[board, core, bitboard] Clean up casts between Rank, File and BitBoard

Let BitBoard::rank and BitBoard::file take a Rank and File directly, instead of a
u8 by reference. And then make the Rank/File::as_index const and return a value
rather than a reference.

All this allows you to convert between Rank, File, and BitBoard at compile tile
(i.e. as a const method) rather than needing to do runtime stuff.

bitboard/src/bitboard.rs

@@ -58,16 +58,12 @@ impl BitBoard {
         BitBoard(bits)
     }
 
-    // TODO: Is &u8 really necessary here?
-    pub fn rank(rank: &u8) -> BitBoard {
-        debug_assert!(*rank < 8);
-        library::RANKS[*rank as usize]
+    pub const fn rank(rank: Rank) -> BitBoard {
+        library::RANKS[rank.as_index()]
     }
 
-    // TODO: Is &u8 really necessary here?
-    pub fn file(file: &u8) -> BitBoard {
-        debug_assert!(*file < 8);
-        library::FILES[*file as usize]
+    pub const fn file(file: File) -> BitBoard {
+        library::FILES[file.as_index()]
     }
 
     pub fn ray(sq: Square, dir: Direction) -> BitBoard {
@@ -313,7 +309,7 @@ impl From<BitBoard> for u64 {
 
 impl From<File> for BitBoard {
     fn from(value: File) -> Self {
-        library::FILES[*value.as_index() as usize]
+        library::FILES[value.as_index()]
     }
 }
 
@@ -325,7 +321,7 @@ impl From<Option<Square>> for BitBoard {
 
 impl From<Rank> for BitBoard {
     fn from(value: Rank) -> Self {
-        library::FILES[*value.as_index() as usize]
+        library::FILES[value.as_index()]
     }
 }
 
@@ -466,21 +462,24 @@ mod tests {
     #[test]
     #[ignore]
     fn display_and_debug() {
-        let bb = BitBoard::file(&0) | BitBoard::file(&3) | BitBoard::rank(&7) | BitBoard::rank(&4);
+        let bb = BitBoard::file(File::A)
+            | BitBoard::file(File::D)
+            | BitBoard::rank(Rank::FIVE)
+            | BitBoard::rank(Rank::EIGHT);
         println!("{}", &bb);
     }
 
     #[test]
     #[allow(clippy::unreadable_literal)]
     fn rank() {
-        assert_eq!(BitBoard::rank(&0).0, 0xFF, "Rank 1");
-        assert_eq!(BitBoard::rank(&1).0, 0xFF00, "Rank 2");
-        assert_eq!(BitBoard::rank(&2).0, 0xFF0000, "Rank 3");
-        assert_eq!(BitBoard::rank(&3).0, 0xFF000000, "Rank 4");
-        assert_eq!(BitBoard::rank(&4).0, 0xFF00000000, "Rank 5");
-        assert_eq!(BitBoard::rank(&5).0, 0xFF0000000000, "Rank 6");
-        assert_eq!(BitBoard::rank(&6).0, 0xFF000000000000, "Rank 7");
-        assert_eq!(BitBoard::rank(&7).0, 0xFF00000000000000, "Rank 8");
+        assert_eq!(BitBoard::rank(Rank::ONE).0, 0xFF, "Rank 1");
+        assert_eq!(BitBoard::rank(Rank::TWO).0, 0xFF00, "Rank 2");
+        assert_eq!(BitBoard::rank(Rank::THREE).0, 0xFF0000, "Rank 3");
+        assert_eq!(BitBoard::rank(Rank::FOUR).0, 0xFF000000, "Rank 4");
+        assert_eq!(BitBoard::rank(Rank::FIVE).0, 0xFF00000000, "Rank 5");
+        assert_eq!(BitBoard::rank(Rank::SIX).0, 0xFF0000000000, "Rank 6");
+        assert_eq!(BitBoard::rank(Rank::SEVEN).0, 0xFF000000000000, "Rank 7");
+        assert_eq!(BitBoard::rank(Rank::EIGHT).0, 0xFF00000000000000, "Rank 8");
     }
 
     #[test]

board/src/movement.rs

@@ -63,10 +63,10 @@ fn pawn_pushes(pawn: BitBoard, color: Color, occupancy: BitBoard) -> BitBoard {
 
     match color {
         Color::White => {
-            let second_rank = BitBoard::rank(&Rank::TWO.into());
+            const SECOND_RANK: BitBoard = BitBoard::rank(Rank::TWO);
 
             let mut pushes = pawn.shift_north_one() & vacancy;
-            if !(pawn & second_rank).is_empty() {
+            if !(pawn & SECOND_RANK).is_empty() {
                 // Double push
                 pushes = pushes | (pushes.shift_north_one() & vacancy);
             }
@@ -74,10 +74,10 @@ fn pawn_pushes(pawn: BitBoard, color: Color, occupancy: BitBoard) -> BitBoard {
             pushes
         }
         Color::Black => {
-            let seventh_rank = BitBoard::rank(&Rank::SEVEN.into());
+            const SEVENTH_RANK: BitBoard = BitBoard::rank(Rank::SEVEN);
 
             let mut pushes = pawn.shift_south_one() & vacancy;
-            if !(pawn & seventh_rank).is_empty() {
+            if !(pawn & SEVENTH_RANK).is_empty() {
                 // Double push
                 pushes = pushes | (pushes.shift_south_one() & vacancy);
             }

core/src/coordinates.rs

@@ -88,8 +88,8 @@ macro_rules! range_bound_struct {
             }
 
             #[must_use]
-            $vis fn as_index(&self) -> &$repr {
-                &self.0
+            $vis const fn as_index(&self) -> usize {
+                self.0 as usize
             }
 
             $vis fn iter(&self) -> impl Iterator<Item = Self> {