How to lookup from and insert into a HashMap efficiently?

Question

I'd like to do the following:

• Lookup a Vec for a certain key, and store it for later use.
• If it doesn't exist, create an empty Vec for the key, but still keep it in the variable.

How to do this efficiently? Naturally I thought I could use match:

use std::collections::HashMap;

// This code doesn't compile.
let mut map = HashMap::new();
let key = "foo";
let values: &Vec<isize> = match map.get(key) {
    Some(v) => v,
    None => {
        let default: Vec<isize> = Vec::new();
        map.insert(key, default);
        &default
    }
};

When I tried it, it gave me errors like:

error[E0502]: cannot borrow `map` as mutable because it is also borrowed as immutable
  --> src/main.rs:11:13
   |
7  |     let values: &Vec<isize> = match map.get(key) {
   |                                     --- immutable borrow occurs here
...
11 |             map.insert(key, default);
   |             ^^^ mutable borrow occurs here
...
15 | }
   | - immutable borrow ends here

I ended up with doing something like this, but I don't like the fact that it performs the lookup twice (map.contains_key and map.get):

// This code does compile.
let mut map = HashMap::new();
let key = "foo";
if !map.contains_key(key) {
    let default: Vec<isize> = Vec::new();
    map.insert(key, default);
}
let values: &Vec<isize> = match map.get(key) {
    Some(v) => v,
    None => {
        panic!("impossiburu!");
    }
};

Is there a safe way to do this with just one match?

Answer 1

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

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

One can use the briefer form via Entry::or_insert_with:

let values = map.entry(key).or_insert_with(|| default);

If default is already computed, or if it's OK/cheap to compute even when it isn't inserted, you can use Entry::or_insert:

let values = map.entry(key).or_insert(default);

If the HashMap's value implements Default, you can use Entry::or_default, although you may need to provide some type hints:

let values = map.entry(key).or_default();

Answer 2

I've used huon's answer and 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 I can do:

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

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