Index post slugs for Relevanssi

relevanssi-index-slug.php

<?php

/**
 * Adds the post slug to the post content, with dashes replaced with spaces.
 * Add this to theme functions.php and rebuild the index.
 *
 * @param string $content The additional post content.
 * @param object $post    The post object.
 */
add_filter( 'relevanssi_content_to_index', 'rlv_index_slug', 10, 2 );
function rlv_index_slug( $content, $post ) {
   $slug_with_spaces = str_replace( '-', ' ', $post->post_name );
   return $content . ' ' . $slug_with_spaces;
}