Introduce #[animation(constant)] for the Animate trait

This allows us to handle fields that should be the same during animations.

components/style/values/animated/effects.rs

@@ -139,21 +139,6 @@ impl ToAnimatedValue for ComputedTextShadowList {
     }
 }
 
-// FIXME(nox): This could be derived if we implement Animate for bool.
-impl Animate for BoxShadow {
-    #[inline]
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        if self.inset != other.inset {
-            return Err(());
-        }
-        Ok(BoxShadow {
-            base: self.base.animate(&other.base, procedure)?,
-            spread: self.spread.animate(&other.spread, procedure)?,
-            inset: self.inset,
-        })
-    }
-}
-
 impl ComputeSquaredDistance for BoxShadow {
     #[inline]
     fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {

components/style/values/generics/basic_shape.rs

@@ -44,11 +44,18 @@ add_impls_for_keyword_enum!(ShapeBox);
 /// A shape source, for some reference box.
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, PartialEq, ToComputedValue, ToCss)]
+#[derive(Animate, Clone, Debug, PartialEq, ToComputedValue, ToCss)]
 pub enum ShapeSource<BasicShape, ReferenceBox, Url> {
+    #[animation(error)]
     Url(Url),
-    Shape(BasicShape, Option<ReferenceBox>),
+    Shape(
+        BasicShape,
+        #[animation(constant)]
+        Option<ReferenceBox>,
+    ),
+    #[animation(error)]
     Box(ReferenceBox),
+    #[animation(error)]
     None,
 }
 
@@ -126,29 +133,6 @@ define_css_keyword_enum!(FillRule:
 );
 add_impls_for_keyword_enum!(FillRule);
 
-// FIXME(nox): This should be derivable, but we need to implement Animate
-// on the T types.
-impl<B, T, U> Animate for ShapeSource<B, T, U>
-where
-    B: Animate,
-    T: Clone + PartialEq,
-{
-    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        match (self, other) {
-            (
-                &ShapeSource::Shape(ref this, ref this_box),
-                &ShapeSource::Shape(ref other, ref other_box),
-            ) if this_box == other_box => {
-                Ok(ShapeSource::Shape(
-                    this.animate(other, procedure)?,
-                    this_box.clone(),
-                ))
-            },
-            _ => Err(()),
-        }
-    }
-}
-
 // FIXME(nox): Implement ComputeSquaredDistance for T types and stop
 // using PartialEq here, this will let us derive this impl.
 impl<B, T, U> ComputeSquaredDistance for ShapeSource<B, T, U>

components/style/values/generics/effects.rs

@@ -11,13 +11,14 @@ use values::specified::url::SpecifiedUrl;
 
 /// A generic value for a single `box-shadow`.
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToAnimatedValue)]
+#[derive(Animate, Clone, Debug, HasViewportPercentage, PartialEq, ToAnimatedValue)]
 pub struct BoxShadow<Color, SizeLength, BlurShapeLength, ShapeLength> {
     /// The base shadow.
     pub base: SimpleShadow<Color, SizeLength, BlurShapeLength>,
     /// The spread radius.
     pub spread: ShapeLength,
     /// Whether this is an inset box shadow.
+    #[animation(constant)]
     pub inset: bool,
 }
 

