can spawn empty bundle from world

[mockersf]

Objective

• Fix world.spawn(())
• Fix World.spawn(()) panics with index out of bounds #6226

Solution

• If the bundle being inserted is (), use world.spawn_empty() instead
• Maybe not the best fix, but it works and it's easy 😄

[alice-i-cecile]

Hmm yeah. I don't love the runtime branching, but maybe it gets optimized away. Does this regress our benchmarks?

[alice-i-cecile]

We should add a regression test for this.

[cart]

I think the only real alternative to branching somewhere in this impl is removing the Bundle impl for () and adding the constraint that Bundle can't be empty.

Note that this PR isn't a full fix. All empty bundles are broken:

// This fails in the same way
#[derive(Bundle)]
struct EmptyBundle {}
let mut world = World::new();
world.spawn(EmptyBundle {});

[cart]

A full (still semi-weird) fix would be to check new_archetype_id == ArchetypeId::EMPTY in get_bundle_spawner, returning None in that case, and handling that with spawn_empty where appropriate.

But maybe we don't need to support empty bundles? Do they even have a purpose anymore? Only use case I can think of is that we don't have a batch spawning api for empty entities:
world.spawn_batch((0..100).map(|_| ())), which could be replaced with some world.spawn_empty_batch(100)

Also: idk if we need to hack around this for 0.9. This case fails with an unhelpful error message, but it still fails and it doesn't really have a use case. Maybe better to just hold off for a full solve.

[cart]

world.spawn_batch((0..100).map(|_| ()))

Notably this also panics atm.

[alice-i-cecile]

Yeah I think I'm fine with removing empty bundles completely. I don't like any of these fixes.

[cart]

The core of the issue is that spawning empty bundles is framed as EmptyArchetype -> NewArchetype from an entity graph perspective, which requires (separate) ownership borrows of both. There is definitely a more satisfying solution that doesn't involve branching everywhere, but it will require rephrasing some things / revisiting assumptions. We could make it work, if we decide empty bundles are desirable. But its a non-trivial amount of work.

[cart]

But yeah we should discuss removing empty bundles.

[cart]

Came up with a reasonable long term fix: #6425