Add Self: Sized bounds to Bundle trait methods

[jnhyatt]

Objective

Fixes #3227.

I'll admit upfront this might be some silly shenanigans.

This came about based on some Rust-fu I was trying to do to enable generic dynamic bundle insertion/removal. Consider the following snippet:

#[derive(Component)]
pub struct InsertThisLater(Box<dyn Bundle>);

This doesn't compile, and from my perspective, understandably: a Bundle that doesn't know its type can't get into the world. A workaround I came up with is this:

pub trait DynBundle {
    fn insert(self: Box<Self>, mut e: EntityWorldMut);
}

pub struct InsertThisLater(Box<DynBundle>);

A component can impl this trait (or derive it) and suddenly you have a working system! Only problem is Rust thinks it's not possible since some of Bundle's methods (erroneously) assume Self: ?Sized.

Solution

Add where Self: Sized to the rest of Bundle's (and DynamicBundle's) trait methods

---

Breaking change?

Technically, I think?

I'm not exactly sure what kind of code you'd have to write to run into this breaking change, but this change expands what you can do with bundles, not the other way around. I'll try and find some weird bit of code that breaks with this, but I don't think it needs a migration guide section, and honestly, I don't think it would be bad to shove it into 0.14.2, however I understand that still breaks the letter of the law and is a solid argument against doing so.

[jnhyatt]

@alice-i-cecile 🙂

[alice-i-cecile]

Neat! Can you please add a doc test to Bundle to verify that Box<dyn Bundle> works, and can be inserted and removed correctly?

[alice-i-cecile]

I've linked the issue I made several years ago for this problem in your PR description. I'm going to put this in 0.15 instead of a 0.14 point release just in case we've missed some terrible edge case on the breaking changes.

[jnhyatt]

To be clear, this isn't a drop-in dynamic bundle solution, however, reading through that issue, it looks like it wasn't far from being pushed through. I can turn this into a full-on dynamic bundle PR, I think my preferred solution would be to add a couple object-safe helper methods on Bundle and then implement them in the derive.

[alice-i-cecile]

I'd prefer to turn this into a full-on dynamic bundle PR IMO :) This is very unintrusive, and there would be a lot of value in a solution there. Probably spin up an ECS folder example?