DIP: @mustuse as function return value annotation

[Richard (Rikki) Andrew Cattermole]

If I understand this correctly, you want the ``@mustuse`` restriction to 
inherit upwards?

This will not work due to the way D's compilation model works. In a 
process not all compiler invocations will see this upward inheritance.

The only restriction I am aware of that can work is to simply disallow 
casting upwards into a class/interface that doesn't have it.