opAssign for structs

[Sean Silva]

In the language definition <http://d-programming-language.org/struct.html>, it says:

> Struct assignment t=s is defined to be semantically equivalent to:
>   t = S.opAssign(s);
> where opAssign is a member function of S:
>   S* opAssign(S s)
>   {   ... bitcopy *this into tmp ...
>       ... bitcopy s into *this ...
>       ... call destructor on tmp ...
>       return this;
>   }

I'm struggling with this on 4 fronts:

1. What is `this`, when opAssign is called off of the type? (does it even make sense to call a member function without an instance?)
2. The return value of opAssign is `S*`, so it would seem that `t` is assigned a pointer value?
3. What is `tmp`, just another stack allocated instance of S?
4. What is the syntax for explicitly calling the destructor? (In C++, it is tmp.~S(), but in D would it be tmp.~this() or what?)