Source

File: wp-includes/post.php

function wp_add_trashed_suffix_to_post_name_for_post( $post ) {
	global $wpdb;

	$post = get_post( $post );

	if ( '__trashed' === substr( $post->post_name, -9 ) ) {
		return $post->post_name;
	}
	add_post_meta( $post->ID, '_wp_desired_post_slug', $post->post_name );
	$post_name = _truncate_post_slug( $post->post_name, 191 ) . '__trashed';
	$wpdb->update( $wpdb->posts, array( 'post_name' => $post_name ), array( 'ID' => $post->ID ) );
	clean_post_cache( $post->ID );
	return $post_name;
}