pub trait InsertImmutable {    fn insert(&self, id: gix_hash::ObjectId) -> bool;} mod trait_impls {    use std::cell::RefCell;     use gix_hash::ObjectId;    use gix_hashtable::HashSet;     use super::InsertImmutable;     impl InsertImmutable for gix_hashtable::sync::ObjectIdMap<()> {        fn insert(&self, id: ObjectId) -> bool {            self.insert(id, ()).is_none()        }    }     impl InsertImmutable for RefCell<HashSet<ObjectId>> {        fn insert(&self, item: ObjectId) -> bool {            self.borrow_mut().insert(item)        }    }}