How to lookup from and insert into a HashMap efficiently?

Question

The entry API is designed for this. In manual form, it might look like

use std::collections::hash_map::Entry;

let values: &Vec<isize> = match map.entry(key) {
    Entry::Occupied(o) => o.into_mut(),
    Entry::Vacant(v) => v.insert(default)
};

Or one can use the briefer form:

map.entry(key).or_insert_with(|| default)

If default is OK/cheap to compute even when it isn't inserted, it can also just be:

map.entry(key).or_insert(default)

I've used @huon and @Shepmaster's answer and just implemented it as a trait:

use std::collections::HashMap;
use std::hash::Hash;

pub trait InsertOrGet<K: Eq + Hash, V: Default> {
    fn insert_or_get(&mut self, item: K) -> &mut V;
}

impl<K: Eq + Hash, V: Default> InsertOrGet<K, V> for HashMap<K, V> {
    fn insert_or_get(&mut self, item: K) -> &mut V {
        return match self.entry(item) {
            std::collections::hash_map::Entry::Occupied(o) => o.into_mut(),
            std::collections::hash_map::Entry::Vacant(v) => v.insert(V::default()),
        };
    }
}

Then somewhere else I can just do:

use crate::utils::hashmap::InsertOrGet;

let new_or_existing_value: &mut ValueType = my_map.insert_or_get(my_key.clone());