Switch labels (null again), some tweaking

[Brian Goetz]

> The benefit is twofold: not only does the user not have to write the 
> stupid cases (imagine if Box had ten slots, would we want to write the 
> 2^10 partial null cases?), but because we throw on the remainder, DA 
> can treat the switch as covering all boxes, and be assured there are 
> no leaks.

More on this last point.  We don't yet have pattern assignment, but 
totality and remainder are key to it.  Suppose we have

     Box<String> bs = ...

and we want to destructure:

     Box(String s) = bs;
     // s had better be DA here

The pattern on the LHS must be total, but can have remainder; the 
statement is allowed to throw on the remainder, because the remainder 
is, by definition, the "silly" matches.  Here, the only remainder is 
null; you can't destructure a null, so if you asked to destructure a 
Box, you probably were assuming that it was a real box.  So this 
assignment NPEs on null (not unlike the corresponding switch).


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