Adding upstream version 115.8.0esr.upstream/115.8.0esr

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

11 files changed, 1631 insertions, 0 deletions

diff --git a/servo/components/style_derive/Cargo.toml b/servo/components/style_derive/Cargo.toml
new file mode 100644
index 0000000000..67d7bcad3c
--- /dev/null
+++ b/servo/components/style_derive/Cargo.toml
@@ -0,0 +1,18 @@
+[package]
+name = "style_derive"
+version = "0.0.1"
+authors = ["The Servo Project Developers"]
+license = "MPL-2.0"
+publish = false
+
+[lib]
+path = "lib.rs"
+proc-macro = true
+
+[dependencies]
+darling = { version = "0.14", default-features = false }
+derive_common = { path = "../derive_common" }
+proc-macro2 = "1"
+quote = "1"
+syn = { version = "1", default-features = false, features = ["clone-impls", "derive", "parsing"] }
+synstructure = "0.12"
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,
+}
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,
+}
diff --git a/servo/components/style_derive/lib.rs b/servo/components/style_derive/lib.rs
new file mode 100644
index 0000000000..079db00c5a
--- /dev/null
+++ b/servo/components/style_derive/lib.rs
@@ -0,0 +1,82 @@
+/* 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/. */
+
+#![recursion_limit = "128"]
+
+#[macro_use]
+extern crate darling;
+extern crate derive_common;
+extern crate proc_macro;
+extern crate proc_macro2;
+#[macro_use]
+extern crate quote;
+#[macro_use]
+extern crate syn;
+extern crate synstructure;
+
+use proc_macro::TokenStream;
+
+mod animate;
+mod compute_squared_distance;
+mod parse;
+mod specified_value_info;
+mod to_animated_value;
+mod to_animated_zero;
+mod to_computed_value;
+mod to_css;
+mod to_resolved_value;
+
+#[proc_macro_derive(Animate, attributes(animate, animation))]
+pub fn derive_animate(stream: TokenStream) -> TokenStream {
+    let input = syn::parse(stream).unwrap();
+    animate::derive(input).into()
+}
+
+#[proc_macro_derive(ComputeSquaredDistance, attributes(animation, distance))]
+pub fn derive_compute_squared_distance(stream: TokenStream) -> TokenStream {
+    let input = syn::parse(stream).unwrap();
+    compute_squared_distance::derive(input).into()
+}
+
+#[proc_macro_derive(ToAnimatedValue)]
+pub fn derive_to_animated_value(stream: TokenStream) -> TokenStream {
+    let input = syn::parse(stream).unwrap();
+    to_animated_value::derive(input).into()
+}
+
+#[proc_macro_derive(Parse, attributes(css, parse))]
+pub fn derive_parse(stream: TokenStream) -> TokenStream {
+    let input = syn::parse(stream).unwrap();
+    parse::derive(input).into()
+}
+
+#[proc_macro_derive(ToAnimatedZero, attributes(animation, zero))]
+pub fn derive_to_animated_zero(stream: TokenStream) -> TokenStream {
+    let input = syn::parse(stream).unwrap();
+    to_animated_zero::derive(input).into()
+}
+
+#[proc_macro_derive(ToComputedValue, attributes(compute))]
+pub fn derive_to_computed_value(stream: TokenStream) -> TokenStream {
+    let input = syn::parse(stream).unwrap();
+    to_computed_value::derive(input).into()
+}
+
+#[proc_macro_derive(ToResolvedValue, attributes(resolve))]
+pub fn derive_to_resolved_value(stream: TokenStream) -> TokenStream {
+    let input = syn::parse(stream).unwrap();
+    to_resolved_value::derive(input).into()
+}
+
+#[proc_macro_derive(ToCss, attributes(css))]
+pub fn derive_to_css(stream: TokenStream) -> TokenStream {
+    let input = syn::parse(stream).unwrap();
+    to_css::derive(input).into()
+}
+
+#[proc_macro_derive(SpecifiedValueInfo, attributes(css, parse, value_info))]
+pub fn derive_specified_value_info(stream: TokenStream) -> TokenStream {
+    let input = syn::parse(stream).unwrap();
+    specified_value_info::derive(input).into()
+}
diff --git a/servo/components/style_derive/parse.rs b/servo/components/style_derive/parse.rs
new file mode 100644
index 0000000000..b1a1213435
--- /dev/null
+++ b/servo/components/style_derive/parse.rs
@@ -0,0 +1,323 @@
+/* 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::to_css::{CssBitflagAttrs, CssVariantAttrs};
+use derive_common::cg;
+use proc_macro2::{Span, TokenStream};
+use quote::TokenStreamExt;
+use syn::{self, DeriveInput, Ident, Path};
+use synstructure::{Structure, VariantInfo};
+
+#[derive(Default, FromVariant)]
+#[darling(attributes(parse), default)]
+pub struct ParseVariantAttrs {
+    pub aliases: Option<String>,
