Source

File: wp-admin/includes/taxonomy.php

function get_terms_to_edit( $post_id, $taxonomy = 'post_tag' ) {
	$post_id = (int) $post_id;
	if ( ! $post_id ) {
		return false;
	}

	$terms = get_object_term_cache( $post_id, $taxonomy );
	if ( false === $terms ) {
		$terms = wp_get_object_terms( $post_id, $taxonomy );
		wp_cache_add( $post_id, wp_list_pluck( $terms, 'term_id' ), $taxonomy . '_relationships' );
	}

	if ( ! $terms ) {
		return false;
	}
	if ( is_wp_error( $terms ) ) {
		return $terms;
	}
	$term_names = array();
	foreach ( $terms as $term ) {
		$term_names[] = $term->name;
	}

	$terms_to_edit = esc_attr( join( ',', $term_names ) );

	/**
	 * Filters the comma-separated list of terms available to edit.
	 *
	 * @since 2.8.0
	 *
	 * @see get_terms_to_edit()
	 *
	 * @param string $terms_to_edit A comma-separated list of term names.
	 * @param string $taxonomy      The taxonomy name for which to retrieve terms.
	 */
	$terms_to_edit = apply_filters( 'terms_to_edit', $terms_to_edit, $taxonomy );

	return $terms_to_edit;
}