Added boxable commands

[jamescarterbell]

Objective

Create a simple method of adding boxed commands to Commands. The cleanest method of adding Box currently isn't possible (adding Box directly), but we can do something close.

The end goal is to allow a performant method of adding commands to the CommandQueue across threads (mainly for sharing some form of Commands in a par_for_each loop).

Solution

By essentially creating and storing the CommandMetas write function alongside the Box in a specialized type, we can copy the data from the box into the CommandQueue, and move it's write function into the new CommandMeta object.

[mcobzarenco]

Just my thoughts - High level, I would really love to see an explanation in the docs as to why you'd want to box your commands, also honest question as I didn't know why / when this is useful

[jamescarterbell]

Came back to look at this, fixed the fmt, think this could open up multithreaded command submissions.

[alice-i-cecile]

Approved, but I want to see what the other folks working in this space have to think about it.

[alice-i-cecile]

@jamescarterbell can you resolve the comments that are addressed to make this easier for the reviewers?

Also clippy is failing :)

[hymm]

I suspect for the parallel use case that #4749 probably avoids contention better than using a channel. (I haven't done any benching, so I wouldn't trust my guess too much.) Even if that is true, I expect that there are other use cases for boxable commands. If the linked PR gets merged we should maybe come up with another example for boxable commands in the docs.

[alice-i-cecile]

I'm happy to merge this once the typos are fixed then. Thanks for the informed opinion @hymm.

[jamescarterbell]

I suspect for the parallel use case that #4749 probably avoids contention better than using a channel. (I haven't done any benching, so I wouldn't trust my guess too much.) Even if that is true, I expect that there are other use cases for boxable commands. If the linked PR gets merged we should maybe come up with another example for boxable commands in the docs.

Yeah I made this before knowing about that PR... Maybe some sort of example with commands in events? Hmm...

[alice-i-cecile]

Yeah 🤔 I think I'd like to try and find another example for now to make this clearer; I want to get this + #4749 in for the next release and having a clear differentiation would be good.

[TheRawMeatball]

Personally I'm not a very big fan of this PR, considering that a Box<dyn FnOnce(&mut World)> would do pretty much everything this does just as well, and because I personally think creating commands without directly adding it to Commands is a bit of an anti-pattern with not much use.

[alice-i-cecile]

@jamescarterbell, my personal feeling is that we should close this out until we have a compelling use case.

[jamescarterbell]

Go for it