Tweak API for fadein() and fadeout() operations.

Previously, we were returning `self` for these operations when impled on
fully-opaque colors. This changes the API to diverge from Less'
implementation such that, for opaque colors, we convert into the alpha
channel equivalent (rgba or hsla) and then perform the color operation
as expected.

Author: vaidehijoshi

src/hsl.rs

@@ -127,12 +127,12 @@ impl Color for HSL {
         self.to_hsla().darken(amount).to_hsl()
     }
 
-    fn fadein(self, _amount: Ratio) -> Self {
-        self
+    fn fadein(self, amount: Ratio) -> Self::Alpha {
+        self.to_hsla().fadein(amount)
     }
 
-    fn fadeout(self, _amount: Ratio) -> Self {
-        self
+    fn fadeout(self, amount: Ratio) -> Self::Alpha {
+        self.to_hsla().fadeout(amount)
     }
 
     fn fade(self, amount: Ratio) -> Self::Alpha {

src/lib.rs

@@ -152,7 +152,7 @@ pub trait Color {
     fn darken(self, amount: Ratio) -> Self;
 
     /// Decreases the transparency (or increase the opacity) of `self`, making it more opaque.
-    /// Has no effect on opaque (non-alpha) colors.
+    /// For opqaue colors, converts into the alpha equivalent of `self`, and then increases the opacity.
     /// For more, see Less' [Color Operations](http://lesscss.org/functions/#color-operations-fadein).
     ///
     /// # Examples
@@ -163,12 +163,12 @@ pub trait Color {
     /// let cornflower_blue = rgb(100, 149, 237);
     ///
     /// assert_eq!(tomato.fadein(percent(25)), rgba(255, 99, 71, 0.5));
-    /// assert_eq!(cornflower_blue.fadein(percent(50)), rgb(100, 149, 237));
+    /// assert_eq!(cornflower_blue.fadein(percent(75)), rgba(100, 149, 237, 1.0));
     /// ```
-    fn fadein(self, amount: Ratio) -> Self;
+    fn fadein(self, amount: Ratio) -> Self::Alpha;
 
     /// Increases the transparency (or decrease the opacity) of `self`, making it less opaque.
-    /// Has no effect on opaque (non-alpha) colors.
+    /// For opqaue colors, converts into the alpha equivalent of `self`, and then decreases the opacity.
     /// For more, see Less' [Color Operations](http://lesscss.org/functions/#color-operations-fadeout).
     ///
     /// # Examples
@@ -179,11 +179,11 @@ pub trait Color {
     /// let cornflower_blue = rgb(100, 149, 237);
     ///
     /// assert_eq!(tomato.fadeout(percent(25)), rgba(255, 99, 71, 0.25));
-    /// assert_eq!(cornflower_blue.fadeout(percent(50)), rgb(100, 149, 237));
+    /// assert_eq!(cornflower_blue.fadeout(percent(75)), rgba(100, 149, 237, 0.25));
     /// ```
-    fn fadeout(self, amount: Ratio) -> Self;
+    fn fadeout(self, amount: Ratio) -> Self::Alpha;
 
-    /// Sets the absolute opacity of `self`.
+    /// Sets the absolute opacity of `self`, and returns the alpha equivalent.
     /// Can be applied to colors whether they already have an opacity value or not.
     /// For more, see Less' [Color Operations](http://lesscss.org/functions/#color-operations-fade).
     ///
@@ -633,12 +633,12 @@ mod css_color_tests {
 
     #[test]
     fn can_fadein() {
-        assert_approximately_eq!(hsl(9, 35, 50).fadein(percent(25)), hsl(9, 35, 50));
+        assert_approximately_eq!(hsl(9, 35, 50).fadein(percent(25)), hsla(9, 35, 50, 1.0));
         assert_approximately_eq!(
             hsla(9, 35, 50, 0.5).fadein(percent(25)),
             hsla(9, 35, 50, 0.75)
         );
-        assert_approximately_eq!(rgb(172, 96, 83).fadein(percent(25)), rgb(172, 96, 83));
+        assert_approximately_eq!(rgb(172, 96, 83).fadein(percent(25)), rgba(172, 96, 83, 1.0));
         assert_approximately_eq!(
             rgba(172, 96, 83, 0.50).fadein(percent(25)),
             rgba(172, 96, 83, 0.75)
@@ -647,8 +647,11 @@ mod css_color_tests {
 
     #[test]
     fn can_fadeout() {
-        assert_approximately_eq!(hsl(9, 35, 50).fadeout(percent(25)), hsl(9, 35, 50));
-        assert_approximately_eq!(rgb(172, 96, 83).fadeout(percent(25)), rgb(172, 96, 83));
+        assert_approximately_eq!(hsl(9, 35, 50).fadeout(percent(25)), hsla(9, 35, 50, 0.75));
+        assert_approximately_eq!(
+            rgb(172, 96, 83).fadeout(percent(25)),
+            rgba(172, 96, 83, 0.75)
+        );
         assert_approximately_eq!(
             hsla(9, 35, 50, 0.60).fadeout(percent(25)),
             hsla(9, 35, 50, 0.35)

src/rgb.rs

@@ -124,12 +124,12 @@ impl Color for RGB {
         self.to_rgba().darken(amount).to_rgb()
     }
 
-    fn fadein(self, _amount: Ratio) -> Self {
-        self
+    fn fadein(self, amount: Ratio) -> RGBA {
+        self.to_rgba().fadein(amount)
     }
 
-    fn fadeout(self, _amount: Ratio) -> Self {
-        self
+    fn fadeout(self, amount: Ratio) -> RGBA {
+        self.to_rgba().fadeout(amount)
     }
 
     fn fade(self, amount: Ratio) -> RGBA {