Despawn entities on relationship component removal when linked spawn is enabled

[Shatur]

What problem does this solve or what need does it fill?

The following code works:

let mut world = World::new();

let mut entity = world.spawn((
    Name::new("Root"),
    children![Name::new("Child1"), Name::new("Child2"),],
));

entity.remove::<Children>();

assert_eq!(world.entities().len(), 3);

But I'm not sure if this behavior is expected/desired. This makes it easy for users to "leak" entities.

What solution would you like?

We have RelationshipTarget::LINKED_SPAWN which works for despawn and cloning. I'd suggest to extend it to the relationship target component removal.

[tim-blackbird]

I think that would make it impossible to remove the relationship without despawning entities?
And, it feels intuitive to me that removing the relationship does exactly that, even for LINKED_SPAWN as it doesn't involve spawning or despawning.

[Shatur]

it feels intuitive to me that removing the relationship

I’ve noticed a few people trying to despawn related entities this way. I think the current approach is a bit error-prone.

I think that would make it impossible to remove the relationship without despawning entities?

We have things like remove_related and replace_related. I know we also have despawn_related, but I think it's better to make despawning implicit to avoid accidentally "leaking" entities, and instead use explicit operations when moving or unrelating entities.

But my main motivation is to enable a more convenient pattern. I'm not sure if you're familiar with BEI, but here I describe how input contexts need to be removed. To put it simply, right now it looks like this:

player
    .remove_with_requires::<OnFoot>()
    .despawn_related::<Actions<OnFoot>>();

But if we had implicit despawn, I could mark relationship targets Actions<C> as a required component for C, so users would only need to do this:

player.remove_with_requires::<OnFoot>();

This isn't specific to BEI - I think it's sometimes convenient to have relationship targets as required components and automatically despawn them when linked_spawn is enabled.