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.

2 files changed, 87 insertions, 9 deletions

diff --git a/test/lisp/emacs-lisp/backtrace-tests.el b/test/lisp/emacs-lisp/backtrace-tests.el
index ba2d33a9d5c..ff26112ab9a 100644
--- a/test/lisp/emacs-lisp/backtrace-tests.el
+++ b/test/lisp/emacs-lisp/backtrace-tests.el
@@ -349,32 +349,74 @@ digit and replace with #[0-9]."
     (buffer-string)))
 
 (ert-deftest backtrace-tests--expand-ellipsis ()
-  "Backtrace buffers ellipsify large forms and can expand the ellipses."
+  "Backtrace buffers ellipsify large forms as buttons which expand the ellipses."
   ;; make a backtrace with an ellipsis
   ;; expand the ellipsis
   (ert-with-test-buffer (:name "variables")
     (let* ((print-level nil)
            (print-length nil)
-           (arg (let ((long (make-list 100 'a))
-                      (deep '(0 (1 (2 (3 (4 (5 (6 (7 (8 (9))))))))))))
-                  (setf (nth 1 long) deep)
-                  long))
+           (backtrace-line-length 300)
+           (arg (make-list 40 (make-string 10 ?a)))
            (results (backtrace-tests--result arg)))
       (backtrace-tests--make-backtrace arg)
       (backtrace-print)
 
-      ;; There should be two ellipses. Find and expand them.
+      ;; There should be an ellipsis. Find and expand it.
       (goto-char (point-min))
       (search-forward "...")
       (backward-char)
       (push-button)
-      (search-forward "...")
-      (backward-char)
-      (push-button)
 
       (should (string= (backtrace-tests--get-substring (point-min) (point-max))
                        results)))))
 
+(ert-deftest backtrace-tests--expand-ellipses ()
+  "Backtrace buffers ellipsify large forms and can expand the ellipses."
+  (ert-with-test-buffer (:name "variables")
+    (let* ((print-level nil)
+           (print-length nil)
+           (backtrace-line-length 300)
+           (arg (let ((outer (make-list 40 (make-string 10 ?a)))
+                      (nested (make-list 40 (make-string 10 ?b))))
+                  (setf (nth 39 nested) (make-list 40 (make-string 10 ?c)))
+                  (setf (nth 39 outer) nested)
+                  outer))
+           (results (backtrace-tests--result-with-locals arg)))
+
+      ;; Make a backtrace with local variables visible.
+      (backtrace-tests--make-backtrace arg)
+      (backtrace-print)
+      (backtrace-toggle-locals '(4))
+
+      ;; There should be two ellipses.
+      (goto-char (point-min))
+      (should (search-forward "..."))
+      (should (search-forward "..."))
+      (should-error (search-forward "..."))
+
+      ;; Expanding the last frame without argument should expand both
+      ;; ellipses, but the expansions will contain one ellipsis each.
+      (let ((buffer-len (- (point-max) (point-min))))
+        (goto-char (point-max))
+        (backtrace-backward-frame)
+        (backtrace-expand-ellipses)
+        (should (> (- (point-max) (point-min)) buffer-len))
+        (goto-char (point-min))
+        (should (search-forward "..."))
+        (should (search-forward "..."))
+        (should-error (search-forward "...")))
+
+      ;; Expanding with argument should remove all ellipses.
+      (goto-char (point-max))
+      (backtrace-backward-frame)
+      (backtrace-expand-ellipses '(4))
+      (goto-char (point-min))
+
+      (should-error (search-forward "..."))
+      (should (string= (backtrace-tests--get-substring (point-min) (point-max))
+                       results)))))
+
+
 (ert-deftest backtrace-tests--to-string ()
   "Backtraces can be produced as strings."
   (let ((frames (ert-with-test-buffer (:name nil)
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.