create colour from borrowed str; handle missing end value; tests

src/colour.rs

@@ -10,8 +10,8 @@ pub struct InvalidRgb;
 
 impl Colour {
     #[inline]
-    pub(crate) fn from(string: String) -> Result<Colour, InvalidRgb> {
+    pub(crate) fn from(string: &str) -> Result<Colour, InvalidRgb> {
-        let values: Vec<&str> = string.split(',').collect();
+        let values: Vec<&str> = string.trim_matches(',').split(',').collect();
 
         if values.len() != 3 {
             return Err(InvalidRgb);
@@ -39,7 +39,7 @@ mod test {
     #[test]
     fn test_colour_from_string() {
         assert_eq!(
-            Colour::from("0,255,0".to_string()).unwrap(),
+            Colour::from("0,255,0").unwrap(),
             Colour { red: 0, green: 255, blue: 0 }
         );
     }
@@ -48,4 +48,19 @@ mod test {
     fn test_colour_to_string() {
         assert_eq!(Colour { red: 22, green: 33, blue: 44, }.to_string(), "22,33,44".to_string());
     }
+
+    #[test]
+    fn test_colour_missing_value() {
+        assert!(Colour::from("0,0").is_err());
+    }
+
+    #[test]
+    fn test_colour_ambiguous_value() {
+        assert!(Colour::from("0,0,").is_err());
+    }
+
+    #[test]
+    fn test_colour_extraneous_value() {
+        assert!(Colour::from("0,0,0,0").is_err());
+    }
 }

src/palette.rs

@@ -23,7 +23,7 @@ impl From<String> for Palette {
 
         let colours = lines[colour_start_index..]
             .iter()
-            .map(|&line| Colour::from(line.to_string()).unwrap())
+            .map(|&line| Colour::from(line).unwrap())
             .collect();
 
         Palette { id, name, colours }