Function with

pub fn with<T, TFn, TRet>(key: &'static LocalKey<Scope<T>>, fun: TFn) -> TRet
where
    TFn: FnOnce(Option<&T>) -> TRet,
    T: ?Sized,

Gets the reference to value from the current scope. Given function will receive None if this is not called within a scope.

See thread_scoped_ref for an example.