Return the passed EntityWorldMut from EntityCommand::apply

[gwafotapa]

For reference here is the trait EntityCommand:

pub trait EntityCommand<Out = ()>: Send + 'static {
    fn apply(self, entity: EntityWorldMut) -> Out;
}

The method consumes the EntityWorldMut it takes. I thought an implementation of the trait with Out as EntityWorldMut should be able to return its owned argument so I could continue using it afterwards. In its simplest form that would be

struct MyEntityCommand;

impl EntityCommand<EntityWorldMut<'_>> for MyEntityCommand {
    fn apply(self, entity: EntityWorldMut) -> EntityWorldMut {
        entity
    }
}

but this doesn't compile. The compiler complains that the lifetimes of the method argument and its return must be different per the trait signature. I've tried

struct MyEntityCommand;

impl<'a> EntityCommand<EntityWorldMut<'a>> for MyEntityCommand {
    fn apply<'b: 'a>(self, entity: EntityWorldMut<'b>) -> EntityWorldMut<'a> {
        entity
    }
}

Here the compiler complains that b is early bound whereas according to the trait signature it should be late bound. Lifting b at the impl level and the method argument may not live as long.

After research I found this post which seems to solve this exact problem. As suggested I've tried lifting the lifetime to the trait to add the feature in Bevy but I've run into many compiler complaints that I am unable to solve for now. I was wondering if I'm on the right track or if I've missed something.

[SkiFire13]

Moving the lifetime is technically possible but it's not something I would suggest doing. What are you actually trying to do?

[gwafotapa]

Something like

    let entity = world.spawn_empty();
    let mut entity = MyEntityCommand.apply(entity);
    entity.insert(MyComponent);

assuming order matters here. Now it's no bother really. I just do

    let entity = world.spawn_empty();
    let id = entity.id();
    MyEntityCommand.apply(entity);
    let mut entity = world.entity_mut(id);
    entity.insert(MyComponent);

But it got me wondering.

[gwafotapa]

Unbeknownst to me, Bevy already got this covered with EntityWorldMut::reborrow_scope!