private static Nullability getNullability(PsiExpression expression) {
    // expression.equals("string") -> expression == NOT_NULL
    if (ExpressionUtils.getCallForQualifier(expression) != null) {
      return Nullability.NOT_NULL;
    }
    // inferred nullability
    Nullability normal = NullabilityUtil.getExpressionNullability(expression, false);
    Nullability dataflow = NullabilityUtil.getExpressionNullability(expression, true);
    if (normal == Nullability.NOT_NULL || dataflow == Nullability.NOT_NULL) {
      return Nullability.NOT_NULL;
    }
    if (normal == Nullability.NULLABLE || dataflow == Nullability.NULLABLE) {
      return Nullability.NULLABLE;
    }
    return Nullability.UNKNOWN;
  }
private static Nullability getNullability(PsiExpression expression)