+    pub condition: Option<Path>,
+}
+
+#[derive(Default, FromField)]
+#[darling(attributes(parse), default)]
+pub struct ParseFieldAttrs {
+    field_bound: bool,
+}
+
+fn parse_bitflags(bitflags: &CssBitflagAttrs) -> TokenStream {
+    let mut match_arms = TokenStream::new();
+    for (rust_name, css_name) in bitflags.single_flags() {
+        let rust_ident = Ident::new(&rust_name, Span::call_site());
+        match_arms.append_all(quote! {
+            #css_name if result.is_empty() => {
+                single_flag = true;
+                Self::#rust_ident
+            },
+        });
+    }
+
+    for (rust_name, css_name) in bitflags.mixed_flags() {
+        let rust_ident = Ident::new(&rust_name, Span::call_site());
+        match_arms.append_all(quote! {
+            #css_name => Self::#rust_ident,
+        });
+    }
+
+    let mut validate_condition = quote! { !result.is_empty() };
+    if let Some(ref function) = bitflags.validate_mixed {
+        validate_condition.append_all(quote! {
+            && #function(&mut result)
+        });
+    }
+
+    // NOTE(emilio): this loop has this weird structure because we run this code
+    // to parse stuff like text-decoration-line in the text-decoration
+    // shorthand, so we need to be a bit careful that we don't error if we don't
+    // consume the whole thing because we find an invalid identifier or other
+    // kind of token. Instead, we should leave it unconsumed.
+    quote! {
+        let mut result = Self::empty();
+        loop {
+            let mut single_flag = false;
+            let flag: Result<_, style_traits::ParseError<'i>> = input.try_parse(|input| {
+                Ok(try_match_ident_ignore_ascii_case! { input,
+                    #match_arms
+                })
+            });
+
+            let flag = match flag {
+                Ok(flag) => flag,
+                Err(..) => break,
+            };
+
+            if single_flag {
+                return Ok(flag);
+            }
+
+            if result.intersects(flag) {
+                return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
+            }
+
+            result.insert(flag);
+        }
+        if #validate_condition {
+            Ok(result)
+        } else {
+            Err(input.new_custom_error(style_traits::StyleParseErrorKind::UnspecifiedError))
+        }
+    }
+}
+
+fn parse_non_keyword_variant(
+    where_clause: &mut Option<syn::WhereClause>,
+    name: &syn::Ident,
+    variant: &VariantInfo,
+    variant_attrs: &CssVariantAttrs,
+    parse_attrs: &ParseVariantAttrs,
+    skip_try: bool,
+) -> TokenStream {
+    let bindings = variant.bindings();
+    assert!(parse_attrs.aliases.is_none());
+    assert!(variant_attrs.function.is_none());
+    assert!(variant_attrs.keyword.is_none());
+    assert_eq!(
+        bindings.len(),
+        1,
+        "We only support deriving parse for simple variants"
+    );
+    let variant_name = &variant.ast().ident;
+    let binding_ast = &bindings[0].ast();
+    let ty = &binding_ast.ty;
+
+    if let Some(ref bitflags) = variant_attrs.bitflags {
+        assert!(skip_try, "Should be the only variant");
+        assert!(
+            parse_attrs.condition.is_none(),
+            "Should be the only variant"
+        );
+        assert!(where_clause.is_none(), "Generic bitflags?");
+        return parse_bitflags(bitflags);
+    }
+
+    let field_attrs = cg::parse_field_attrs::<ParseFieldAttrs>(binding_ast);
+    if field_attrs.field_bound {
+        cg::add_predicate(where_clause, parse_quote!(#ty: crate::parser::Parse));
+    }
+
+    let mut parse = if skip_try {
+        quote! {
+            let v = <#ty as crate::parser::Parse>::parse(context, input)?;
+            return Ok(#name::#variant_name(v));
+        }
+    } else {
+        quote! {
+            if let Ok(v) = input.try(|i| <#ty as crate::parser::Parse>::parse(context, i)) {
+                return Ok(#name::#variant_name(v));
+            }
+        }
+    };
+
+    if let Some(ref condition) = parse_attrs.condition {
+        parse = quote! {
+            if #condition(context) {
+                #parse
+            }
+        };
+
+        if skip_try {
+            // We're the last variant and we can fail to parse due to the
+            // condition clause. If that happens, we need to return an error.
