bug#25393: FEATURE REQUEST: *Backtrace* -- C source code def highlight + jump to def.

[Keith David Bershatsky]

As a feature request, the Emacs team may wish to consider implementing the 
ability for users to highlight C source code definitions in the *Backtrace* 
buffer and jump to the function definitions.  The highlighting of said 
functions makes the *Backtrace* buffer prettier, and it's handy to jump to the 
definitions with buttons.  A check should probably be included to see whether 
the `find-function-C-source-directory' contains C source code files before 
creating highlighting/buttons for jumping:

Here is the link to the related thread:  
http://emacs.stackexchange.com/questions/29875/debugging-debugger-mode-how-to-highlight-the-culprit

In my use-case, it wasn't very obvious to me that `/` was the cause of my 
backtrace error message because it was not highlighted -- this is true even 
though I am aware that the problem is always near the top of the debugger 
buffer.  It is just not a function I use a lot and it didn't really look like a 
function because it was just regular text coloration.

    (require 'debug)

    (defun debugger-make-xrefs (&optional buffer)
      "Attach cross-references to function names in the `*Backtrace*' buffer."
      (interactive "b")
      (with-current-buffer (or buffer (current-buffer))
        (save-excursion
          (setq buffer (current-buffer))
          (let ((inhibit-read-only t)
          (old-end (point-min)) (new-end (point-min)))
      (if debugger-previous-backtrace
          (let (old-start new-start (all-match t))
            (goto-char (point-max))
            (with-temp-buffer
        (insert debugger-previous-backtrace)
        (while (and all-match (not (bobp)))
          (setq old-end (point))
          (forward-line -1)
          (setq old-start (point))
          (with-current-buffer buffer
            (setq new-end (point))
            (forward-line -1)
            (setq new-start (point)))
          (if (not (zerop
              (let ((case-fold-search nil))
                (compare-buffer-substrings
                 (current-buffer) old-start old-end
                 buffer new-start new-end))))
              (setq all-match nil))))
            (delete-region new-end (point-max))
            (goto-char (point-max))
            (insert (substring debugger-previous-backtrace
             (- old-end (point-min))))
            (narrow-to-region (point-min) new-end)))
      (goto-char (point-min))
      (while (progn
         (goto-char (+ (point) 2))
         (skip-syntax-forward "^w_")
         (not (eobp)))
        (let* ((beg (point))
               (end (progn (skip-syntax-forward "w_") (point)))
               (fn (function-called-at-point)) ;; MODIFICATION
               (sym (intern-soft (buffer-substring-no-properties beg end)))
                ;; MODIFICATION
               (file 
                 (if fn
                   (let* (
                      (function fn)
                      (advised (and (symbolp function)
                          (featurep 'nadvice)
                          (advice--p (advice--symbol-function function))))
                       ;; If the function is advised, use the symbol that has 
the
                       ;; real definition, if that symbol is already set up.
                       (real-function
                        (or (and advised
                                        (advice--cd*r (advice--symbol-function 
function)))
                            function))
                       ;; Get the real definition.
                       (def (if (symbolp real-function)
                         (or (symbol-function real-function)
                             (signal 'void-function (list real-function)))
                       real-function))
                       (aliased (or (symbolp def)
                             ;; Advised & aliased function.
                             (and advised (symbolp real-function)
                            (not (eq 'autoload (car-safe def))))))
                       (file-name (find-lisp-object-file-name function (if 
aliased 'defun
                                                                            
def))))
                     file-name)
                   (and sym (symbol-file sym 'defun)))))
          (when (or fn file) ;; MODIFICATION
            (goto-char beg)
            (re-search-forward "\\(\\sw\\|\\s_\\)+")
            (help-xref-button 0 'help-function-def sym file)))
        (forward-line 1))
      (widen))
          (setq debugger-previous-backtrace (buffer-string)))))