Add an Entities::iter() method to iterate over all entities when you only have &World

[alice-i-cecile]

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

The ordinary approach for getting the list of entities (query for Query<Entity>) doesn't work in contexts where you only have read-only access to the World.

What solution would you like?

Add an Entities::iter() or World::iter_entities() method.

What alternative(s) have you considered?

The following snippet works

        for archetype in world.archetypes().iter() {
            builder.extract_entities(archetype.entities().iter().copied());

Borrowed from @mockersf in https://github.com/bevyengine/bevy/pull/6227/files#diff-717fbf581c75ed1fc65c9755af2bf8a2ddc97c02e89f4044f86efa70a2ea4260L45

Additional context

It's not entirely clear if Entities::iter() can be implemented directly without digging in; I'm not sure if we have all of the information needed there.

[hymm]

FYI I was able to implement this on Entities, but I prefer #6242 as my method requires filtering out entities with invalid archetypes and creating the Entity from the EntityMeta. Iterating over the archetypes feels more straightforward.

impl Entities {
    /// Returns an iterator over entities in valid archetypes
    pub fn iter(&'_ self) -> impl Iterator<Item = Entity> + '_ {
        self.meta
            .iter()
            .enumerate()
            .filter(|(_, meta)| meta.location.archetype_id != ArchetypeId::INVALID)
            .map(|(index, meta)| Entity {
                generation: meta.generation,
                id: index as u32,
            })
    }
}

[mockersf]

closed by #6242