+            parse = quote! {
+                #parse
+                Err(input.new_custom_error(style_traits::StyleParseErrorKind::UnspecifiedError))
+            };
+        }
+    }
+
+    parse
+}
+
+pub fn derive(mut input: DeriveInput) -> TokenStream {
+    let mut where_clause = input.generics.where_clause.take();
+    for param in input.generics.type_params() {
+        cg::add_predicate(
+            &mut where_clause,
+            parse_quote!(#param: crate::parser::Parse),
+        );
+    }
+
+    let name = &input.ident;
+    let s = Structure::new(&input);
+
+    let mut saw_condition = false;
+    let mut match_keywords = quote! {};
+    let mut non_keywords = vec![];
+
+    let mut effective_variants = 0;
+    for variant in s.variants().iter() {
+        let css_variant_attrs = cg::parse_variant_attrs_from_ast::<CssVariantAttrs>(&variant.ast());
+        if css_variant_attrs.skip {
+            continue;
+        }
+        effective_variants += 1;
+
+        let parse_attrs = cg::parse_variant_attrs_from_ast::<ParseVariantAttrs>(&variant.ast());
+
+        saw_condition |= parse_attrs.condition.is_some();
+
+        if !variant.bindings().is_empty() {
+            non_keywords.push((variant, css_variant_attrs, parse_attrs));
+            continue;
+        }
+
+        let identifier = cg::to_css_identifier(
+            &css_variant_attrs
+                .keyword
+                .unwrap_or_else(|| variant.ast().ident.to_string()),
+        );
+        let ident = &variant.ast().ident;
+
+        let condition = match parse_attrs.condition {
+            Some(ref p) => quote! { if #p(context) },
+            None => quote! {},
+        };
+
+        match_keywords.extend(quote! {
+            #identifier #condition => Ok(#name::#ident),
+        });
+
+        let aliases = match parse_attrs.aliases {
+            Some(aliases) => aliases,
+            None => continue,
+        };
+
+        for alias in aliases.split(',') {
+            match_keywords.extend(quote! {
+                #alias #condition => Ok(#name::#ident),
+            });
+        }
+    }
+
+    let needs_context = saw_condition || !non_keywords.is_empty();
+
+    let context_ident = if needs_context {
+        quote! { context }
+    } else {
+        quote! { _ }
+    };
+
+    let has_keywords = non_keywords.len() != effective_variants;
+
+    let mut parse_non_keywords = quote! {};
+    for (i, (variant, css_attrs, parse_attrs)) in non_keywords.iter().enumerate() {
+        let skip_try = !has_keywords && i == non_keywords.len() - 1;
+        let parse_variant = parse_non_keyword_variant(
+            &mut where_clause,
+            name,
+            variant,
+            css_attrs,
+            parse_attrs,
+            skip_try,
+        );
+        parse_non_keywords.extend(parse_variant);
+    }
+
+    let parse_body = if needs_context {
+        let parse_keywords = if has_keywords {
+            quote! {
+                let location = input.current_source_location();
+                let ident = input.expect_ident()?;
+                match_ignore_ascii_case! { &ident,
+                    #match_keywords
+                    _ => Err(location.new_unexpected_token_error(
+                        cssparser::Token::Ident(ident.clone())
+                    ))
+                }
+            }
+        } else {
+            quote! {}
+        };
+
+        quote! {
+            #parse_non_keywords
+            #parse_keywords
+        }
+    } else {
+        quote! { Self::parse(input) }
+    };
+
+    let has_non_keywords = !non_keywords.is_empty();
+
+    input.generics.where_clause = where_clause;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    let parse_trait_impl = quote! {
+        impl #impl_generics crate::parser::Parse for #name #ty_generics #where_clause {
+            #[inline]
+            fn parse<'i, 't>(
+                #context_ident: &crate::parser::ParserContext,
+                input: &mut cssparser::Parser<'i, 't>,
+            ) -> Result<Self, style_traits::ParseError<'i>> {
+                #parse_body
+            }
+        }
+    };
+
+    if needs_context {
+        return parse_trait_impl;
+    }
+
+    assert!(!has_non_keywords);
+
+    // TODO(emilio): It'd be nice to get rid of these, but that makes the
+    // conversion harder...
