Safe cast away from immutable

[Iakh via Digitalmars-d]

import std.stdio;

struct S
{
    void[] arr;

    auto f() pure @safe
    {
        int[] a = new int[4];
        arr = a;
        return a;
    }
}
void main() @safe
{
    S s;
    immutable a = s.f();
    int[] b = (cast(int[])s.arr);
    writeln(a);
    b[0] = 1;
    writeln(a);
}

http://dpaste.dzfl.pl/13751913d2ff