Three steps
Create new layer,
Copy mask from different layer
Invert mask.
So if I want to do this from layer 3:
Create new layer 4.
On new layer 4 right click and select "copy mask from layer (select layer 3 in this case)"
Once copied to layer 4, right click and select invert layer.
This is now I do it, there are probably ways to do via keyboard shortcuts, however I rarely use them.
Paul Caldwell