WordPress | Hack to get CPT to have no post-type slug

cpt-remove-slug-hack.php

<?php

/** 
 * NOTE, this causes PAGES AND POSTS TO NOT WORK! 
 */

/**
 * NOTE, this also requires your post type (e.g., via CPT UI) to have a Rewrite to True,
 * and a Custom Rewrite Slug of / (forward slash) ... which probably isn't best practice. 
 * Sources:
 * https://kellenmace.com/remove-custom-post-type-slug-from-permalinks/
 * https://wordpress.stackexchange.com/a/282040/91826
 * https://wordpress.org/support/topic/cpt-without-any-front-slug-at-all/
 * https://core.trac.wordpress.org/ticket/34136
 */

/**
 * 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 need to be unique across all posts.
 * By default, WordPress only accounts for posts and pages where the slug is /post-name/.
 */
function gc_add_cpt_post_names_to_main_query( $query ) {
	// Bail if this is not the main query.
	if ( ! $query->is_main_query() ) {
		return;
	}
	// Bail if this query doesn't match our very specific rewrite rule.
	if ( ! isset( $query->query['page'] ) || 2 !== count( $query->query ) ) {
		return;
	}
	// Bail if we're not querying based on the post name.
	if ( empty( $query->query['name'] ) ) {
		return;
	}
	// Add CPT to the list of post types WP will include when it queries based on the post name.
	$query->set( 'post_type', array( 'post', 'page', 'fs_local' ) );
}
add_action( 'pre_get_posts', 'gc_add_cpt_post_names_to_main_query' );

/**
 * Force check the slug of the custom post type, and also the posts and page.
 * Compare them and make sure they aren't a duplicate slug from the other post type.
 */
function gc_prevent_slug_duplicates( $slug, $post_ID, $post_status, $post_type, $post_parent, $original_slug ) {
	$check_post_types = array(
		'post',
		'page',
		'fs_local'
	);

	if ( ! in_array( $post_type, $check_post_types ) ) {
		return $slug;
	}

	if ( 'fs_local' == $post_type ) {
		// Saving a fs_local post, check for duplicates in POST or PAGE post types
		$post_match = get_page_by_path( $slug, 'OBJECT', 'post' );
		$page_match = get_page_by_path( $slug, 'OBJECT', 'page' );

		if ( $post_match || $page_match ) {
			$slug .= '-duplicate';
		}
	} else {
		// Saving a POST or PAGE, check for duplicates in fs_local post type
		$fs_local_match = get_page_by_path( $slug, 'OBJECT', 'fs_local' );

		if ( $fs_local_match ) {
			$slug .= '-duplicate';
		}
	}

	return $slug;
}
add_filter( 'wp_unique_post_slug', 'gc_prevent_slug_duplicates', 10, 6 );