Limit integers printed as characters (Bug#16828)

* lisp/simple.el (eval-expression-print-maximum-character): New
variable.
(eval-expression-print-format): Only display value as character if
it's less than or equal to `eval-expression-print-maximum-character'.
(eval-expression-get-print-arguments): Check
eval-expression-print-maximum-character, allow negative arg to
override it.
(eval-expression):
* lisp/progmodes/elisp-mode.el (elisp--eval-last-sexp):
(elisp--eval-last-sexp-print-value): Handle new variable.
* doc/emacs/building.texi (Lisp Eval): Document new variable and
behavior.
* etc/NEWS: Announce it.
* test/lisp/progmodes/elisp-mode-tests.el
(eval-last-sexp-print-format-small-int)
(eval-last-sexp-print-format-small-int-echo)
(eval-last-sexp-print-format-large-int)
(eval-last-sexp-print-format-large-int-echo):
* test/lisp/simple-tests.el (eval-expression-print-format-small-int)
(eval-expression-print-format-small-int-echo)
(eval-expression-print-format-large-int)
(eval-expression-print-format-large-int-echo): New tests.

6 files changed, 164 insertions, 31 deletions

diff --git a/doc/emacs/building.texi b/doc/emacs/building.texi
index ba8eae07591..f7eb8fe9eaf 100644
--- a/doc/emacs/building.texi
+++ b/doc/emacs/building.texi
@@ -1485,7 +1485,9 @@ expression.)
 Emacs Lisp expression preceding point in the buffer, and displays the
 value in the echo area.  When the result of an evaluation is an
 integer, it is displayed together with the value in other formats
-(octal, hexadecimal, and character).
+(octal, hexadecimal, and character if
+@code{eval-expression-print-maximum-character}, described below,
+allows it).
 
   If @kbd{M-:} or @kbd{C-x C-e} is given a prefix argument, it inserts
 the value into the current buffer at point, rather than displaying it
@@ -1493,8 +1495,10 @@ in the echo area.  If the prefix argument is zero, any integer output
 is inserted together with its value in other formats (octal,
 hexadecimal, and character).  Such a prefix argument also prevents
 abbreviation of the output according to the variables
-@code{eval-expression-print-level} and @code{eval-expression-print-length}
-(see below).
+@code{eval-expression-print-level} and
+@code{eval-expression-print-length} (see below).  Similarly, a prefix
+argument of @code{-1} overrides the effect of
+@code{eval-expression-print-length}.
 
 @kindex C-M-x @r{(Emacs Lisp mode)}
 @findex eval-defun
@@ -1524,6 +1528,7 @@ eval-buffer} is similar but evaluates the entire buffer.
 
 @vindex eval-expression-print-level
 @vindex eval-expression-print-length
+@vindex eval-expression-print-maximum-character
 @vindex eval-expression-debug-on-error
   The options @code{eval-expression-print-level} and
 @code{eval-expression-print-length} control the maximum depth and
@@ -1533,6 +1538,8 @@ before abbreviating them.  Supplying a zero prefix argument to
 printed in full.  @code{eval-expression-debug-on-error} controls
 whether evaluation errors invoke the debugger when these commands are
 used; its default is @code{t}.
+@code{eval-expression-print-maximum-character} prevents integers which
+are larger than it from being displayed as characters.
 
 @node Lisp Interaction
 @section Lisp Interaction Buffers
diff --git a/etc/NEWS b/etc/NEWS
index 340718ecbc4..aa579b11c7a 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -352,6 +352,10 @@ environment variable on a remote machine to emacsclient, and
 use the local Emacs to edit remote files via Tramp.  See the node
 "emacsclient Options" in the user manual for the details.
 
