Replace ConvertToString by std::fmt::Display

src/main.rs

-trait ConvertToString {
-    /// Returns a string representation of self
-    fn to_string(&self) -> String;
-}
-
 #[derive(Clone, Copy, Debug, PartialEq)]
 enum Square {
     Empty,
@@ -10,12 +5,12 @@ enum Square {
     Circle,
 }
 
-impl ConvertToString for Square {
-    fn to_string(&self) -> String {
+impl std::fmt::Display for Square {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            Square::Empty => String::from("."),
-            Square::Cross => String::from("X"),
-            Square::Circle => String::from("O"),
+            Square::Empty => write!(f, "."),
+            Square::Cross => write!(f, "X"),
+            Square::Circle => write!(f, "O"),
         }
     }
 }
@@ -34,28 +29,23 @@ impl Board {
 
     /// Display the board to standard output
     fn display(&self) {
-        println!("{}", self.to_string());
+        println!("{}", self);
     }
 }
 
-impl ConvertToString for Board {
-    fn to_string(&self) -> String {
-        let mut out = String::new();
+impl std::fmt::Display for Board {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         for row in 0..3 {
-            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(),
-            );
+            writeln!(f, " {} | {} | {}",
+                    self.board[row * 3],
+                    self.board[row * 3 + 1],
+                    self.board[row * 3 + 2],
+                )?;
             if row == 0 || row == 1 {
-                out.push_str("\n---+---+---\n");
+                writeln!(f, "---+---+---")?;
             }
         }
-        out
+        Ok(())
     }
 }
 
@@ -96,20 +86,21 @@ mod tests {
 ---+---+---
  . | . | .
 ---+---+---
- . | . | .";
-        assert_eq!(expected, Board::new().to_string());
+ . | . | .
+";
+        assert_eq!(expected, format!("{}", Board::new()));
     }
 
     #[test]
     fn empty_square_to_string_dot() {
-        assert_eq!(".", Square::Empty.to_string());
+        assert_eq!(".", format!("{}", Square::Empty));
     }
     #[test]
     fn cross_square_to_string_x() {
-        assert_eq!("X", Square::Cross.to_string());
+        assert_eq!("X", format!("{}", Square::Cross));
     }
     #[test]
     fn circle_square_to_string_big_o() {
-        assert_eq!("O", Square::Circle.to_string());
+        assert_eq!("O", format!("{}", Square::Circle));
     }
 }