How to lazily create map entry whose construction uses self in Rust

Question

I\'m trying to implement a lazy-construction / memoized evaluation / caching idiom in Rust.

There\'s an outer type which has a bunch of data and an accessor method.

Answer 1

The answer is that it depends on specifically which state you need access to in the or_insert_with closure. The problem is that or_insert_with definitely cannot have access to the map itself because the entry api takes a mutable borrow of the map.

If all you need for ContainedThing::create is just the size of the map, then you'll just need to calculate the map size ahead of time.

impl Thing {
    pub fn get(&mut self, key: i32) -> &ContainedThing {
        let map_size = self.map.len();
        self.map.entry(&key).or_insert_with(|| {
            // The call to entry takes a mutable reference to the map,
            // so you cannot borrow map again in here
            ContainedThing::create(map_size)
        })
    }
}

I think the spirit of the question was more about general strategies, though, so let's assume there's some other state within Thing that is also required to create ContainedThing.

struct Thing {
    map: HashMap,
    some_other_stuff: AnotherType, //assume that this other state is also required in order to create ContainedThing
}

impl Thing {
    pub fn get(&mut self, key: i32) -> &ContainedThing {
        //this is the borrow of self
        let Thing {
            ref mut map,
            ref mut some_other_stuff,
        } = *self;

        let map_size = map.len();
        map.entry(key).or_insert_with(|| {
            // map.entry now only borrows map instead of self
            // Here you're free to borrow any other members of Thing apart from map
            ContainedThing::create(map_size, some_other_stuff)
        })
    }
}

Whether that's really cleaner than your other solution of manually checking if self.map.contains_key(&key) is up for debate. Destructuring tends to be the strategy that I go for, though, to allow borrowing specific members of self instead of the entire struct.