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.

4 files changed, 17 insertions, 5 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.
diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index 70b557ad36b..4308d4085fa 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,8 @@
+2014-03-18  Juanma Barranquero  <lekktu@gmail.com>
+
+        * custom.el (defcustom): Doc fix: recommend avoiding destructive
+        modification of the value argument of :set (bug#16755).
+
 2014-03-18  Stefan Monnier  <monnier@iro.umontreal.ca>
 
         * simple.el (newline-and-indent): Do autofill (bug#17031).
diff --git a/lisp/custom.el b/lisp/custom.el
index 09431c454ec..c30ad7cb213 100644
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -232,9 +232,10 @@ The following keywords are meaningful:
         given in the `defcustom' call.  The default is
         `custom-initialize-reset'.
 :set    VALUE should be a function to set the value of the symbol
-        when using the Customize user interface.
-        It takes two arguments, the symbol to set and the value to
-        give it.  The default choice of function is `set-default'.
+        when using the Customize user interface.  It takes two arguments,
+        the symbol to set and the value to give it.  The function should
+        not modify its value argument destructively.  The default choice
+        of function is `set-default'.
 :get    VALUE should be a function to extract the value of symbol.
         The function takes one argument, a symbol, and should return
         the current value for that symbol.  The default choice of function