Support operations for render layers and fix equality comparisons (#13310)

# Objective

Allow combining render layers with a more-ergonomic syntax than
`RenderLayers::from_iter(a.iter().chain(b.iter()))`.

## Solution

Add the `or` operation (and corresponding `const` method) to allow
computing the union of a set of render layers. While we're here, also
added `and` and `xor` operations. Someone might find them useful

## Testing

Added a simple unit test.

crates/bevy_render/src/view/visibility/render_layers.rs

@@ -22,7 +22,10 @@ pub type Layer = usize;
 /// Entities without this component belong to layer `0`.
 #[derive(Component, Clone, Reflect, PartialEq, Eq, PartialOrd, Ord)]
 #[reflect(Component, Default, PartialEq)]
-pub struct RenderLayers(SmallVec<[u64; 1]>);
+pub struct RenderLayers(SmallVec<[u64; INLINE_BLOCKS]>);
+
+/// The number of memory blocks stored inline
+const INLINE_BLOCKS: usize = 1;
 
 impl Default for &RenderLayers {
     fn default() -> Self {
@@ -47,8 +50,7 @@ impl FromIterator<Layer> for RenderLayers {
 impl Default for RenderLayers {
     /// By default, this structure includes layer `0`, which represents the first layer.
     fn default() -> Self {
-        let (_, bit) = Self::layer_info(0);
-        RenderLayers(SmallVec::from_const([bit]))
+        Self::layer(0)
     }
 }
 
@@ -57,15 +59,17 @@ impl RenderLayers {
     pub const fn layer(n: Layer) -> Self {
         let (buffer_index, bit) = Self::layer_info(n);
         assert!(
-            buffer_index < 1,
+            buffer_index < INLINE_BLOCKS,
             "layer is out of bounds for const construction"
         );
-        RenderLayers(SmallVec::from_const([bit]))
+        let mut buffer = [0; INLINE_BLOCKS];
+        buffer[buffer_index] = bit;
+        RenderLayers(SmallVec::from_const(buffer))
     }
 
     /// Create a new `RenderLayers` that belongs to no layers.
     pub const fn none() -> Self {
-        RenderLayers(SmallVec::from_const([0]))
+        RenderLayers(SmallVec::from_const([0; INLINE_BLOCKS]))
     }
 
     /// Create a `RenderLayers` from a list of layers.
@@ -91,6 +95,11 @@ impl RenderLayers {
         let (buffer_index, bit) = Self::layer_info(layer);
         if buffer_index < self.0.len() {
             self.0[buffer_index] &= !bit;
+            // Drop trailing zero memory blocks.
+            // NOTE: This is not just an optimization, it is necessary for the derived PartialEq impl to be correct.
+            if buffer_index == self.0.len() - 1 {
+                self = self.shrink();
+            }
         }
         self
     }
@@ -154,6 +163,81 @@ impl RenderLayers {
             Some(layer - 1)
         })
     }
+
+    /// Returns the set of [layers](Layer) shared by two instances of [`RenderLayers`].
+    ///
+    /// This corresponds to the `self & other` operation.
+    pub fn intersection(&self, other: &Self) -> Self {
+        self.combine_blocks(other, |a, b| a & b).shrink()
+    }
+
+    /// Returns all [layers](Layer) included in either instance of [`RenderLayers`].
+    ///
+    /// This corresponds to the `self | other` operation.
+    pub fn union(&self, other: &Self) -> Self {
+        self.combine_blocks(other, |a, b| a | b) // doesn't need to be shrunk, if the inputs are nonzero then the result will be too
+    }
+
+    /// Returns all [layers](Layer) included in exactly one of the instances of [`RenderLayers`].
+    ///
+    /// This corresponds to the "exclusive or" (XOR) operation: `self ^ other`.
+    pub fn symmetric_difference(&self, other: &Self) -> Self {
+        self.combine_blocks(other, |a, b| a ^ b).shrink()
+    }
+
+    /// Deallocates any trailing-zero memory blocks from this instance
+    fn shrink(mut self) -> Self {
+        let mut any_dropped = false;
+        while self.0.len() > INLINE_BLOCKS && self.0.last() == Some(&0) {
+            self.0.pop();
+            any_dropped = true;
+        }
+        if any_dropped && self.0.len() <= INLINE_BLOCKS {
+            self.0.shrink_to_fit();
+        }
+        self
+    }
+
+    /// Creates a new instance of [`RenderLayers`] by applying a function to the memory blocks
+    /// of self and another instance.
+    ///
+    /// If the function `f` might return `0` for non-zero inputs, you should call [`Self::shrink`]
+    /// on the output to ensure that there are no trailing zero memory blocks that would break
+    /// this type's equality comparison.
+    fn combine_blocks(&self, other: &Self, mut f: impl FnMut(u64, u64) -> u64) -> Self {
+        let mut a = self.0.iter();
+        let mut b = other.0.iter();
+        let mask = std::iter::from_fn(|| {
+            let a = a.next().copied();
+            let b = b.next().copied();
+            if a.is_none() && b.is_none() {
+                return None;
+            }
+            Some(f(a.unwrap_or_default(), b.unwrap_or_default()))
+        });
+        Self(mask.collect())
+    }
+}
+
+impl std::ops::BitAnd for RenderLayers {
+    type Output = Self;
+    fn bitand(self, rhs: Self) -> Self::Output {
+        self.intersection(&rhs)
+    }
+}
+
+impl std::ops::BitOr for RenderLayers {
+    type Output = Self;
+    fn bitor(self, rhs: Self) -> Self::Output {
+        self.union(&rhs)
+    }
+}
+
+impl std::ops::BitXor for RenderLayers {
+    type Output = Self;
+    fn bitxor(self, rhs: Self) -> Self::Output {
+        self.symmetric_difference(&rhs)
+    }
 }
 
 #[cfg(test)]
@@ -242,4 +326,33 @@ mod rendering_mask_tests {
         let out = layers.iter().collect::<Vec<_>>();
         assert_eq!(tricky_layers, out, "tricky layers roundtrip");
     }
+
+    const MANY: RenderLayers = RenderLayers(SmallVec::from_const([u64::MAX]));
+
+    #[test]
+    fn render_layer_ops() {
+        let a = RenderLayers::from_layers(&[2, 4, 6]);
+        let b = RenderLayers::from_layers(&[1, 2, 3, 4, 5]);
+
+        assert_eq!(
+            a.clone() | b.clone(),
+            RenderLayers::from_layers(&[1, 2, 3, 4, 5, 6])
+        );
+        assert_eq!(a.clone() & b.clone(), RenderLayers::from_layers(&[2, 4]));
+        assert_eq!(a ^ b, RenderLayers::from_layers(&[1, 3, 5, 6]));
+
+        assert_eq!(RenderLayers::none() & MANY, RenderLayers::none());
+        assert_eq!(RenderLayers::none() | MANY, MANY);
+        assert_eq!(RenderLayers::none() ^ MANY, MANY);
+    }
+
+    #[test]
+    fn render_layer_shrink() {
+        // Since it has layers greater than 64, the instance should take up two memory blocks
+        let layers = RenderLayers::from_layers(&[1, 77]);
+        assert!(layers.0.len() == 2);
+        // When excluding that layer, it should drop the extra memory block
+        let layers = layers.without(77);
+        assert!(layers.0.len() == 1);
+    }
 }