core.thread functions are not marked @nogc For example: thread_isMainThread
Hmm due to Thread being a class the compiler thinks it can't be @nogc, so as a workaround sth. like `assumeNogc` could be it used. See e.g: https://github.com/nordlow/phobos-next/blob/master/src/dbgio.d#L13
Several functions in core.thread are now marked @nogc. If you have other specific functions in mind, please make other issues for them.