"Improperly reduced borrows" can be updated soon.

[petercs-sophos]

https://doc.rust-lang.org/nomicon/lifetime-mismatch.html#improperly-reduced-borrows states the code below will not compile:

fn get_default<'m, K, V>(map: &'m mut HashMap<K, V>, key: K) -> &'m mut V
where
    K: Clone + Eq + Hash,
    V: Default,
{
    match map.get_mut(&key) {
        Some(value) => value,
        None => {
            map.insert(key.clone(), V::default());
            map.get_mut(&key).unwrap()
        }
    }
}

However, on nightly it will now compile due to Polonius Alpha being enabled (https://blog.rust-lang.org/2026/08/04/enabling-polonius-alpha-on-nightly/), it may be worth mentioning this.

Additionally, the name of the function should probably be get_mut_or_default as it is in the article.