lib/module: add mkChangedOptionModule function

lib/modules.nix

@@ -598,6 +598,32 @@ rec {
                (mergeFn config)));
     };
 
+  /* Single "from" version of mkMergedOptionModule.
+     Return a module that causes a warning to be shown if the "from" option is
+     defined; the defined value can be used in the "mergeFn" to set the "to"
+     value.
+     This function can be used to change an option into another that has a
+     different type.
+
+     "mergeFn" takes the module "config" as a parameter and must return a value of
+     "to" option type.
+
+       mkChangedOptionModule [ "a" "b" "c" ] [ "x" "y" "z" ]
+         (config:
+           let value = getAttrFromPath [ "a" "b" "c" ] config;
+           in
+           if   value > 100 then "high"
+           else "normal")
+
+     - options.a.b.c is a removed int option
+     - options.x.y.z is a new str option that supersedes a.b.c
+
+     This show a warning if a.b.c is set, and set the value of x.y.z to the
+     result of the change function
+  */
+  mkChangedOptionModule = from: to: changeFn:
+    mkMergedOptionModule [ from ] to changeFn;
+
   /* Like ‘mkRenamedOptionModule’, but doesn't show a warning. */
   mkAliasOptionModule = from: to: doRename {
     inherit from to;