Why does this mixin fail to compile?

[cc]

I've been using patterns such as this with no difficulty.  
ctfe+mixins == it just works.
I don't know if there's some character limit where eventually the 
ctfe string return will give up, but for moderate programs it 
seems fine.

```d
// Copy fields from another struct, wrapping non-primary key 
elements in Nullable!T
private static string UpdaterFields(STRUCT)() {
	string[] s;
	static foreach (idx, field; STRUCT.tupleof) {{
		enum NAME = field.stringof;
		enum bool isPrimary = hasUDA!(field, PRIMARY);
		static if (isPrimary) {
			enum typestr = format("typeof(STRUCT.%s)", NAME);
		} else {
			enum typestr = format("Nullable!(typeof(STRUCT.%s))", NAME);
		}
		string[] udas;
		static foreach (uda; __traits(getAttributes, field)) {
			udas ~= "@("~uda.stringof~")";
		}
		auto udastr = udas.length ? udas.join(" ")~" " : "";
		s ~= format("%s%s %s;", udastr, typestr, NAME);
	}}
	return s.join("\n");
}
pragma(msg, UpdaterFields!STRUCT);
mixin(UpdaterFields!STRUCT);
```