Make Ref copyable

[ItsDoot]

Objective

Trying to write a bevy_immediate widget with the following signature:

fn meter<T: Display>(self, name: impl Into<String>, current: Ref<T>, max: Ref<T>) -> Self;

Where it would be called like so (pseudocode):

#[derive(Component)]
struct Health {
    current: f32,
    max: f32
}

fn render(ui: &mut ImmEntity, health: Ref<Health>) {
    ui.meter("Health", health.map(|v| &v.current), health.map(|v| &v.max));
}

However, this results in a compiler error as there's currently no way to split borrows on a Ref<T> (there is no Ref::clone).

Solution

• Make Ref both Clone and Copy, easily allowing split borrows.

Note: Res<T> is not Clone or Copy to make cloning the T out easier, but comparatively Ref<T> is much less commonly used (users primarily write &T in their query). And even further, I believe the amount of users who both use Ref<T> and who need to clone the T out will be quite small (we don't even have a Ref::clone inherent function, if that proves anything).

Alternatively, we could add Ref::clone as an inherent function, but the UX on that isn't great.

Testing

The rust compiler keeps us in check here.

[SkiFire13]

Note that this is technically a breaking change since it will break code calling .clone() on it (as you already mentioned).

I also wonder if we can improve the situation by e.g. adding a separate method to copy a Ref<T> and making some methods on it take &self instead of self (for instance your example would work if map took &self)

[ItsDoot]

Note that this is technically a breaking change

Added a migration guide.

[ItsDoot]

Additionally, without Copy/Clone it would need Ref::reborrow to go from &Ref -> Ref, which would be a bit odd since EntityRef doesn't have or need a reborrow function.

[alice-i-cecile]

Initial impressions on this are positive: I think this is a reasonable tradeoff. Needs a comment about the inconsistency though.

[alice-i-cecile]

@PPakalns, can you take a look at this quick? Is this pattern reasonable to use downstream in bevy_immediate?