Safety, undefined behavior, @safe, @trusted

[Walter Bright]

Steven Schveighoffer wrote:
> That is, I have a mutable reference x, I want to make it immutable.  How 
> do you write a function to do that?
> 
> i.e.:
> 
> @safe void foo()
> {
>    x = new X();
>    x.modifyState(5);
>    immutable(X) ix = ???; // how to write this part
> }

If you, the writer of foo(), know that there are no other mutable 
references to x you can cast it to immutable - but you'll have to mark 
the function as @trusted.