Introduce a macro to define Interpolate trait for tuple struct which has Option<T>.

The computed value of word-spacing is T(pub Option<LengthOrPercentage>),
the computed value of letter-spacing is similar to this, T(pub Option<Au>).
It would be nice to have re-usable macro for these kind of struct.

components/style/properties/helpers.mako.rs

@@ -736,3 +736,26 @@
     %>
 </%def>
 
+/// Macro for defining Interpolate trait for tuple struct which has Option<T>,
+/// e.g. struct T(pub Option<Au>).
+<%def name="impl_interpolate_for_option_tuple(value_for_none)">
+    impl Interpolate for T {
+        #[inline]
+        fn interpolate(&self, other: &Self, progress: f64) -> Result<Self, ()> {
+            match (self, other) {
+                (&T(Some(ref this)), &T(Some(ref other))) => {
+                    Ok(T(this.interpolate(other, progress).ok()))
+                },
+                (&T(Some(ref this)), &T(None)) => {
+                    Ok(T(this.interpolate(&${value_for_none}, progress).ok()))
+                },
+                (&T(None), &T(Some(ref other))) => {
+                    Ok(T(${value_for_none}.interpolate(other, progress).ok()))
+                },
+                (&T(None), &T(None)) => {
+                    Ok(T(None))
+                },
+            }
+        }
+    }
+</%def>

components/style/properties/longhand/inherited_text.mako.rs

@@ -445,26 +445,7 @@ ${helpers.single_keyword("text-align-last",
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
         pub struct T(pub Option<Au>);
 
-        impl Interpolate for T {
+        ${helpers.impl_interpolate_for_option_tuple('Au(0)')}
-            #[inline]
-            fn interpolate(&self, other: &Self, progress: f64) -> Result<Self, ()> {
-                match (self, other) {
-                    (&T(Some(ref this)), &T(Some(ref other))) => {
-                        Ok(T(this.interpolate(other, progress).ok()))
-                    },
-                    (&T(Some(ref this)), &T(None)) => {
-                        Ok(T(this.interpolate(&Au(0), progress).ok()))
-                    },
-                    (&T(None), &T(Some(ref other))) => {
-                        Ok(T(Au(0).interpolate(other, progress).ok()))
-                    },
-                    (&T(None), &T(None)) => {
-                        Ok(T(None))
-                    },
-                }
-            }
-        }
-
     }
 
     impl ToCss for computed_value::T {