implement std::error for FromHexError.

Author: mswift42

palette/src/rgb/rgb.rs

@@ -972,6 +972,30 @@ impl From<&'static str> for FromHexError {
         FromHexError::HexFormatError(err)
     }
 }
+impl core::fmt::Display for FromHexError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match &*self {
+            FromHexError::ParseIntError(e) => {
+                write!(f, "ParseIntError: {}", e)
+            }
+            FromHexError::HexFormatError(s) => {
+                write!(f, "HexFormatError: {}, please use format '#fff', 'fff', '#ffffff' or\
+                'ffffff'.", s)
+            }
+        }
+    }
+}
+
+#[cfg(feature="std")]
+impl std::error::Error for FromHexError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match &*self {
+            FromHexError::HexFormatError(_s) => None,
+            FromHexError::ParseIntError(e) => Some(e),
+        }
+    }
+}
+
 
 impl<S: RgbStandard> FromStr for Rgb<S, u8> {
     type Err = FromHexError;
@@ -1158,6 +1182,10 @@ mod test {
         assert_eq!(c.unwrap(), Rgb::<Srgb,u8>::new(18, 52, 86));
         let c = Rgb::<Srgb,u8>::from_str("#iii");
         assert!(c.is_err());
+        assert_eq!(
+            format!("{}", c.err().unwrap()),
+            "ParseIntError: invalid digit found in string"
+        );
         let c = Rgb::<Srgb,u8>::from_str("#08f");
         assert_eq!(c.unwrap(), Rgb::<Srgb,u8>::new(0, 136, 255));
         let c = Rgb::<Srgb,u8>::from_str("08f");
@@ -1166,6 +1194,11 @@ mod test {
         assert_eq!(c.unwrap(), Rgb::<Srgb,u8>::new(255,255,255));
         let c = Rgb::<Srgb,u8>::from_str("#12");
         assert!(c.is_err());
+        assert_eq!(
+            format!("{}", c.err().unwrap()), 
+            "HexFormatError: invalid hex code format, \
+        please use format \'#fff\', \'fff\', \'#ffffff\' or\'ffffff\'."
+        );
         let c = Rgb::<Srgb,u8>::from_str("da0bce");
         assert_eq!(c.unwrap(), Rgb::<Srgb,u8>::new(218,11,206));
         let c = Rgb::<Srgb,u8>::from_str("f034e6");