+    let methods_impl = quote! {
+        impl #name {
+            /// Parse this keyword.
+            #[inline]
+            pub fn parse<'i, 't>(
+                input: &mut cssparser::Parser<'i, 't>,
+            ) -> Result<Self, style_traits::ParseError<'i>> {
+                let location = input.current_source_location();
+                let ident = input.expect_ident()?;
+                Self::from_ident(ident.as_ref()).map_err(|()| {
+                    location.new_unexpected_token_error(
+                        cssparser::Token::Ident(ident.clone())
+                    )
+                })
+            }
+
+            /// Parse this keyword from a string slice.
+            #[inline]
+            pub fn from_ident(ident: &str) -> Result<Self, ()> {
+                match_ignore_ascii_case! { ident,
+                    #match_keywords
+                    _ => Err(()),
+                }
+            }
+        }
+    };
+
+    quote! {
+        #parse_trait_impl
+        #methods_impl
+    }
+}
diff --git a/servo/components/style_derive/specified_value_info.rs b/servo/components/style_derive/specified_value_info.rs
new file mode 100644
index 0000000000..e29f2bc416
--- /dev/null
+++ b/servo/components/style_derive/specified_value_info.rs
@@ -0,0 +1,195 @@
+/* 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::parse::ParseVariantAttrs;
+use crate::to_css::{CssFieldAttrs, CssInputAttrs, CssVariantAttrs};
+use derive_common::cg;
+use proc_macro2::TokenStream;
+use quote::TokenStreamExt;
+use syn::{Data, DeriveInput, Fields, Ident, Type};
+
+pub fn derive(mut input: DeriveInput) -> TokenStream {
+    let css_attrs = cg::parse_input_attrs::<CssInputAttrs>(&input);
+    let mut types = vec![];
+    let mut values = vec![];
+
+    let input_ident = &input.ident;
+    let input_name = || cg::to_css_identifier(&input_ident.to_string());
+    if let Some(function) = css_attrs.function {
+        values.push(function.explicit().unwrap_or_else(input_name));
+    // If the whole value is wrapped in a function, value types of
+    // its fields should not be propagated.
+    } else {
+        let mut where_clause = input.generics.where_clause.take();
+        for param in input.generics.type_params() {
+            cg::add_predicate(
+                &mut where_clause,
+                parse_quote!(#param: style_traits::SpecifiedValueInfo),
+            );
+        }
+        input.generics.where_clause = where_clause;
+
+        match input.data {
+            Data::Enum(ref e) => {
+                for v in e.variants.iter() {
+                    let css_attrs = cg::parse_variant_attrs::<CssVariantAttrs>(&v);
+                    let info_attrs = cg::parse_variant_attrs::<ValueInfoVariantAttrs>(&v);
+                    let parse_attrs = cg::parse_variant_attrs::<ParseVariantAttrs>(&v);
+                    if css_attrs.skip {
+                        continue;
+                    }
+                    if let Some(aliases) = parse_attrs.aliases {
+                        for alias in aliases.split(',') {
+                            values.push(alias.to_string());
+                        }
+                    }
+                    if let Some(other_values) = info_attrs.other_values {
+                        for value in other_values.split(',') {
+                            values.push(value.to_string());
+                        }
+                    }
+                    let ident = &v.ident;
+                    let variant_name = || cg::to_css_identifier(&ident.to_string());
+                    if info_attrs.starts_with_keyword {
+                        values.push(variant_name());
+                        continue;
+                    }
+                    if let Some(keyword) = css_attrs.keyword {
+                        values.push(keyword);
+                        continue;
+                    }
+                    if let Some(function) = css_attrs.function {
+                        values.push(function.explicit().unwrap_or_else(variant_name));
+                    } else if !derive_struct_fields(&v.fields, &mut types, &mut values) {
+                        values.push(variant_name());
+                    }
+                }
+            },
+            Data::Struct(ref s) => {
+                if let Some(ref bitflags) = css_attrs.bitflags {
+                    for (_rust_name, css_name) in bitflags.single_flags() {
+                        values.push(css_name)
