When a field is marked with @AllNonNull, field.all!"a !is null" is asserted.
When a field is marked with @NonEmpty, !field.empty is asserted.
When a field is marked with @NonInit, field !is T.init is asserted.
When a field is marked with @NonNull, field !is null is asserted.
GenerateInvariants is a mixin string that automatically generates an invariant{} block for each field with a condition.