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, 36 insertions, 0 deletions

diff --git a/test/lisp/emacs-lisp/cl-print-tests.el b/test/lisp/emacs-lisp/cl-print-tests.el
index 7594d2466b5..a469b5526c0 100644
--- a/test/lisp/emacs-lisp/cl-print-tests.el
+++ b/test/lisp/emacs-lisp/cl-print-tests.el
@@ -233,5 +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.