Required components accept const values (#16720) (#18309)

# Objective

Const values should be more ergonomic to insert, since this is too
verbose
``` rust
#[derive(Component)]
#[require(
    LockedAxes(||LockedAxes::ROTATION_LOCKED),
)]
pub struct CharacterController;
```
instead, users can now abbreviate that nonsense like this
``` rust
#[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:
```rust
#[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 #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

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_ecs/macros/src/component.rs

@@ -611,6 +611,12 @@ impl Parse for Require {
                 let func = content.parse::<Path>()?;
                 Some(RequireFunc::Path(func))
             }
+        } else if input.peek(Token![=]) {
+            let _t: syn::Token![=] = input.parse()?;
+            let label: Ident = input.parse()?;
+            let tokens: TokenStream = quote::quote! (|| #path::#label).into();
+            let func = syn::parse(tokens).unwrap();
+            Some(RequireFunc::Closure(func))
         } else {
             None
         };

crates/bevy_ecs/src/component.rs

@@ -180,11 +180,43 @@ use thiserror::Error;
 /// }
 ///
 /// # let mut world = World::default();
+/// // This will implicitly also insert C with the init_c() constructor
+/// let id = world.spawn(A).id();
+/// assert_eq!(&C(10), world.entity(id).get::<C>().unwrap());
+///
 /// // This will implicitly also insert C with the `|| C(20)` constructor closure
 /// let id = world.spawn(B).id();
 /// assert_eq!(&C(20), world.entity(id).get::<C>().unwrap());
 /// ```
 ///
+/// For convenience sake, you can abbreviate enum labels or constant values, with the type inferred to match that of the component you are requiring:
+///
+/// ```
+/// # use bevy_ecs::prelude::*;
+/// #[derive(Component)]
+/// #[require(B = One, C = ONE)]
+/// struct A;
+///
+/// #[derive(Component, PartialEq, Eq, Debug)]
+/// enum B {
+///    Zero,
+///    One,
+///    Two
+/// }
+///
+/// #[derive(Component, PartialEq, Eq, Debug)]
+/// struct C(u8);
+///
+/// impl C {
+///     pub const ONE: Self = Self(1);
+/// }
+///
+/// # let mut world = World::default();
+/// let id = world.spawn(A).id();
+/// assert_eq!(&B::One, world.entity(id).get::<B>().unwrap());
+/// assert_eq!(&C(1), world.entity(id).get::<C>().unwrap());
+/// ````
+///
 /// Required components are _recursive_. This means, if a Required Component has required components,
 /// those components will _also_ be inserted if they are missing:
 ///