dpi: derive common traits for PixelUnit

Derive `Debug`, `Copy`, `Clone`	, `PartialEq`, `Serialize`,
`Deserialize` for `PixelUnit`, which follows other types in dpi crate.

dpi/CHANGELOG.md

@@ -11,6 +11,8 @@ Unreleased` header.
 
 # Unreleased
 
+- Derive `Debug`, `Copy`, `Clone`, `PartialEq`, `Serialize`, `Deserialize` traits for `PixelUnit`.
+
 # 0.1.0
 
 - Add `LogicalUnit`, `PhysicalUnit` and `PixelUnit` types and related functions.

dpi/src/lib.rs

@@ -314,6 +314,8 @@ impl<P: Pixel> From<PhysicalUnit<P>> for f64 {
 }
 
 /// A pixel unit that's either physical or logical.
+#[derive(Debug, Copy, Clone, PartialEq)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub enum PixelUnit {
     Physical(PhysicalUnit<i32>),
     Logical(LogicalUnit<f64>),
@@ -1205,4 +1207,38 @@ mod tests {
         let _ = format!("{:?}", Size::Physical((1, 2).into()).clone());
         let _ = format!("{:?}", Position::Physical((1, 2).into()).clone());
     }
+
+    #[test]
+    fn ensure_copy_trait() {
+        fn is_copy<T: Copy>() {}
+
+        is_copy::<LogicalUnit<i32>>();
+        is_copy::<PhysicalUnit<f64>>();
+        is_copy::<PixelUnit>();
+
+        is_copy::<LogicalSize<i32>>();
+        is_copy::<PhysicalSize<f64>>();
+        is_copy::<Size>();
+
+        is_copy::<LogicalPosition<i32>>();
+        is_copy::<PhysicalPosition<f64>>();
+        is_copy::<Position>();
+    }
+
+    #[test]
+    fn ensure_partial_eq_trait() {
+        fn is_partial_eq<T: PartialEq>() {}
+
+        is_partial_eq::<LogicalUnit<i32>>();
+        is_partial_eq::<PhysicalUnit<f64>>();
+        is_partial_eq::<PixelUnit>();
+
+        is_partial_eq::<LogicalSize<i32>>();
+        is_partial_eq::<PhysicalSize<f64>>();
+        is_partial_eq::<Size>();
+
+        is_partial_eq::<LogicalPosition<i32>>();
+        is_partial_eq::<PhysicalPosition<f64>>();
+        is_partial_eq::<Position>();
+    }
 }