Question about interface implementation

[Theo]

see comments in the code below. they are my questions.

But feel free to ignore the comment about 'private(this)' ;-)


interface Ship
{
     @safe void setSpeed(int speed);
     @safe int getSpeed();
}

class PirateShip : Ship
{
     private int speed = 0; // If only I had 'private(this)' !!

     // how do I know this method is actually an implementation of 
an interface method
     // and not a method specific to this class?
     public void setSpeed(int speed)
     {
         this.speed = speed;
     }

    // how do I know this method is actually an implementation of 
an interface method
    // and not a method specific to this class?
    public int getSpeed()
     {
         return speed;
     }
}


class MerchantShip : Ship
{
     private int speed = 0; // If only I had 'private(this)' !!

     // how do I know this method is actually an implementation of 
an interface method
     // and not a method specific to this class?
     // AND ... how come I can change a @safe interface method 
into a @trusted one?
     public @trusted void setSpeed(int speed)
     {
         int *s = void; // Mmmm.. and my interface all had @safe 
methods!!
         this.speed = speed;
     }

     // how do I know this method is actually an implementation of 
an interface method
     // and not a method specific to this class?
     // AND ... how come I can change a @safe interface method 
into a @trusted one?
     public @trusted int getSpeed()
     {
         int *s = void; // Mmmm.. and my interface all had @safe 
methods!!
         return speed;
     }

}