1 files changed, 125 insertions, 0 deletions
diff --git a/servo/components/style_derive/compute_squared_distance.rs b/servo/components/style_derive/compute_squared_distance.rs
new file mode 100644
index 0000000000..022ab115ee
--- /dev/null
+++ b/servo/components/style_derive/compute_squared_distance.rs
@@ -0,0 +1,125 @@
+/* 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 crate::animate::{AnimationFieldAttrs, AnimationInputAttrs, AnimationVariantAttrs};
+use derive_common::cg;
+use proc_macro2::TokenStream;
+use quote::TokenStreamExt;
+use syn::{DeriveInput, WhereClause};
+use synstructure;
+
+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::distance::ComputeSquaredDistance),
+            );
+        }
+    }
+
+    let (mut match_body, needs_catchall_branch) = {
+        let s = synstructure::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::distance::ComputeSquaredDistance for #name #ty_generics #where_clause {
+            #[allow(unused_variables, unused_imports)]
+            #[inline]
+            fn compute_squared_distance(
+                &self,
+                other: &Self,
+            ) -> Result<crate::values::distance::SquaredDistance, ()> {
+                if std::mem::discriminant(self) != std::mem::discriminant(other) {
+                    return Err(());
+                }
+                match (self, other) {
+                    #match_body
+                }
+            }
+        }
+    }
+}
+
+fn derive_variant_arm(
+    variant: &synstructure::VariantInfo,
+    mut 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 sum = if this_info.is_empty() {
+        quote! { crate::values::distance::SquaredDistance::from_sqrt(0.) }
+    } else {
+        let mut sum = quote!();
+        sum.append_separated(this_info.iter().zip(&other_info).map(|(this, other)| {
+            let field_attrs = cg::parse_field_attrs::<DistanceFieldAttrs>(&this.ast());
+            if field_attrs.field_bound {
+                let ty = &this.ast().ty;
+                cg::add_predicate(
+                    &mut where_clause,
+                    parse_quote!(#ty: crate::values::distance::ComputeSquaredDistance),
+                );
+            }
+
+            let animation_field_attrs =
+                cg::parse_field_attrs::<AnimationFieldAttrs>(&this.ast());
+
+            if animation_field_attrs.constant {
+                quote! {
+                    {
+                        if #this != #other {
+                            return Err(());
+                        }
+                        crate::values::distance::SquaredDistance::from_sqrt(0.)
+                    }
+                }
+            } else {
+                quote! {
+                    crate::values::distance::ComputeSquaredDistance::compute_squared_distance(#this, #other)?
+                }
+            }
+        }), quote!(+));
+        sum
+    };
+
+    return quote! {
+        (&#this_pattern, &#other_pattern) => Ok(#sum),
+    };
+}
+
+#[derive(Default, FromField)]
+#[darling(attributes(distance), default)]
+struct DistanceFieldAttrs {
+    field_bound: bool,
+}