Pure Contract bug? (unnecessarily strict)

[Era Scarecrow]

 Maybe someone's brought this up, but i seem to have the compiler complaining to me that my function isn't 'pure' by calling a non-pure function, specifically to!string().

 However the unpure functions are only accessed in the contracts, and only if it failed seriously. Is this already planned to be worked on? I thought i read the contracts shouldn't be considered as part of it since they are totally excluded during the release builds (and shouldn't have any side effects). 

Error: pure function 'offset' cannot call impure function 'to'


	@property const pure int offset(int field)
	in {
		assert(field < notes.length);
	}
	out (o) {
		assert(o >= 0, "Negative value! Check structure:" ~ name ~ "\nReq:" ~ requ ~ "\nsize:" ~ to!string(size) ~ "\n");
	}
	body { ... }