components/style_derive/animate.rs

@@ -28,9 +28,30 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
         let mut computations = quote!();
         let iter = result_info.iter().zip(this_info.iter().zip(&other_info));
         computations.append_all(iter.map(|(result, (this, other))| {
-            where_clause.add_trait_bound(this.field.ty.clone());
-            quote! {
-                let #result = ::values::animated::Animate::animate(#this, #other, procedure)?;
+            let field_attrs = cg::parse_field_attrs::<AnimateFieldAttrs>(&result.field);
+            if field_attrs.constant {
+                if cg::is_parameterized(&result.field.ty, where_clause.params) {
+                    where_clause.inner.predicates.push(cg::where_predicate(
+                        result.field.ty.clone(),
+                        &["std", "cmp", "PartialEq"],
+                    ));
+                    where_clause.inner.predicates.push(cg::where_predicate(
+                        result.field.ty.clone(),
+                        &["std", "clone", "Clone"],
+                    ));
+                }
+                quote! {
+                    if #this != #other {
+                        return Err(());
+                    }
+                    let #result = ::std::clone::Clone::clone(#this);
+                }
+            } else {
+                where_clause.add_trait_bound(result.field.ty.clone());
+                quote! {
+                    let #result =
+                        ::values::animated::Animate::animate(#this, #other, procedure)?;
+                }
             }
         }));
         Some(quote! {
@@ -67,3 +88,9 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
 pub struct AnimateAttrs {
     pub error: bool,
 }
+
+#[derive(Default, FromField)]
+#[darling(attributes(animation), default)]
+pub struct AnimateFieldAttrs {
+    pub constant: bool,
+}

components/style_derive/cg.rs

@@ -2,12 +2,12 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use darling::FromVariant;
+use darling::{FromField, FromVariant};
 use quote::{ToTokens, Tokens};
 use std::borrow::Cow;
 use std::collections::HashSet;
 use std::iter;
-use syn::{self, AngleBracketedParameterData, Body, DeriveInput, Ident};
+use syn::{self, AngleBracketedParameterData, Body, DeriveInput, Field, Ident};
 use syn::{ImplGenerics, Path, PathParameters, PathSegment, PolyTraitRef};
 use syn::{QSelf, TraitBoundModifier, Ty, TyGenerics, TyParam, TyParamBound};
 use syn::{Variant, WhereBoundPredicate, WherePredicate};
@@ -15,15 +15,15 @@ use syn::visit::{self, Visitor};
 use synstructure::{self, BindOpts, BindStyle, BindingInfo};
 
 pub struct WhereClause<'input, 'path> {
-    clause: syn::WhereClause,
-    params: &'input [TyParam],
+    pub inner: syn::WhereClause,
+    pub params: &'input [TyParam],
     trait_path: &'path [&'path str],
     bounded_types: HashSet<Ty>,
 }
 
 impl<'input, 'path> ToTokens for WhereClause<'input, 'path> {
     fn to_tokens(&self, tokens: &mut Tokens) {
-        self.clause.to_tokens(tokens);
+        self.inner.to_tokens(tokens);
     }
 }
 
@@ -31,7 +31,7 @@ impl<'input, 'path> WhereClause<'input, 'path> {
     pub fn add_trait_bound(&mut self, ty: Ty) {
         if is_parameterized(&ty, self.params) && !self.bounded_types.contains(&ty) {
             self.bounded_types.insert(ty.clone());
-            self.clause.predicates.push(where_predicate(ty, self.trait_path));
+            self.inner.predicates.push(where_predicate(ty, self.trait_path));
         }
     }
 }
@@ -116,13 +116,23 @@ where
     }
 }
 
+pub fn parse_field_attrs<A>(field: &Field) -> A
+where
+    A: FromField,
+{
+    match A::from_field(field) {
+        Ok(attrs) => attrs,
+        Err(e) => panic!("failed to parse field attributes: {}", e),
+    }
+}
+
 pub fn parse_variant_attrs<A>(variant: &Variant) -> A
 where
     A: FromVariant,
 {
     match A::from_variant(variant) {
         Ok(attrs) => attrs,
-        Err(e) => panic!("failed to parse attributes: {}", e),
+        Err(e) => panic!("failed to parse variant attributes: {}", e),
     }
 }
 
@@ -144,7 +154,7 @@ pub fn trait_parts<'input, 'path>(
 ) -> (ImplGenerics<'input>, TyGenerics<'input>, WhereClause<'input, 'path>) {
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
     let where_clause = WhereClause {
-        clause: where_clause.clone(),
+        inner: where_clause.clone(),
         params: &input.generics.ty_params,
         trait_path,
         bounded_types: HashSet::new()