When is a class's destructor called?

[Mike]

I'm trying to implement a very minimal D runtime for the ARM 
Cortex-M platform.  I've been quite successful so far, but I'm 
now getting into reference types and I need to understand better 
what D does under the hood.

I have a working malloc/free implementation, but don't have a 
garbage collector.  I'm not yet sure if I want a garbage 
collector, so at the moment, I'm trying to implement something 
simple.

Consider the following code modeled after the example at 
http://dlang.org/memory.html#newdelete:

*************************************
class X
{
     __gshared uint _x;

     new(size_t nBytes)
     {
	Trace.WriteLine("Constructing");
	
         void* p;

         p = malloc(nBytes);

         if (p == null)
         {
	    //TODO:
             //throw new OutOfMemoryError();
         }

         return p;
     }

     ~this()
     {
	Trace.WriteLine("Destructor");
     }

     delete(void* p)
     {
         if (p != null)
         {
             free(p);
         }

         Trace.WriteLine("Destroying");
     }
}

void DoX()
{
     X x = new X();
     x._x = 123;
     Trace.WriteLine(x._x);

     //Why doesn't x's destructor get called here.
}

//My entry point
void main()
{
     DoX();

     Trace.WriteLine("Done");
     while(true) { }
}
**************************************

The output is as follows:
------------------------------
Constructing
56
Done

x's destructor never gets called.  What do I need to implement to 
have the destructor called when x goes out of scope?

I'm using GDC 4.8 cross-compiled for arm-none-eabi on a Linux 
64-bit host.

Thanks,
Mike