Add derives to mouse structs

[aevyrie]

This is a proposed fix for issue #269
This adds Copy, as well as any other derives where possible.

[cart]

This is a stylistic choice, but I think anything that is a "value type" should impl Copy. Ex: Vec2, MouseScrollUnit, and f32 all meet my definition of "value type".

I think by default, most structs don't meet my bar for Copy and should instead require explicit clones. MouseWheel and MouseMotion both have the potential to grow in the future (ex: with a device id field). Anything that has the potential to grow should be clone() unless theres a really good reason.

I'm happy to discuss it, but I think my preference here is to just impl Copy for MouseScrollUnit

[aevyrie]

I think by default, most structs don't meet my bar for Copy and should instead require explicit clones.

That's interesting! I thought that best practice was to implement Copy where possible because it can be much more performant than clone() in many cases. (Edit: the compiler can often determine when a clone should be a bytewise copy, but it is not a guarantee)

The major downside to implementing Copy, as far as I can see, is that if a non-Copy field is added to the struct in the future, it will cause a breaking change by forcing the removal of Copy and invalidating any code that relies on it instead of clone(). However, it sounds like breaking changes are a non-issue for Bevy right now.

For something that is probably being used in many places every frame, does the potential performance price of clone() matter to you? If not, I'll make the changes you requested!

[cart]

Yeah you might be right about it being a performance optimization, but i would still prefer it if we made the change. Explicit clones make you think about type ownership, which I think is worth the potential perf cost (but its probably all in the noise anyway).

[aevyrie]

Sounds good! This is probably a case of premature optimization anyway. 😄

[cart]

Build failures appear to be azure's mirrors of the ubuntu repos. We probably just need to wait a bit.