Validate en-passant square's rank in 'ChessBoard'

src/board/chess_board/error.rs

@@ -9,7 +9,7 @@ pub enum InvalidError {
     InvalidPawnPosition,
     /// Castling rights do not match up with the state of the board.
     InvalidCastlingRights,
-    /// En-passant target square is not empty and behind an opponent's pawn.
+    /// En-passant target square is not empty, behind an opponent's pawn, on the correct rank.
     InvalidEnPassant,
     /// The two kings are next to each other.
     NeighbouringKings,
@@ -33,7 +33,7 @@ impl std::fmt::Display for InvalidError {
                 "Castling rights do not match up with the state of the board."
             }
             Self::InvalidEnPassant => {
-                "En-passant target square is not empty and behind an opponent's pawn."
+                "En-passant target square is not empty, behind an opponent's pawn, on the correct rank."
             }
             Self::NeighbouringKings => "The two kings are next to each other.",
             Self::OpponentInCheck => "The opponent is currently in check.",

src/board/chess_board/mod.rs

@@ -297,12 +297,22 @@ impl ChessBoard {
             }
         }
 
-        // The current en-passant target square must be empty, right behind an opponent's pawn.
+        // En-passant validation
         if let Some(square) = self.en_passant() {
+            // Must be empty
             if !(self.combined_occupancy() & square).is_empty() {
                 return Err(InvalidError::InvalidEnPassant);
             }
-            let opponent_pawns = self.occupancy(Piece::Pawn, !self.current_player());
+
+            let opponent = !self.current_player();
+
+            // Must be on the opponent's third rank
+            if (square & opponent.third_rank().into_bitboard()).is_empty() {
+                return Err(InvalidError::InvalidEnPassant);
+            }
+
+            // Must be behind a pawn
+            let opponent_pawns = self.occupancy(Piece::Pawn, opponent);
             let double_pushed_pawn = self
                 .current_player()
                 .backward_direction()