++++
+** The new variable 'eval-expression-print-maximum-character' prevents
+large integers from being displayed as characters.
+
 
 * Editing Changes in Emacs 26.1
 
diff --git a/lisp/progmodes/elisp-mode.el b/lisp/progmodes/elisp-mode.el
index c2fdba47a09..6c6fb925040 100644
--- a/lisp/progmodes/elisp-mode.el
+++ b/lisp/progmodes/elisp-mode.el
@@ -1120,24 +1120,25 @@ output with no limit on the length and level of lists, and
 include additional formats for integers \(octal, hexadecimal, and
 character)."
   (pcase-let*
-      ((`(,insert-value ,no-truncate ,char-print)
+      ((`(,insert-value ,no-truncate ,char-print-limit)
         (eval-expression-get-print-arguments eval-last-sexp-arg-internal)))
     ;; Setup the lexical environment if lexical-binding is enabled.
     (elisp--eval-last-sexp-print-value
      (eval (eval-sexp-add-defvars (elisp--preceding-sexp)) lexical-binding)
-     (if insert-value (current-buffer) t) no-truncate char-print)))
+     (if insert-value (current-buffer) t) no-truncate char-print-limit)))
 
 (defun elisp--eval-last-sexp-print-value
-    (value output &optional no-truncate char-print)
+    (value output &optional no-truncate char-print-limit)
   (let* ((unabbreviated (let ((print-length nil) (print-level nil))
                           (prin1-to-string value)))
+         (eval-expression-print-maximum-character char-print-limit)
          (print-length (unless no-truncate eval-expression-print-length))
          (print-level  (unless no-truncate eval-expression-print-level))
          (beg (point))
          end)
     (prog1
 	(prin1 value output)
-      (let ((str (and char-print (eval-expression-print-format value))))
+      (let ((str (and char-print-limit (eval-expression-print-format value))))
 	(if str (princ str output)))
       (setq end (point))
       (when (and (bufferp output)
@@ -1175,14 +1176,17 @@ POS specifies the starting position where EXP was found and defaults to point."
 
 (defun eval-last-sexp (eval-last-sexp-arg-internal)
   "Evaluate sexp before point; print value in the echo area.
-Interactively, with prefix argument, print output into current buffer.
+Interactively, with a non `-' prefix argument, print output into
+current buffer.
 
-Normally, this function truncates long output according to the value
-of the variables `eval-expression-print-length' and
+Normally, this function truncates long output according to the
+value of the variables `eval-expression-print-length' and
 `eval-expression-print-level'.  With a prefix argument of zero,
 however, there is no such truncation.  Such a prefix argument
 also causes integers to be printed in several additional formats
-\(octal, hexadecimal, and character).
+\(octal, hexadecimal, and character when the prefix argument is
+-1 or the integer is `eval-expression-print-maximum-character' or
+less).
 
 If `eval-expression-debug-on-error' is non-nil, which is the default,
 this command arranges for all errors to enter the debugger."
diff --git a/lisp/simple.el b/lisp/simple.el
index 3af62657dbf..ea3a495fbc3 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -1450,15 +1450,24 @@ If nil, don't change the value of `debug-on-error'."
   :type 'boolean
   :version "21.1")
 
+(defcustom eval-expression-print-maximum-character 127
+  "The largest integer that will be displayed as a character.
+This affects printing by `eval-expression' (via
+`eval-expression-print-format')."
+  :group 'lisp
+  :type 'integer
+  :version "26.1")
+
 (defun eval-expression-print-format (value)
   "If VALUE in an integer, return a specially formatted string.
 This string will typically look like \" (#o1, #x1, ?\\C-a)\".
 If VALUE is not an integer, nil is returned.
-This function is used by functions like `prin1' that display the
-result of expression evaluation."
+This function is used by commands like `eval-expression' that
+display the result of expression evaluation."
   (when (integerp value)
     (let ((char-string
            (and (characterp value)
+                (<= value eval-expression-print-maximum-character)
                 (char-displayable-p value)
                 (prin1-char value))))
       (if char-string
@@ -1484,32 +1493,40 @@ result of expression evaluation."
 
 (defun eval-expression-get-print-arguments (prefix-argument)
   "Get arguments for commands that print an expression result.
-Returns a list (INSERT-VALUE NO-TRUNCATE CHAR-PRINT)
+Returns a list (INSERT-VALUE NO-TRUNCATE CHAR-PRINT-LIMIT)
 based on PREFIX-ARG.  This function determines the interpretation
 of the prefix argument for `eval-expression' and
 `eval-last-sexp'."
   (let ((num (prefix-numeric-value prefix-argument)))
-    (list (not (memq prefix-argument '(nil)))
+    (list (not (memq prefix-argument '(- nil)))
           (= num 0)
-          (cond ((not (memq prefix-argument '(0 nil))) nil)
-                (t t)))))
+          (cond ((not (memq prefix-argument '(0 -1 - nil))) nil)
+                ((= num -1) most-positive-fixnum)
+                (t eval-expression-print-maximum-character)))))
 