+                    }
+                    for (_rust_name, css_name) in bitflags.mixed_flags() {
+                        values.push(css_name)
+                    }
+                } else if !derive_struct_fields(&s.fields, &mut types, &mut values) {
+                    values.push(input_name());
+                }
+            },
+            Data::Union(_) => unreachable!("union is not supported"),
+        }
+    }
+
+    let info_attrs = cg::parse_input_attrs::<ValueInfoInputAttrs>(&input);
+    if let Some(other_values) = info_attrs.other_values {
+        for value in other_values.split(',') {
+            values.push(value.to_string());
+        }
+    }
+
+    let mut types_value = quote!(0);
+    types_value.append_all(types.iter().map(|ty| {
+        quote! {
+            | <#ty as style_traits::SpecifiedValueInfo>::SUPPORTED_TYPES
+        }
+    }));
+
+    let mut nested_collects = quote!();
+    nested_collects.append_all(types.iter().map(|ty| {
+        quote! {
+            <#ty as style_traits::SpecifiedValueInfo>::collect_completion_keywords(_f);
+        }
+    }));
+
+    if let Some(ty) = info_attrs.ty {
+        types_value.append_all(quote! {
+            | style_traits::CssType::#ty
+        });
+    }
+
+    let append_values = if values.is_empty() {
+        quote!()
+    } else {
+        let mut value_list = quote!();
+        value_list.append_separated(values.iter(), quote! { , });
+        quote! { _f(&[#value_list]); }
+    };
+
+    let name = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    quote! {
+        impl #impl_generics style_traits::SpecifiedValueInfo for #name #ty_generics
+        #where_clause
+        {
+            const SUPPORTED_TYPES: u8 = #types_value;
+
+            fn collect_completion_keywords(_f: &mut FnMut(&[&'static str])) {
+                #nested_collects
+                #append_values
+            }
+        }
+    }
+}
+
+/// Derive from the given fields. Return false if the fields is a Unit,
+/// true otherwise.
+fn derive_struct_fields<'a>(
+    fields: &'a Fields,
+    types: &mut Vec<&'a Type>,
+    values: &mut Vec<String>,
+) -> bool {
+    let fields = match *fields {
+        Fields::Unit => return false,
+        Fields::Named(ref fields) => fields.named.iter(),
+        Fields::Unnamed(ref fields) => fields.unnamed.iter(),
+    };
+    types.extend(fields.filter_map(|field| {
+        let info_attrs = cg::parse_field_attrs::<ValueInfoFieldAttrs>(field);
+        if let Some(other_values) = info_attrs.other_values {
+            for value in other_values.split(',') {
+                values.push(value.to_string());
+            }
+        }
+        let css_attrs = cg::parse_field_attrs::<CssFieldAttrs>(field);
+        if css_attrs.represents_keyword {
+            let ident = field
+                .ident
+                .as_ref()
+                .expect("only named field should use represents_keyword");
+            values.push(cg::to_css_identifier(&ident.to_string()));
+            return None;
+        }
+        if let Some(if_empty) = css_attrs.if_empty {
+            values.push(if_empty);
+        }
+        if !css_attrs.skip {
+            Some(&field.ty)
+        } else {
+            None
+        }
+    }));
+    true
+}
+
+#[derive(Default, FromDeriveInput)]
+#[darling(attributes(value_info), default)]
+struct ValueInfoInputAttrs {
+    ty: Option<Ident>,
+    other_values: Option<String>,
+}
+
+#[derive(Default, FromVariant)]
+#[darling(attributes(value_info), default)]
+struct ValueInfoVariantAttrs {
+    starts_with_keyword: bool,
+    other_values: Option<String>,
+}
+
+#[derive(Default, FromField)]
+#[darling(attributes(value_info), default)]
+struct ValueInfoFieldAttrs {
+    other_values: Option<String>,
+}
diff --git a/servo/components/style_derive/to_animated_value.rs b/servo/components/style_derive/to_animated_value.rs
new file mode 100644
index 0000000000..45282f0c44
--- /dev/null
+++ b/servo/components/style_derive/to_animated_value.rs
@@ -0,0 +1,35 @@
+/* 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 proc_macro2::TokenStream;
+use syn::DeriveInput;
+use synstructure::BindStyle;
+use to_computed_value;
+
+pub fn derive(input: DeriveInput) -> TokenStream {
+    let trait_impl = |from_body, to_body| {
+        quote! {
+             #[inline]
+             fn from_animated_value(from: Self::AnimatedValue) -> Self {
+                 #from_body
+             }
+
