style: Drop fallback attribute from animate and distance.

Still keep the discriminant checks to avoid generating terrible code.

Differential Revision: https://phabricator.services.mozilla.com/D62329

components/style/values/animated/mod.rs

@@ -109,9 +109,6 @@ pub fn animate_multiplicative_factor(
 /// If a variant is annotated with `#[animation(error)]`, the corresponding
 /// `match` arm returns an error.
 ///
-/// If the two values are not similar, an error is returned unless a fallback
-/// function has been specified through `#[animate(fallback)]`.
-///
 /// Trait bounds for type parameter `Foo` can be opted out of with
 /// `#[animation(no_bound(Foo))]` on the type definition, trait bounds for
 /// fields can be opted into with `#[animation(field_bound)]` on the field.

components/style/values/distance.rs

@@ -19,9 +19,6 @@ use std::ops::Add;
 /// If a variant is annotated with `#[animation(error)]`, the corresponding
 /// `match` arm returns an error.
 ///
-/// If the two values are not similar, an error is returned unless a fallback
-/// function has been specified through `#[distance(fallback)]`.
-///
 /// Trait bounds for type parameter `Foo` can be opted out of with
 /// `#[animation(no_bound(Foo))]` on the type definition, trait bounds for
 /// fields can be opted into with `#[distance(field_bound)]` on the field.

components/style_derive/animate.rs

@@ -6,12 +6,11 @@ use darling::util::PathList;
 use derive_common::cg;
 use proc_macro2::TokenStream;
 use quote::TokenStreamExt;
-use syn::{DeriveInput, Path, WhereClause};
+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 input_attrs = cg::parse_input_attrs::<AnimateInputAttrs>(&input);
 
     let no_bound = animation_input_attrs.no_bound.unwrap_or_default();
     let mut where_clause = input.generics.where_clause.take();
@@ -44,11 +43,6 @@ pub fn derive(mut input: DeriveInput) -> TokenStream {
     let name = &input.ident;
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
 
-    let fallback = match input_attrs.fallback {
-        Some(fallback) => quote! { #fallback(self, other, procedure) },
-        None => quote! { Err(()) },
-    };
-
     quote! {
         impl #impl_generics crate::values::animated::Animate for #name #ty_generics #where_clause {
             #[allow(unused_variables, unused_imports)]
@@ -59,7 +53,7 @@ pub fn derive(mut input: DeriveInput) -> TokenStream {
                 procedure: crate::values::animated::Procedure,
             ) -> Result<Self, ()> {
                 if std::mem::discriminant(self) != std::mem::discriminant(other) {
-                    return #fallback;
+                    return Err(());
                 }
                 match (self, other) {
                     #match_body
@@ -118,12 +112,6 @@ fn derive_variant_arm(
     }
 }
 
-#[darling(attributes(animate), default)]
-#[derive(Default, FromDeriveInput)]
-struct AnimateInputAttrs {
-    fallback: Option<Path>,
-}
-
 #[darling(attributes(animation), default)]
 #[derive(Default, FromDeriveInput)]
 pub struct AnimationInputAttrs {

components/style_derive/compute_squared_distance.rs

@@ -6,12 +6,11 @@ use crate::animate::{AnimationFieldAttrs, AnimationInputAttrs, AnimationVariantA
 use derive_common::cg;
 use proc_macro2::TokenStream;
 use quote::TokenStreamExt;
-use syn::{DeriveInput, Path, WhereClause};
+use syn::{DeriveInput, WhereClause};
 use synstructure;
 
 pub fn derive(mut input: DeriveInput) -> TokenStream {
     let animation_input_attrs = cg::parse_input_attrs::<AnimationInputAttrs>(&input);
-    let input_attrs = cg::parse_input_attrs::<DistanceInputAttrs>(&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() {
@@ -43,11 +42,6 @@ pub fn derive(mut input: DeriveInput) -> TokenStream {
         match_body.append_all(quote! { _ => unsafe { debug_unreachable!() } });
     }
 
-    let fallback = match input_attrs.fallback {
-        Some(fallback) => quote! { #fallback(self, other) },
-        None => quote! { Err(()) },
-    };
-
     let name = &input.ident;
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
 
@@ -60,7 +54,7 @@ pub fn derive(mut input: DeriveInput) -> TokenStream {
                 other: &Self,
             ) -> Result<crate::values::distance::SquaredDistance, ()> {
                 if std::mem::discriminant(self) != std::mem::discriminant(other) {
-                    return #fallback;
+                    return Err(());
                 }
                 match (self, other) {
                     #match_body
@@ -124,12 +118,6 @@ fn derive_variant_arm(
     };
 }
 
-#[darling(attributes(distance), default)]
-#[derive(Default, FromDeriveInput)]
-struct DistanceInputAttrs {
-    fallback: Option<Path>,
-}
-
 #[darling(attributes(distance), default)]
 #[derive(Default, FromField)]
 struct DistanceFieldAttrs {