Review NullableWalker.GetDefaultState

[jcouv]

        private bool? GetDefaultState(ref LocalState state, int slot)
        {
...

            switch (symbol.Kind)
            {
...
                case SymbolKind.Field:
                case SymbolKind.Property:
                case SymbolKind.Event:
                    {
                        // PROTOTYPE(NullableReferenceTypes): State of containing struct should not be important.
                        // And if it is important, what about fields of structs that are fields of other structs, etc.?
                        int containingSlot = variable.ContainingSlot;
                        if (containingSlot > 0 &&
                            variableBySlot[containingSlot].Symbol.GetTypeOrReturnType().TypeKind == TypeKind.Struct &&
                            state[containingSlot] == null)
                        {
                            return null;
                        }
                        return !symbol.GetTypeOrReturnType().IsNullable;
                    }
                default:
                    throw ExceptionUtilities.UnexpectedValue(symbol.Kind);
            }
        }