Casting to interface not allowed in @safe code?

[Jim via Digitalmars-d-learn]

Hi,

consider this:
interface Base
{
  void setup();
}

interface FeatureX
{
  void x();
}

class Foo: Base, FeatureX
{
  void setup(){};
  void x(){};
}

void main()
{
  Base foo = new Foo(); // This would be the result of a factory 
class

  (cast(FeatureX)foo).x(); // 1
}

1) error: casting to interface FeatureX is not allowed in @safe 
code.

Question: How to call foo.x in @safe code ?