src/main.rs

+trait ConvertToString {
+    /// Returns a string representation of self
+    fn to_string(&self) -> String;
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
 enum Square {
     Empty,
     Cross,
     Circle,
 }
 
+impl ConvertToString for Square {
+    fn to_string(&self) -> String {
+        match self {
+            Square::Empty => String::from("."),
+            Square::Cross => String::from("X"),
+            Square::Circle => String::from("O"),
+        }
+    }
+}
+
 struct Board {
     board: [Square; 9],
 }
@@ -12,41 +28,34 @@ impl Board {
     /// Creates a new empty board
     fn new() -> Self {
         Self {
-            board: [
-                Square::Empty,
-                Square::Cross,
-                Square::Circle,
-                Square::Empty,
-                Square::Empty,
-                Square::Empty,
-                Square::Empty,
-                Square::Empty,
-                Square::Empty,
-            ],
+            board: [Square::Empty; 9],
         }
     }
 
     /// Display the board to standard output
     fn display(&self) {
+        println!("{}", self.to_string());
+    }
+}
+
+impl ConvertToString for Board {
+    fn to_string(&self) -> String {
+        let mut out = String::new();
         for row in 0..3 {
-            println!(
-                " {} | {} | {}",
-                square_to_char(&self.board[row * 3]),
-                square_to_char(&self.board[row * 3 + 1]),
-                square_to_char(&self.board[row * 3 + 2])
+            out.push_str(
+                format!(
+                    " {} | {} | {}",
+                    self.board[row * 3].to_string(),
+                    self.board[row * 3 + 1].to_string(),
+                    self.board[row * 3 + 2].to_string()
+                )
+                .as_str(),
             );
             if row == 0 || row == 1 {
-                println!("---+---+---");
+                out.push_str("\n---+---+---\n");
             }
         }
-    }
-}
-
-fn square_to_char(s: &Square) -> char {
-    match s {
-        Square::Empty => '.',
-        Square::Cross => 'X',
-        Square::Circle => 'O',
+        out
     }
 }
 
@@ -61,7 +70,7 @@ fn main() {
 // A module where the tests are located, more on modules later
 mod tests {
     use super::*;
-    
+
     #[test]
     fn should_pass() {
         assert!(true);
@@ -77,14 +86,30 @@ mod tests {
         let board = Board::new();
 
         for square in board.board {
-
-            // Cannot do that due to 'trait' issues, more on that
-            // later
-            //            assert_eq!(square, Square::Empty);
-            match square {
-                Square::Empty => (),
-                Square::Cross | Square::Circle => panic!("Square is not empty"),
-            }
+            assert_eq!(square, Square::Empty);
         }
     }
+
+    #[test]
+    fn empty_board_to_string() {
+        let expected = " . | . | .
+---+---+---
+ . | . | .
+---+---+---
+ . | . | .";
+        assert_eq!(expected, Board::new().to_string());
+    }
+
+    #[test]
+    fn empty_square_to_string_dot() {
+        assert_eq!(".", Square::Empty.to_string());
+    }
+    #[test]
+    fn cross_square_to_string_x() {
+        assert_eq!("X", Square::Cross.to_string());
+    }
+    #[test]
+    fn circle_square_to_string_big_o() {
+        assert_eq!("O", Square::Circle.to_string());
+    }
 }