Fix Swift 3 guard warnings

.swiftlint.yml

@@ -12,6 +12,7 @@ disabled_rules: # rule identifiers to exclude from running
     - type_body_length
     - function_body_length
     - comma
+    - conditional_binding_cascade
 
 opt_in_rules: # some rules are only opt-in
     - empty_count

Sources/Game.swift

@@ -171,14 +171,17 @@ public final class Game {
             #else
                 let parts = fen.characters.split(" ").map(String.init)
             #endif
-            guard parts.count == 6,
-                let board = Board(fen: parts[0])
-                where parts[1].characters.count == 1,
+            guard
+                parts.count == 6,
+                let board = Board(fen: parts[0]),
+                parts[1].characters.count == 1,
                 let playerTurn = parts[1].characters.first.flatMap(Color.init),
-                rights = CastlingRights(string: parts[2]),
-                halfmoves = UInt(parts[4]),
-                fullmoves = UInt(parts[5]) where fullmoves > 0
-                else { return nil }
+                let rights = CastlingRights(string: parts[2]),
+                let halfmoves = UInt(parts[4]),
+                let fullmoves = UInt(parts[5]),
+                fullmoves > 0 else {
+                    return nil
+            }
             var target: Square? = nil
             let targetStr = parts[3]
             let targetChars = targetStr.characters
@@ -335,7 +338,7 @@ public final class Game {
 
     /// The target move location for an en passant.
     public var enPassantTarget: Square? {
-        guard let (move, piece, _, _, _, _) = _moveHistory.last where piece.kind.isPawn else {
+        guard let (move, piece, _, _, _, _) = _moveHistory.last, piece.kind.isPawn else {
             return nil
         }
         guard abs(move.rankChange) == 2 else {
@@ -427,7 +430,7 @@ public final class Game {
         if considerHalfmoves && halfmoves >= 100 {
             return 0
         }
-        guard let piece = board[square] where piece.color == playerTurn else {
+        guard let piece = board[square], piece.color == playerTurn else {
             return 0
         }
         if kingIsDoubleChecked {

Sources/Square.swift

@@ -474,7 +474,7 @@ extension Square {
 
     /// Create a square from `file` and `rank`. Returns `nil` if either is `nil`.
     public init?(file: File?, rank: Rank?) {
-        guard let file = file, rank = rank else {
+        guard let file = file, let rank = rank else {
             return nil
         }
         self.init(file: file, rank: rank)