Add new command to expand all "..."s in a backtrace frame

* doc/lispref/debugging.texi (Backtraces): Document new keybinding.
* lisp/emacs-lisp/backtrace.el (backtrace-line-length): Add the
option of unlimited line length.
(backtrace--match-ellipsis-in-string): Add a comment to explain
why this function is necessary.
(backtrace-mode-map): Add keybinding for 'backtrace-expand-ellipses'.
(backtrace-expand-ellipsis): Use 'cl-print-to-string-with-limit'.
(backtrace-expand-ellipses): New command.
(backtrace-print-to-string): Use 'cl-print-to-string-with-limit'.
Tag the printed forms with a gensym instead of the values of
print-length and print-level.
(backtrace--print): Add 'stream' argument.
* test/lisp/emacs-lisp/backtrace-tests.el
(backtrace-tests--expand-ellipsis): Make the test less dependent
on the implementation.
(backtrace-tests--expand-ellipses): New test.

Move the fitting of a printed representation into a limited number of
characters using appropriate values of print-level and print-length
from 'backtrace-print-to-string' to cl-print.el for future use by
other parts of Emacs.
* lisp/emacs-lisp/cl-print.el (cl-print-to-string-with-limit): New
function.
* test/lisp/emacs-lisp/cl-print-tests.el
(cl-print-tests-print-to-string-with-limit): New test.

1 files changed, 3 insertions, 0 deletions

diff --git a/doc/lispref/debugging.texi b/doc/lispref/debugging.texi
index 5230854cc7a..87429a67ba9 100644
--- a/doc/lispref/debugging.texi
+++ b/doc/lispref/debugging.texi
@@ -457,6 +457,9 @@ 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