Function set_writable_state_dir 

pub fn set_writable_state_dir<P>(dir: P)
where
    P: Into<PathBuf>,

Anchor the runtime-writable state (saves/ + settings) at dir, leaving the read-only content (data/) at the installed state dir. A shipped application installs this when its content dir is not writable (a read-only install such as Program Files), redirecting only what it writes at runtime to a per-user directory. When unset, writable state stays beside data/.