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https://kellenmace.com/remove-custom-post-type-slug-from-permalinks/
    
        /**
         * Remove the slug from published post permalinks. Only affect our custom post type, though.
         */
        function gln_remove_cpt_slug( $post_link, $post, $leavename ) {
         
            if ( 'rsvp' != $post->post_type || 'publish' != $post->post_status ) {
                return $post_link;
            }
         
            $post_link = str_replace( '/' . $post->post_type . '/', '/', $post_link );
         
            return $post_link;
        }
        add_filter( 'post_type_link', 'gln_remove_cpt_slug', 10, 3 );

        /**
         * Have WordPress match postname to any of our public post types (post, page, race)
         * All of our public post types can have /post-name/ as the slug, so they better be unique across all posts
         * By default, core only accounts for posts and pages where the slug is /post-name/
         */
        function gln_parse_request_trick( $query ) {
         
            // Only noop the main query
            if ( ! $query->is_main_query() )
                return;
         
            // Only noop our very specific rewrite rule match
            if ( 2 != count( $query->query ) || ! isset( $query->query['page'] ) ) {
                return;
            }
         
            // 'name' will be set if post permalinks are just post_name, otherwise the page rule will match
            if ( ! empty( $query->query['name'] ) ) {
                $query->set( 'post_type', array( 'post', 'page', 'rsvp' ) );
            }
        }
        add_action( 'pre_get_posts', 'gln_parse_request_trick' );