adding const fn with_a8() to Color

piet/src/color.rs

@@ -187,8 +187,36 @@ impl Color {
         Color::from_rgba32_u32((self.as_rgba_u32() & !0xff) | a)
     }
 
+    /// Change just the red value of a color.
+    ///
+    /// The `r` value represents red as a `u8` from 0 to 255.
+    pub const fn with_r8(self, r: u8) -> Color {
+        Color::from_rgba32_u32((self.as_rgba_u32() & !0xff000000) | (r as u32) << 24)
+    }
+
+    /// Change just the green value of a color.
+    ///
+    /// The `g` value represents green as a `u8` from 0 to 255.
+    pub const fn with_g8(self, g: u8) -> Color {
+        Color::from_rgba32_u32((self.as_rgba_u32() & !0xff0000) | (g as u32) << 16)
+    }
+
+    /// Change just the blue value of a color.
+    ///
+    /// The `b` value represents blue as a `u8` from 0 to 255.
+    pub const fn with_b8(self, b: u8) -> Color {
+        Color::from_rgba32_u32((self.as_rgba_u32() & !0xff00) | (b as u32) << 8)
+    }
+
+    /// Change just the alpha value of a color.
+    ///
+    /// The `a` value represents alpha as a `u8` from 0 to 255.
+    pub const fn with_a8(self, a: u8) -> Color {
+        Color::from_rgba32_u32((self.as_rgba_u32() & !0xff) | a as u32)
+    }
+
     /// Convert a color value to a 32-bit rgba value.
-    pub fn as_rgba_u32(self) -> u32 {
+    pub const fn as_rgba_u32(self) -> u32 {
         match self {
             Color::Rgba32(rgba) => rgba,
         }
@@ -355,4 +383,14 @@ mod tests {
         assert!(Color::from_hex_str("x0f").is_err());
         assert!(Color::from_hex_str("#0afa1").is_err());
     }
+
+    #[test]
+    fn change_subcolor_values() {
+        let color = Color::from_rgba32_u32(0x11aa22bb);
+
+        assert_eq!(color.with_r8(0xff), Color::from_rgba32_u32(0xffaa22bb));
+        assert_eq!(color.with_g8(0xff), Color::from_rgba32_u32(0x11ff22bb));
+        assert_eq!(color.with_b8(0xff), Color::from_rgba32_u32(0x11aaffbb));
+        assert_eq!(color.with_a8(0xff), Color::from_rgba32_u32(0x11aa22ff));
+    }
 }