 		   (forward-line (if (eq (car args) 'debug)
                                      ;; Remove debug--implement-debug-on-entry
                                      ;; and the advice's `apply' frame.

lisp/emacs-lisp/edebug.el

@@ -3798,7 +3798,9 @@ Otherwise call `debug' normally."
 	  (forward-line 1)
 	  (delete-region last-ok-point (point)))
 
-	 ((looking-at "^  edebug")
+	 ((looking-at (if debugger-stack-frame-as-list
+                          "^  (edebug"
+                        "^  edebug"))
 	  (forward-line 1)
 	  (delete-region last-ok-point (point))
 	  )))

src/eval.c

@@ -3421,13 +3421,17 @@ Output stream used is value of `standard-output'.  */)
       else
 	{
 	  tem = backtrace_function (pdl);
+	  if (debugger_stack_frame_as_list)
+	    write_string ("(");
 	  Fprin1 (tem, Qnil);	/* This can QUIT.  */
-	  write_string ("(");
+	  if (!debugger_stack_frame_as_list)
+	    write_string ("(");
 	  {
 	    ptrdiff_t i;
 	    for (i = 0; i < backtrace_nargs (pdl); i++)
 	      {
-		if (i) write_string (" ");
+		if (i || debugger_stack_frame_as_list)
+		  write_string(" ");
 		Fprin1 (backtrace_args (pdl)[i], Qnil);
 	      }
 	  }
@@ -3850,6 +3854,10 @@ This is nil when the debugger is called under circumstances where it
 might not be safe to continue.  */);
   debugger_may_continue = 1;
 
+  DEFVAR_BOOL ("debugger-stack-frame-as-list", debugger_stack_frame_as_list,
+	       doc: /* Non-nil means display call stack frames as lists. */);
+  debugger_stack_frame_as_list = 0;
+
   DEFVAR_LISP ("debugger", Vdebugger,
 	       doc: /* Function to call to invoke debugger.
 If due to frame exit, args are `exit' and the value being returned;