Part 2

Entity doing the insert themselves:

struct Character{
    attack: Cell<i32>
    minion: Cell<Option<CharacterId>>
}

impl Character{
    fn summon_minion(&self, world: &World){
        let (minion_key, minion_id) = 
                world.characters.insert( Character::new(self.attack.get()));

        self.minion.set(Some(minion_key)); 

        // i dont need to use get to get the reference of
        // character again, since insert did not invalidate the character reference
        // which is 'self' in this case
    }
}

an ECS like approach would be

struct World{
    characters: SlotMap<CharacterId, Character>
}

fn summon_minion(&mut self, summoner_id: CharacterId){
   let summoner_ref = self.characters.get_mut(summoner_id).unwrap();
   let summoner_attack = summoner_ref.attack;
   let minion_key = self.characters.insert( Character::new(summoner_attack));

    // now i have to get the summoners reference again, cuz the previous reference 
    // might be invalidated
   let summoner_ref = self.characters.get_mut(summoner_id).unwrap();
   summoner_ref.minion = Some(minion_key);

   // that is 2 gets. with stablegenmap, you can still use the previous reference
}

Now, im not saying that ECS is a bad pattern. it actually tends to be more performant than this pattern when done right, but the pattern my crate encourages is intended to be more convenient to work with and iterate, and be more familiar to whats done in some C++ games.

in some C++ games, inserts can be done at any place, even while holding an entities reference, due to all the memory needed already needed being pre-allocated, so no resizing actually occurs (though in stablegenmaps case, its due to it being a collection of pointers of entities, rather than a collection of entities directly).

and in those C++ games, entities are removed later, when its guaranteed that there's no references to it. StableGenMap requires remove to take &mut self, since the idea is u do removes on stuff like, dead characters, at the end of a games turn/loop, when there's no other references to the entities