Consider the following case:

   class Foo(f : => Unit){
      lazy val bar = f;
   }

Once bar has been evaluated, f is no longer needed, but instances of Foo will retain a reference to it anyway (and anything it in turn has captured), potentially preventing all sorts of things from being garbage collected. This is bad, and will become particularly important once #240 is done and we have lazy parameters.

It would be nice if the following was also handled correctly:

   class Foo(f : => Unit){
      lazy val bar = f;
      lazy val baz = f;
   }

But this is somewhat less important and handling the case of only one lazy val will probably capture most of the use cases.