Source

File: wp-includes/post.php

function _truncate_post_slug( $slug, $length = 200 ) {
	if ( strlen( $slug ) > $length ) {
		$decoded_slug = urldecode( $slug );
		if ( $decoded_slug === $slug ) {
			$slug = substr( $slug, 0, $length );
		} else {
			$slug = utf8_uri_encode( $decoded_slug, $length );
		}
	}

	return rtrim( $slug, '-' );
}