Add 'NUM_VARIANTS' constant to all 'board' enums

src/board/castle_rights.rs

@@ -14,10 +14,13 @@ pub enum CastleRights {
 }
 
 impl CastleRights {
+    /// The number of [CastleRights] variants.
+    pub const NUM_VARIANTS: usize = 4;
+
     /// Convert from a castle rights index into a [CastleRights] type.
     #[inline(always)]
     pub fn from_index(index: usize) -> Self {
-        assert!(index < 4);
+        assert!(index < Self::NUM_VARIANTS);
         // SAFETY: we know the value is in-bounds
         unsafe { Self::from_index_unchecked(index) }
     }

src/board/color.rs

@@ -8,10 +8,13 @@ pub enum Color {
 }
 
 impl Color {
+    /// The number of [Color] variants.
+    pub const NUM_VARIANTS: usize = 2;
+
     /// Convert from a piece index into a [Color] type.
     #[inline(always)]
     pub fn from_index(index: usize) -> Self {
-        assert!(index < 2);
+        assert!(index < Self::NUM_VARIANTS);
         // SAFETY: we know the value is in-bounds
         unsafe { Self::from_index_unchecked(index) }
     }

src/board/file.rs

@@ -15,7 +15,10 @@ pub enum File {
 }
 
 impl File {
-    const ALL: [Self; 8] = [
+    /// The number of [File] variants.
+    pub const NUM_VARIANTS: usize = 8;
+
+    const ALL: [Self; Self::NUM_VARIANTS] = [
         Self::A,
         Self::B,
         Self::C,

src/board/rank.rs

@@ -15,7 +15,10 @@ pub enum Rank {
 }
 
 impl Rank {
-    const ALL: [Self; 8] = [
+    /// The number of [Rank] variants.
+    pub const NUM_VARIANTS: usize = 8;
+
+    const ALL: [Self; Self::NUM_VARIANTS] = [
         Self::First,
         Self::Second,
         Self::Third,

src/board/square.rs

@@ -23,8 +23,11 @@ impl std::fmt::Display for Square {
 }
 
 impl Square {
+    /// The number of [Square] variants.
+    pub const NUM_VARIANTS: usize = 64;
+
     #[rustfmt::skip]
-    const ALL: [Self; 64] = [
+    const ALL: [Self; Self::NUM_VARIANTS] = [
         Self::A1, Self::A2, Self::A3, Self::A4, Self::A5, Self::A6, Self::A7, Self::A8,
         Self::B1, Self::B2, Self::B3, Self::B4, Self::B5, Self::B6, Self::B7, Self::B8,
         Self::C1, Self::C2, Self::C3, Self::C4, Self::C5, Self::C6, Self::C7, Self::C8,