Auto merge of #17774 - servo:derive-all-the-things, r=emilio

Prepare some code for future derivation 🌊

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components/style/properties/helpers.mako.rs

@@ -129,6 +129,8 @@
 
             % if animation_value_type == "ComputedValue":
                 use properties::animated_properties::Animatable;
+                use values::animated::ToAnimatedZero;
+
                 impl Animatable for T {
                     fn add_weighted(&self, other: &Self, self_portion: f64, other_portion: f64)
                         -> Result<Self, ()> {
@@ -149,6 +151,11 @@
                         self.0.compute_squared_distance(&other.0)
                     }
                 }
+
+                impl ToAnimatedZero for T {
+                    #[inline]
+                    fn to_animated_zero(&self) -> Result<Self, ()> { Err(()) }
+                }
             % endif
 
             pub type Iter<'a, 'cx, 'cx_a> = ComputedVecIter<'a, 'cx, 'cx_a, super::single_value::SpecifiedValue>;