Recommend not modifying :set's value arg in defcustom (bug#16755).

* doc/lispref/customize.texi (Variable Definitions):
* lisp/custom.el (defcustom): Recommend avoiding
  destructive modification of the value argument of :set.

2 files changed, 8 insertions, 2 deletions

diff --git a/doc/lispref/ChangeLog b/doc/lispref/ChangeLog
index 723bd1188c6..2749a521c9a 100644
--- a/doc/lispref/ChangeLog
+++ b/doc/lispref/ChangeLog
@@ -1,3 +1,8 @@
+2014-03-18  Juanma Barranquero  <lekktu@gmail.com>
+
+        * customize.texi (Variable Definitions): Recommend avoiding
+        destructive modification of the value argument of :set (bug#16755).
+
 2014-03-18  Stefan Monnier  <monnier@iro.umontreal.ca>
 
         * modes.texi (Auto-Indentation): Mention electric-indent variables.
diff --git a/doc/lispref/customize.texi b/doc/lispref/customize.texi
index 094beef01ec..0c6497fb4ef 100644
--- a/doc/lispref/customize.texi
+++ b/doc/lispref/customize.texi
@@ -353,8 +353,9 @@ option when using the Customize interface.  The function
 @var{setfunction} should take two arguments, a symbol (the option
 name) and the new value, and should do whatever is necessary to update
 the value properly for this option (which may not mean simply setting
-the option as a Lisp variable).  The default for @var{setfunction} is
-@code{set-default}.
+the option as a Lisp variable); preferably, though, it should not
+modify its value argument destructively.  The default for
+@var{setfunction} is @code{set-default}.
 
 If you specify this keyword, the variable's documentation string
 should describe how to do the same job in hand-written Lisp code.