1 files changed, 135 insertions, 0 deletions
diff --git a/servo/components/style_derive/animate.rs b/servo/components/style_derive/animate.rs
new file mode 100644
index 0000000000..9549100ad0
--- /dev/null
+++ b/servo/components/style_derive/animate.rs
@@ -0,0 +1,135 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use darling::util::PathList;
+use derive_common::cg;
+use proc_macro2::TokenStream;
+use quote::TokenStreamExt;
+use syn::{DeriveInput, WhereClause};
+use synstructure::{Structure, VariantInfo};
+
+pub fn derive(mut input: DeriveInput) -> TokenStream {
+    let animation_input_attrs = cg::parse_input_attrs::<AnimationInputAttrs>(&input);
+
+    let no_bound = animation_input_attrs.no_bound.unwrap_or_default();
+    let mut where_clause = input.generics.where_clause.take();
+    for param in input.generics.type_params() {
+        if !no_bound.iter().any(|name| name.is_ident(&param.ident)) {
+            cg::add_predicate(
+                &mut where_clause,
+                parse_quote!(#param: crate::values::animated::Animate),
+            );
+        }
+    }
+    let (mut match_body, needs_catchall_branch) = {
+        let s = Structure::new(&input);
+        let needs_catchall_branch = s.variants().len() > 1;
+        let match_body = s.variants().iter().fold(quote!(), |body, variant| {
+            let arm = derive_variant_arm(variant, &mut where_clause);
+            quote! { #body #arm }
+        });
+        (match_body, needs_catchall_branch)
+    };
+
+    input.generics.where_clause = where_clause;
+
+    if needs_catchall_branch {
+        // This ideally shouldn't be needed, but see
+        // https://github.com/rust-lang/rust/issues/68867
+        match_body.append_all(quote! { _ => unsafe { debug_unreachable!() } });
+    }
+
+    let name = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    quote! {
+        impl #impl_generics crate::values::animated::Animate for #name #ty_generics #where_clause {
+            #[allow(unused_variables, unused_imports)]
+            #[inline]
+            fn animate(
+                &self,
+                other: &Self,
+                procedure: crate::values::animated::Procedure,
+            ) -> Result<Self, ()> {
+                if std::mem::discriminant(self) != std::mem::discriminant(other) {
+                    return Err(());
+                }
+                match (self, other) {
+                    #match_body
+                }
+            }
+        }
+    }
+}
+
+fn derive_variant_arm(
+    variant: &VariantInfo,
+    where_clause: &mut Option<WhereClause>,
+) -> TokenStream {
+    let variant_attrs = cg::parse_variant_attrs_from_ast::<AnimationVariantAttrs>(&variant.ast());
+    let (this_pattern, this_info) = cg::ref_pattern(&variant, "this");
+    let (other_pattern, other_info) = cg::ref_pattern(&variant, "other");
+
+    if variant_attrs.error {
+        return quote! {
+            (&#this_pattern, &#other_pattern) => Err(()),
+        };
+    }
+
+    let (result_value, result_info) = cg::value(&variant, "result");
+    let mut computations = quote!();
+    let iter = result_info.iter().zip(this_info.iter().zip(&other_info));
+    computations.append_all(iter.map(|(result, (this, other))| {
+        let field_attrs = cg::parse_field_attrs::<AnimationFieldAttrs>(&result.ast());
+        if field_attrs.field_bound {
+            let ty = &this.ast().ty;
+            cg::add_predicate(
+                where_clause,
+                parse_quote!(#ty: crate::values::animated::Animate),
+            );
+        }
+        if field_attrs.constant {
+            quote! {
+                if #this != #other {
+                    return Err(());
+                }
+                let #result = std::clone::Clone::clone(#this);
+            }
+        } else {
+            quote! {
+                let #result =
+                    crate::values::animated::Animate::animate(#this, #other, procedure)?;
+            }
+        }
+    }));
+
+    quote! {
+        (&#this_pattern, &#other_pattern) => {
+            #computations
+            Ok(#result_value)
+        }
+    }
+}
+
+#[derive(Default, FromDeriveInput)]
+#[darling(attributes(animation), default)]
+pub struct AnimationInputAttrs {
+    pub no_bound: Option<PathList>,
+}
+
+#[derive(Default, FromVariant)]
+#[darling(attributes(animation), default)]
+pub struct AnimationVariantAttrs {
+    pub error: bool,
+    // Only here because of structs, where the struct definition acts as a
+    // variant itself.
+    pub no_bound: Option<PathList>,
+}
+
+#[derive(Default, FromField)]
+#[darling(attributes(animation), default)]
+pub struct AnimationFieldAttrs {
+    pub constant: bool,
+    pub field_bound: bool,
+}