Auto merge of #16278 - hiikezoe:make-letter-spacing-animatable, r=emilio

Make letter-spacing animatable.

This is a PR of https://bugzilla.mozilla.org/show_bug.cgi?id=1353921

From the spec: 'normal' value computes to zero.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

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- [X] There are tests for these changes in web-platform-test (web-animations/animation-model/animation-types/interpolation-per-property.html)

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components/style/properties/gecko.mako.rs

@@ -2882,6 +2882,15 @@ fn static_assert() {
         }
     }
 
+    pub fn clone_letter_spacing(&self) -> longhands::letter_spacing::computed_value::T {
+        use properties::longhands::letter_spacing::computed_value::T;
+        match self.gecko.mLetterSpacing.as_value() {
+            CoordDataValue::Normal => T(None),
+            CoordDataValue::Coord(coord) => T(Some(Au(coord))),
+            _ => unreachable!("Unexpected computed value for letter-spacing"),
+        }
+    }
+
     <%call expr="impl_coord_copy('letter_spacing', 'mLetterSpacing')"></%call>
 
     pub fn set_word_spacing(&mut self, v: longhands::word_spacing::computed_value::T) {

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

@@ -404,8 +404,7 @@ ${helpers.single_keyword("text-align-last",
     % endif
 </%helpers:longhand>
 
-// FIXME: This prop should be animatable.
-<%helpers:longhand name="letter-spacing" boxed="True" animatable="False"
+<%helpers:longhand name="letter-spacing" boxed="True" animatable="True"
                    spec="https://drafts.csswg.org/css-text/#propdef-letter-spacing">
     use std::fmt;
     use style_traits::ToCss;
@@ -438,9 +437,32 @@ ${helpers.single_keyword("text-align-last",
 
     pub mod computed_value {
         use app_units::Au;
+        use properties::animated_properties::Interpolate;
+
         #[derive(Debug, Clone, PartialEq)]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
         pub struct T(pub Option<Au>);
+
+        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(&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 {