Implement SystemSet for BoxedSystemSet

[yyogo]

Objective

• Fix ECS: SystemConfig::after(BoxedSystemSet) does not work #8813

Solution

Explicitly implement SystemSet (implying IntoSystemSet) for BoxedSystemSet

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Rationale

BoxedSystemSet doesn't "need" to implement SystemSet because you can call the methods on the trait object reference. However, that means it can't be IntoSystemSet, since it can't unwrap the internal type, and you can't call .into_system_set(self) on dyn SystemSet, because it isn't sized.

The trivial solution is to impl SystemSet for Box<dyn SystemSet>. But that introduces a problem because IntoSystemSetConfig is implemented separately for SystemSet and BoxedSystemSet, and for SystemSet it boxes it before passing to the constructor. So to avoid this double-boxing I just added a default into_boxed() method that turns a normal, non-trait-object system set into a BoxedSystemSet, and override it for BoxedSystemSet to just return itself.

[alice-i-cecile]

I think enabling this sort of use case is important, but we've had troubles with this sort of thing before causing subtle confusion and ergonomics issues. Can you explain what you mean by "non-dispatchable"?

[yyogo]

Can you explain what you mean by "non-dispatchable"?

I just mean a trait method with a Self: Sized bound, so it can't be called on a trait object (that's what the reference calls them).

I've updated the description a bit, hope my reasoning is clearer. Can you point me to a similar case that caused trouble in the past?

[alice-i-cecile]

#8436 was the case I was thinking of :) Thanks for the clearer description!

[yyogo]

Ah, I see. I'll take a closer look and set this to draft in the meantime