Required components accept const values (#16720)

[Wuketuke]

Objective

Const values should be more ergonomic to insert, since this is too verbose

#[derive(Component)]
#[require(
    LockedAxes(||LockedAxes::ROTATION_LOCKED),
)]
pub struct CharacterController;

instead, users can now abbreviate that nonsense like this

#[derive(Component)]
#[require(
    LockedAxes = ROTATION_LOCKED),
)]
pub struct CharacterController;

it also works for enum labels.
I chose to omit the type, since were trying to reduce typing here. The alternative would have been this:

#[require(
    LockedAxes = LockedAxes::ROTATION_LOCKED),
)]

This of course has its disadvantages, since the const has to be associated, but the old closure method is still possible, so I dont think its a problem.

• Fixes Required components should accept constant and enum values of the appropriate type #16720

Testing

I added one new test in the docs, which also explain the new change. I also saw that the docs for the required components on line 165 was missing an assertion, so I added it back in

[Brezak]

We could parse out an expression instead and support more ways to set the value.

[Wuketuke]

yes, but you would somehow need to extract the type from the expression, so it will be messy

[Carter0]

Not really a macro expert but this seems reasonable. I think it would be really awesome if we had a required components example showcasing this work, but I can do that in another PR too

[alice-i-cecile]

A bit implicit but I think it's worth it. Using constants here should be dead easy.

[cart]

I would have preferred the less implicit:

#[derive(Component)]
#[require(
    LockedAxes = LockedAxes::ROTATION_LOCKED),
)]
pub struct CharacterController;

That would also support this pattern:

const CHARACTER_CONTROLLER_LOCKED_AXES: LockedAxes = LockedAxes::from_bits(...);

#[derive(Component)]
#[require(
    LockedAxes = CHARACTER_CONTROLLER_LOCKED_AXES,
)]
pub struct CharacterController;

The current implementation is unintuitive and will be confusingly broken when people try to do something like the above example.

[cart]

Actually I think we should go even further and do this:

require(
   Transform, // becomes Transform::default
   LockedAxes::ROTATION_LOCKED, // becomes || LockedAxes::ROTATION_LOCKED
   Health(10), // becomes || Health(10)
   Defense = || Defense(8), // becomes  || Defense(8)
   Speed = my_speed, // becomes my_speed
   Position { x: 10, y: 20 }, // becomes || Position { x: 10, y: 20 }
)

This would be a breaking change, but I think its what pretty much everyone actually wants. I'll take a stab at this