Write some tests - fixed 2 errors in the only 2 "real" lines of code I wrote...

card_stuffs/src/lib.rs

@@ -109,27 +109,27 @@ impl fmt::Display for Card {
     }
 }
 
-#[derive(Error, Debug)]
+#[derive(Error, Debug, PartialEq)]
 pub enum StackingError {
     #[error("Trying to stack the same coloured suit")]
     SameColour,
     #[error("{0} is not \"next\" to {1}")]
-    NotAdjacent(Card, Card),
+    NotAdjacent(String, String),
 }
 
 impl Card {
-    pub fn can_be_placed_on_top(&self, top: Card) -> bool {
+    pub fn can_be_placed_on_top(&self, top: Card) -> Result<(), StackingError> {
         // Can't be the same Colour
-        if self.suit.colour() != top.suit.colour() {
-            return false
+        if self.suit.colour() == top.suit.colour() {
+            return Err(StackingError::SameColour);
         }
 
-        // Needs to be adjascent
-        if self.value == Value::King || self.value.indexed_values() != top.value.indexed_values() + 1 {
-            return false
+        // Needs to be adjacent
+        if self.value == Value::King || self.value.indexed_values() + 1 != top.value.indexed_values() {
+            return Err(StackingError::NotAdjacent(self.to_string(), top.to_string()));
         }
 
-        true
+        Ok(())
     }
 }
 
@@ -139,8 +139,38 @@ mod tests {
     use super::*;
 
     #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
+    fn basic_card_stacking() {
+        let testing_card = Card {
+            suit: Suit::Heart,
+            value: Value::Five,
+        };
+        let bad_same_suit = Card {
+            suit: Suit::Heart,
+            value: Value::Six,
+        };
+        assert_eq!(testing_card.can_be_placed_on_top(bad_same_suit), Err(StackingError::SameColour));
+        let bad_same_colour = Card {
+            suit: Suit::Diamond,
+            value: Value::Six,
+        };
+        assert_eq!(testing_card.can_be_placed_on_top(bad_same_colour), Err(StackingError::SameColour));
+        let should_stack_card = Card {
+            suit: Suit::Club,
+            value: Value::Six,
+        };
+        assert_eq!(testing_card.can_be_placed_on_top(should_stack_card), Ok(()));
+        let value_too_high = Card {
+            suit: Suit::Club,
+            value: Value::Seven,
+        };
+        let not_adj_error = testing_card.can_be_placed_on_top(value_too_high);
+        if let Err(e) = not_adj_error {
+            match e {
+                StackingError::NotAdjacent(_, _) => assert!(true),
+                StackingError::SameColour => assert!(false, "Colour is different - incorrect error"),
+            }
+        } else {
+            assert!(false, "Cards are not adjacent - should be an error")
+        }
     }
 }