How to lookup from and insert into a HashMap efficiently?

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 = match map.get(key) {
    Some(v) => v,
    None => {
        let default: Vec = 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 = 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 = Vec::new();
    map.insert(key, default);
}
let values: &Vec = match map.get(key) {
    Some(v) => v,
    None => {
        panic!("impossiburu!");
    }
};

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

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();


Stack Overflow Q#28512394, score: 273