 ;; We define this, rather than making `eval' interactive,
 ;; for the sake of completion of names like eval-region, eval-buffer.
-(defun eval-expression (exp &optional insert-value no-truncate char-print)
+(defun eval-expression (exp &optional insert-value no-truncate char-print-limit)
   "Evaluate EXP and print value in the echo area.
-When called interactively, read an Emacs Lisp expression and evaluate it.
-Value is also consed on to front of the variable `values'.
-If the resulting value is an integer, it will be printed in
-several additional formats (octal, hexadecimal, and character).
-Optional argument INSERT-VALUE non-nil (interactively, with
-prefix argument) means insert the result into the current buffer
-instead of printing it in the echo area.
-
-Normally, this function truncates long output according to the value
-of the variables `eval-expression-print-length' and
-`eval-expression-print-level'.  With a prefix argument of zero,
-however, there is no such truncation.
+When called interactively, read an Emacs Lisp expression and
+evaluate it.  Value is also consed on to front of the variable
+`values'.  Optional argument INSERT-VALUE non-nil (interactively,
+with a non `-' prefix argument) means insert the result into the
+current buffer instead of printing it in the echo area.
+
+Normally, this function truncates long output according to the
+value of the variables `eval-expression-print-length' and
+`eval-expression-print-level'.  When NO-TRUNCATE is
+non-nil (interactively, with a prefix argument of zero), however,
+there is no such truncation.
+
+If the resulting value is an integer, and CHAR-PRINT-LIMIT is
+non-nil (interactively, unless given a positive prefix argument)
+it will be printed in several additional formats (octal,
+hexadecimal, and character).  The character format is only used
+if the value is below CHAR-PRINT-LIMIT (interactively, if the
+prefix argument is -1 or the value is below
+`eval-expression-print-maximum-character').
 
 Runs the hook `eval-expression-minibuffer-setup-hook' on entering the
 minibuffer.
@@ -1535,11 +1552,12 @@ this command arranges for all errors to enter the debugger."
 
   (let ((print-length (unless no-truncate eval-expression-print-length))
         (print-level  (unless no-truncate eval-expression-print-level))
+        (eval-expression-print-maximum-character char-print-limit)
         (deactivate-mark))
     (let ((out (if insert-value (current-buffer) t)))
       (prog1
           (prin1 (car values) out)
-        (let ((str (and char-print
+        (let ((str (and char-print-limit
                         (eval-expression-print-format (car values)))))
           (when str (princ str out)))))))
 
diff --git a/test/lisp/progmodes/elisp-mode-tests.el b/test/lisp/progmodes/elisp-mode-tests.el
index 5edb590b1e5..03ae8481ee5 100644
--- a/test/lisp/progmodes/elisp-mode-tests.el
+++ b/test/lisp/progmodes/elisp-mode-tests.el
@@ -132,6 +132,54 @@
       (call-interactively #'eval-last-sexp)
       (should (equal (current-message) "t")))))
 
