Kotlin/Native implements strict mutability checks, ensuring
important invariant that object is either immutable or
accessible from the single thread at the moment (mutable XOR global).
Immutability is the runtime property in Kotlin/Native, and can be applied
to an arbitrary object subgraph using konan.worker.freeze function.
It makes all objects reachable from the given one immutable, and
such a transition is a one way operation (object cannot be unfrozen later).
Some naturally immutable objects, such as kotlin.String, kotlin.Int and
other primitive types, along with AtomicInt and AtomicReference are frozen
by default. If mutating operation is applied to a frozen object,
an InvalidMutabilityException is thrown.
To achieve mutable XOR global invariant all globally visible state (currently,
object singletons and enums) are automatically frozen. If an object freezing
is not desirable, konan.ThreadLocal annotation could be used, which will make
object state thread local, and thus, mutable (but changed state not visible to
other threads).
Class AtomicReference could be used to publish changed frozen state to
other threads, and thus build patterns like shared caches.