Struct destructors not always called?

[Jeremy DeHaan via Digitalmars-d-learn]

I was playing around with some code today and I noticed that in 
some cases struct destructors are not called.

for example:
impost std.stdio;

SomeStruct global;

void main()
{
   SomeStruct inMain;
   writeln(global.thing);
   writeln(inMain.thing);
   writeln(getSomeInt());
}

int getSomeInt()
{
    static SomeStruct inner;
    return inner.thing;
}

struct SomeStruct
{
    int thing = 100;
    ~this()
    {
        writeln("destructor");
    }



output is
100
100
100
destructor

Only inMain's destructor is ever called, or at least it is the 
only one that ever prints "destructor" to the console. Are there 
special rules for structs that I'm unaware of?