Implement System for Box<dyn System>

[hankjordan]

Objective

• Implement System for Box<dyn System>
• Implement System and ReadOnlySystem for Box<dyn System>
• Add BoxedReadOnlySystem type alias
• Closes Implement System for Box<dyn System> #18624

Solution

• See above

Testing

• Added module test
• Should the implementation of type_id return the type_id of the Box or the underlying type_id impl?

[alice-i-cecile]

We've resisted similar things in the past (impl Reflect for Box<dyn Reflect>) out of concern for confusing double-boxing. I don't have a strong opinion on whether we should follow that precedent here, but I wanted to raise it.

[chescock]

We've resisted similar things in the past (impl Reflect for Box<dyn Reflect>) out of concern for confusing double-boxing. I don't have a strong opinion on whether we should follow that precedent here, but I wanted to raise it.

I don't have a strong opinion here, either, but I can see how being able to use combinators like pipe with already-boxed systems could be useful.

• Should the implementation of type_id return the type_id of the Box or the underlying type_id impl?

I'd vote for using the underlying impl, as you're doing now. After all, the whole point of that method is to get the actual type of a dyn System!

[chescock]

Looking at impls for Box in std, it seems more common to make them generic in the contents of the box, like

Suggested change

	impl<In, Out> System for Box<dyn System<In = In, Out = Out>>
	where
	In: SystemInput + 'static,
	Out: 'static,
	{
	type In = In;
	type Out = Out;
	impl<S: System + ?Sized> System for Box<S> {
	type In = S::In;
	type Out = S::Out;

Then, since dyn System: System, that impl covers Box<dyn System>.