pub trait InsertImmutable<Item: Eq + std::hash::Hash> {    fn insert(&self, item: Item) -> bool;} mod trait_impls {    use std::{cell::RefCell, hash::Hash};     use dashmap::DashSet;    use hash_hasher::{HashBuildHasher, HashedSet};     use super::InsertImmutable;     impl<T: Eq + Hash> InsertImmutable<T> for DashSet<T, HashBuildHasher> {        fn insert(&self, item: T) -> bool {            self.insert(item)        }    }     impl<T: Eq + Hash> InsertImmutable<T> for RefCell<HashedSet<T>> {        fn insert(&self, item: T) -> bool {            self.borrow_mut().insert(item)        }    }}