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.

1 files changed, 30 insertions, 0 deletions

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 ()