Byte-compiler warning about mutation of constant values

When we can easily detect mutation of constants (quoted lists, strings
and vectors), warn.  For example,

  (setcdr '(1 . 2) 3)
  (nreverse [1 2 3])
  (put-text-property 0 3 'face 'highlight "moo")

Such code can result in surprising behaviour and problems that
are difficult to debug.

* lisp/emacs-lisp/bytecomp.el (byte-compile-form, mutating-fns):
Add the warning and a list of functions to warn about.
* etc/NEWS: Announce.
* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-test--with-suppressed-warnings): Add test cases.

3 files changed, 103 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 3bef9d2ed2a..7d033b0b13e 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -510,6 +510,26 @@ This warning can be suppressed using 'with-suppressed-warnings' with
 the warning name 'suspicious'.
 
 ---
+*** Warn about mutation of constant values.
+The compiler now warns about code that modifies program constants in
+some obvious cases.  Examples:
+
+    (setcar '(1 2) 7)
+    (aset [3 4] 0 8)
+    (aset "abc" 1 ?d)
+
+Such code may have unpredictable behaviour because the constants are
+part of the program, not data structures generated afresh during
+execution, and the compiler does not expect them to change.
+
+To avoid the warning, operate on an object created by the program
+(maybe a copy of the constant), or use a non-destructive operation
+instead.
+
+This warning can be suppressed using 'with-suppressed-warnings' with
+the warning name 'suspicious'.
+
+---
 *** Warn about more ignored function return values.
 The compiler now warns when the return value from certain functions is
 implicitly ignored.  Example:
diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index 6c804056ee7..d17f1c93a76 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -3488,6 +3488,22 @@ lambda-expression."
                                       (format-message "; use `%s' instead."
                                                       interactive-only))
                                      (t "."))))
+        (let ((mutargs (function-get (car form) 'mutates-arguments)))
+          (when mutargs
+            (dolist (idx (if (eq mutargs 'all-but-last)
+                             (number-sequence 1 (- (length form) 2))
+                           mutargs))
+              (let ((arg (nth idx form)))
+                (when (and (or (and (eq (car-safe arg) 'quote)
+                                    (consp (nth 1 arg)))
+                               (arrayp arg))
+                           (byte-compile-warning-enabled-p
+                            'suspicious (car form)))
+                  (byte-compile-warn-x form "`%s' on constant %s (arg %d)"
+                                       (car form)
+                                       (if (consp arg) "list" (type-of arg))
+                                       idx))))))
+
         (if (eq (car-safe (symbol-function (car form))) 'macro)
             (byte-compile-report-error
              (format-message "`%s' defined after use in %S (missing `require' of a library file?)"
@@ -3557,6 +3573,43 @@ lambda-expression."
   (dolist (fn important-return-value-fns)
     (put fn 'important-return-value t)))
 
+(let ((mutating-fns
+       ;; FIXME: Should there be a function declaration for this?
+       ;;
+       ;; (FUNC . ARGS) means that FUNC mutates arguments whose indices are
+       ;; in the list ARGS, starting at 1, or all but the last argument if
+       ;; ARGS is `all-but-last'.
+       '(
+         (setcar 1) (setcdr 1) (aset 1)
+         (nreverse 1)
+         (nconc . all-but-last)
+         (nbutlast 1) (ntake 2)
+         (sort 1)
+         (delq 2) (delete 2)
+         (delete-dups 1) (delete-consecutive-dups 1)
+         (plist-put 1)
+         (fillarray 1)
+         (store-substring 1)
+         (clear-string 1)
+
+         (add-text-properties 4) (put-text-property 5) (set-text-properties 4)
+         (remove-text-properties 4) (remove-list-of-text-properties 4)
+         (alter-text-property 5)
+         (add-face-text-property 5) (add-display-text-property 5)
+
+         (cl-delete 2) (cl-delete-if 2) (cl-delete-if-not 2)
+         (cl-delete-duplicates 1)
+         (cl-nsubst 3) (cl-nsubst-if 3) (cl-nsubst-if-not 3)
+         (cl-nsubstitute 3) (cl-nsubstitute-if 3) (cl-nsubstitute-if-not 3)
+         (cl-nsublis 2)
+         (cl-nunion 1 2) (cl-nintersection 1 2) (cl-nset-difference 1 2)
+         (cl-nset-exclusive-or 1 2)
+         (cl-nreconc 1)
+         (cl-sort 1) (cl-stable-sort 1) (cl-merge 2 3)
+         )))
+  (dolist (entry mutating-fns)
+    (put (car entry) 'mutates-arguments (cdr entry))))
+
 
 (defun byte-compile-normal-call (form)
   (when (and (symbolp (car form))
diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
index 222065c2e4e..9136a6cd9b3 100644
--- a/test/lisp/emacs-lisp/bytecomp-tests.el
+++ b/test/lisp/emacs-lisp/bytecomp-tests.el
@@ -1518,6 +1518,36 @@ literals (Bug#20852)."
         ))
    '((empty-body with-suppressed-warnings))
    "Warning: `with-suppressed-warnings' with empty body")
+
+  (test-suppression
+   '(defun zot ()
+      (setcar '(1 2) 3))
+   '((suspicious setcar))
+   "Warning: `setcar' on constant list (arg 1)")
+
+  (test-suppression
+   '(defun zot ()
+      (aset [1 2] 1 3))
+   '((suspicious aset))
+   "Warning: `aset' on constant vector (arg 1)")
+
+  (test-suppression
+   '(defun zot ()
+      (aset "abc" 1 ?d))
+   '((suspicious aset))
+   "Warning: `aset' on constant string (arg 1)")
+
+  (test-suppression
+   '(defun zot (x y)
+      (nconc x y '(1 2) '(3 4)))
+   '((suspicious nconc))
+   "Warning: `nconc' on constant list (arg 3)")
+
+  (test-suppression
+   '(defun zot ()
+      (put-text-property 0 2 'prop 'val "abc"))
+   '((suspicious put-text-property))
+   "Warning: `put-text-property' on constant string (arg 5)")
   )
 
 (ert-deftest bytecomp-tests--not-writable-directory ()