Renders the `core/block` block on server.

function render_block_core_block( $attributes ) {
	static $seen_refs = array();

	if ( empty( $attributes['ref'] ) ) {
		return '';
	}

	$reusable_block = get_post( $attributes['ref'] );
	if ( ! $reusable_block || 'wp_block' !== $reusable_block->post_type ) {
		return '';
	}

	if ( isset( $seen_refs[ $attributes['ref'] ] ) ) {
		// WP_DEBUG_DISPLAY must only be honored when WP_DEBUG. This precedent
		// is set in `wp_debug_mode()`.
		$is_debug = defined( 'WP_DEBUG' ) && WP_DEBUG &&
			defined( 'WP_DEBUG_DISPLAY' ) && WP_DEBUG_DISPLAY;

		return $is_debug ?
			// translators: Visible only in the front end, this warning takes the place of a faulty block.
			__( '[block rendering halted]' ) :
			'';
	}

	if ( 'publish' !== $reusable_block->post_status || ! empty( $reusable_block->post_password ) ) {
		return '';
	}

	$seen_refs[ $attributes['ref'] ] = true;

	// Handle embeds for reusable blocks.
	global $wp_embed;
	$content = $wp_embed->run_shortcode( $reusable_block->post_content );
	$content = $wp_embed->autoembed( $content );

	$content = do_blocks( $content );
	unset( $seen_refs[ $attributes['ref'] ] );
	return $content;
}