Code shared from the Rust Playground

playground.rs

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

trait EntryExt<'a, K, V>: 'a {
    fn or_insert_with2<F: FnOnce(&K) -> V>(self, f: F) -> &'a mut V;
}

impl<'a, K, V> EntryExt<'a, K, V> for Entry<'a, K, V> {
    fn or_insert_with2<F: FnOnce(&K) -> V>(self, f: F) -> &'a mut V {
        match self {
            Entry::Occupied(entry) => entry.into_mut(),
            Entry::Vacant(entry) => {
                let v = f(entry.key());
                entry.insert(v)
            }
        }
    }
}

fn main() {
    let mut map: HashMap<i32, i32> = HashMap::new();
    println!("{}", map.entry(10).or_insert_with2(|i| i * i));
}