How to create mixin template and pass name of a variable?

[Bosak]

On Friday, 2 August 2013 at 11:37:27 UTC, Bosak wrote:
> I want to create a mixin template such that:
>
> mixin template ArgNull(alias arg, string name)
> {
>     if(arg is null)
>         throw new Exception(name~" cannot be null.");
> }
>
> But is there a way to do that with only one template argument. 
> And then use it like:
>
> string text = null;
> mixin ArgNull!(text);
>
> And the above mixin to make:
>
> if(text is null)
>     throw new Exception("text cannot be null.");

I tested my code above and it seems it doesn't work either. I 
came with an idea to
write:

mixin template ArgNull(string arg)
{
     if(mixin(arg) is null)
         throw new Exception(arg~" cannot be null.");
}

But it seems that I can't put an if in a template like that. Is 
there a way to
implement this?