+(ert-deftest eval-last-sexp-print-format-small-int ()
+  (with-temp-buffer
+    (let ((current-prefix-arg '(4)))
+      (erase-buffer) (insert "?A")
+      (call-interactively #'eval-last-sexp)
+      (should (equal (buffer-string) "?A65")))
+    (let ((current-prefix-arg 0))
+      (erase-buffer) (insert "?A")
+      (call-interactively #'eval-last-sexp)
+      (should (equal (buffer-string) "?A65 (#o101, #x41, ?A)")))))
+
+(ert-deftest eval-last-sexp-print-format-small-int-echo ()
+  (skip-unless (not noninteractive))
+  (with-temp-buffer
+    (let ((current-prefix-arg nil))
+      (erase-buffer) (insert "?A") (message nil)
+      (call-interactively #'eval-last-sexp)
+      (should (equal (current-message) "65 (#o101, #x41, ?A)")))))
+
+(ert-deftest eval-last-sexp-print-format-large-int ()
+  (with-temp-buffer
+    (let ((eval-expression-print-maximum-character ?A))
+      (let ((current-prefix-arg '(4)))
+        (erase-buffer) (insert "?B")
+        (call-interactively #'eval-last-sexp)
+        (should (equal (buffer-string) "?B66")))
+      (let ((current-prefix-arg 0))
+        (erase-buffer) (insert "?B")
+        (call-interactively #'eval-last-sexp)
+        (should (equal (buffer-string) "?B66 (#o102, #x42)")))
+      (let ((current-prefix-arg -1))
+        (erase-buffer) (insert "?B")
+        (call-interactively #'eval-last-sexp)
+        (should (equal (buffer-string) "?B66 (#o102, #x42, ?B)"))))))
+
+(ert-deftest eval-last-sexp-print-format-large-int-echo ()
+  (skip-unless (not noninteractive))
+  (with-temp-buffer
+    (let ((eval-expression-print-maximum-character ?A))
+      (let ((current-prefix-arg nil))
+        (erase-buffer) (insert "?B") (message nil)
+        (call-interactively #'eval-last-sexp)
+        (should (equal (current-message) "66 (#o102, #x42)")))
+      (let ((current-prefix-arg '-))
+        (erase-buffer) (insert "?B") (message nil)
+        (call-interactively #'eval-last-sexp)
+        (should (equal (current-message) "66 (#o102, #x42, ?B)"))))))
+
 ;;; xref
 
 (defun xref-elisp-test-descr-to-target (xref)
diff --git a/test/lisp/simple-tests.el b/test/lisp/simple-tests.el
index b74e28ccaf1..180dcc0a209 100644
--- a/test/lisp/simple-tests.el
+++ b/test/lisp/simple-tests.el
@@ -396,5 +396,57 @@ See Bug#21722."
         (call-interactively #'eval-expression)
         (should (equal (current-message) "t"))))))
 
+(ert-deftest eval-expression-print-format-small-int ()
+  (with-temp-buffer
+    (cl-letf (((symbol-function 'read--expression) (lambda (&rest _) ?A)))
+      (let ((current-prefix-arg '(4)))
+        (erase-buffer)
+        (call-interactively #'eval-expression)
+        (should (equal (buffer-string) "65")))
+      (let ((current-prefix-arg 0))
+        (erase-buffer)
+        (call-interactively #'eval-expression)
+        (should (equal (buffer-string) "65 (#o101, #x41, ?A)"))))))
+
+(ert-deftest eval-expression-print-format-small-int-echo ()
+  (skip-unless (not noninteractive))
+  (with-temp-buffer
+    (cl-letf (((symbol-function 'read--expression) (lambda (&rest _) ?A)))
+      (let ((current-prefix-arg nil))
+        (message nil)
+        (call-interactively #'eval-expression)
+        (should (equal (current-message) "65 (#o101, #x41, ?A)"))))))
+
+(ert-deftest eval-expression-print-format-large-int ()
+  (with-temp-buffer
+    (cl-letf (((symbol-function 'read--expression) (lambda (&rest _) ?B))
+              (eval-expression-print-maximum-character ?A))
+      (let ((current-prefix-arg '(4)))
+        (erase-buffer)
+        (call-interactively #'eval-expression)
+        (should (equal (buffer-string) "66")))
+      (let ((current-prefix-arg 0))
+        (erase-buffer)
+        (call-interactively #'eval-expression)
+        (should (equal (buffer-string) "66 (#o102, #x42)")))
+      (let ((current-prefix-arg -1))
+        (erase-buffer)
+        (call-interactively #'eval-expression)
+        (should (equal (buffer-string) "66 (#o102, #x42, ?B)"))))))
+
+(ert-deftest eval-expression-print-format-large-int-echo ()
+  (skip-unless (not noninteractive))
+  (with-temp-buffer
+    (cl-letf (((symbol-function 'read--expression) (lambda (&rest _) ?B))
+              (eval-expression-print-maximum-character ?A))
+      (let ((current-prefix-arg nil))
+        (message nil)
+        (call-interactively #'eval-expression)
+        (should (equal (current-message) "66 (#o102, #x42)")))
+      (let ((current-prefix-arg '-))
+        (message nil)
+        (call-interactively #'eval-expression)
+        (should (equal (current-message) "66 (#o102, #x42, ?B)"))))))
+
 (provide 'simple-test)
 ;;; simple-test.el ends here