$attributes['custom_country'] = [
            'group' => 'General',
            'type' => 'int',
            'label' => 'Country',
            'input' => 'select',
            'source' => VendorModuleModelResourceModelAttributeSourceCountry::class,
            'global' => ScopedAttributeInterface::SCOPE_STORE,
            'required' => '0',
            'user_defined' => false,
            'default' => '',
            'unique' => false,
            'position' => '50',
            'note' => '',
            'visible' => '1',
            'wysiwyg_enabled' => '0',
        ];

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace VendorModuleModelResourceModelAttributeSource;

class Custom extends MagentoEavModelEntityAttributeSourceTable {
    /**
     * @var MagentoEavModelConfig
     */
    protected $eavConfig;

    /**
     * [__construct description]
     * @param MagentoEavModelResourceModelEntityAttributeOptionCollectionFactory $attrOptionCollectionFactory [description]
     * @param MagentoEavModelResourceModelEntityAttributeOptionFactory            $attrOptionFactory           [description]
     * @param MagentoEavModelConfig                                                  $eavConfig                   [description]
     */
    public function __construct(
        MagentoEavModelResourceModelEntityAttributeOptionCollectionFactory $attrOptionCollectionFactory,
        MagentoEavModelResourceModelEntityAttributeOptionFactory $attrOptionFactory,
        MagentoEavModelConfig $eavConfig
    ) {
        $this->eavConfig = $eavConfig;
        parent::__construct($attrOptionCollectionFactory, $attrOptionFactory);
    }

    public function getAllOptions($withEmpty = true, $defaultValues = false) {
        $attribute = $this->eavConfig->getAttribute('custom_eav_entity', 'attr_code');
        return $attribute->getSource()->getAllOptions($withEmpty,$defaultValues); // here returns error
    }

    public function getOptionsArray($withEmpty = true) {
        $options = [];
        foreach ($this->getAllOptions($withEmpty) as $option) {
            $options[$option['value']] = $option['label'];
        }
        return $options;
    }
}
?>