How do I create a module-local immutable class object?

[bearophile]

Andrej Mitrovic:

> I need to have an object which is initialized only once, so I thought
> I could use immutable for that. But I can't do this:
> 
> private class Foo {}
> immutable Foo foo;
> 
> static this()
> {
>     foo = new Foo;
> }
> 
> void main() {}

private class Foo {}
immutable Foo foo1;

static this() {
    foo1 = new immutable(Foo);
}

void main() {
    auto foo2 = new immutable(Foo);
}

Bye,
bearophile