Why is a static struct's dtor called at the exit of a function?

[Andrej Mitrovic]

What are the current semantics of static struct definitions in module
scope? It doesn't seem to have any effect. I was thinking about this:

module test;

int z;

static struct Foo
{
    void foo()
    {
        z = 5;  // maybe make this an error if the struct definition is static?
    }
}

Maybe it could be useful as protection against modifying globals.