Smart fill is filling a certain defined area with a shape. Inkscape has it, in the image the orange shape has been filled with the "fill bounded areas" tool.
Click image for larger version. 

Name:	inkscape_fill.jpg 
Views:	58 
Size:	79.1 KB 
ID:	130610

Come to think of it; it would be handy if Xara would have this tool also.

But then, development of Xara has been slow and we (users) have suggested several improvements who are on the wishing list ....