Which built-in WordPress files are safe to edit?

Consider everything that comes with a vanilla WordPress installation a core file. Core files will be overwritten on updates, so it’s not safe to edit them. E.g. wp-config.php isn’t a core file, because the vanilla installation comes only with the sample version of that file.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)