import std.traits;

​

auto assumePure(T)(T t) {

    enum attrs = functionAttributes!T | FunctionAttribute.pure_;

    return cast(SetFunctionAttributes!(T, functionLinkage!T, attrs)) t;

}

​

int thing = 3;

​

void modify()

{

    thing = 4;

}

​

pure unittest {

    assumePure(&modify)();

}