+             #[inline]
+             fn to_animated_value(self) -> Self::AnimatedValue {
+                 #to_body
+             }
+        }
+    };
+
+    to_computed_value::derive_to_value(
+        input,
+        parse_quote!(crate::values::animated::ToAnimatedValue),
+        parse_quote!(AnimatedValue),
+        BindStyle::Move,
+        |_| Default::default(),
+        |binding| quote!(crate::values::animated::ToAnimatedValue::from_animated_value(#binding)),
+        |binding| quote!(crate::values::animated::ToAnimatedValue::to_animated_value(#binding)),
+        trait_impl,
+    )
+}
diff --git a/servo/components/style_derive/to_animated_zero.rs b/servo/components/style_derive/to_animated_zero.rs
new file mode 100644
index 0000000000..008e94cbcf
--- /dev/null
+++ b/servo/components/style_derive/to_animated_zero.rs
@@ -0,0 +1,65 @@
+/* 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;
+use synstructure;
+
+pub fn derive(mut input: syn::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::ToAnimatedZero),
+            );
+        }
+    }
+
+    let to_body = synstructure::Structure::new(&input).each_variant(|variant| {
+        let attrs = cg::parse_variant_attrs_from_ast::<AnimationVariantAttrs>(&variant.ast());
+        if attrs.error {
+            return Some(quote! { Err(()) });
+        }
+        let (mapped, mapped_bindings) = cg::value(variant, "mapped");
+        let bindings_pairs = variant.bindings().iter().zip(mapped_bindings);
+        let mut computations = quote!();
+        computations.append_all(bindings_pairs.map(|(binding, mapped_binding)| {
+            let field_attrs = cg::parse_field_attrs::<AnimationFieldAttrs>(&binding.ast());
+            if field_attrs.constant {
+                quote! {
+                    let #mapped_binding = std::clone::Clone::clone(#binding);
+                }
+            } else {
+                quote! {
+                    let #mapped_binding =
+                        crate::values::animated::ToAnimatedZero::to_animated_zero(#binding)?;
+                }
+            }
+        }));
+        computations.append_all(quote! { Ok(#mapped) });
+        Some(computations)
+    });
+    input.generics.where_clause = where_clause;
+
+    let name = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    quote! {
+        impl #impl_generics crate::values::animated::ToAnimatedZero for #name #ty_generics #where_clause {
+            #[allow(unused_variables)]
+            #[inline]
+            fn to_animated_zero(&self) -> Result<Self, ()> {
+                match *self {
+                    #to_body
+                }
+            }
+        }
+    }
+}
diff --git a/servo/components/style_derive/to_computed_value.rs b/servo/components/style_derive/to_computed_value.rs
new file mode 100644
index 0000000000..5e0f595c6b
--- /dev/null
+++ b/servo/components/style_derive/to_computed_value.rs
@@ -0,0 +1,205 @@
+/* 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 derive_common::cg;
+use proc_macro2::TokenStream;
+use syn::{DeriveInput, Ident, Path};
+use synstructure::{BindStyle, BindingInfo};
+
+pub fn derive_to_value(
+    mut input: DeriveInput,
+    trait_path: Path,
+    output_type_name: Ident,
+    bind_style: BindStyle,
+    // Returns whether to apply the field bound for a given item.
+    mut binding_attrs: impl FnMut(&BindingInfo) -> ToValueAttrs,
+    // Returns a token stream of the form: trait_path::from_foo(#binding)
+    mut call_from: impl FnMut(&BindingInfo) -> TokenStream,
+    mut call_to: impl FnMut(&BindingInfo) -> TokenStream,
+    // Returns a tokenstream of the form:
+    // fn from_function_syntax(foobar) -> Baz {
+    //     #first_arg
+    // }
+    //
+    // fn to_function_syntax(foobar) -> Baz {
+    //     #second_arg
+    // }
+    mut trait_impl: impl FnMut(TokenStream, TokenStream) -> TokenStream,
+) -> TokenStream {
+    let name = &input.ident;
+
+    let mut where_clause = input.generics.where_clause.take();
+    cg::propagate_clauses_to_output_type(
+        &mut where_clause,
+        &input.generics,
+        &trait_path,
+        &output_type_name,
+    );
+
+    let moves = match bind_style {
+        BindStyle::Move | BindStyle::MoveMut => true,
+        BindStyle::Ref | BindStyle::RefMut => false,
+    };
+
+    let params = input.generics.type_params().collect::<Vec<_>>();
+    for param in &params {
+        cg::add_predicate(&mut where_clause, parse_quote!(#param: #trait_path));
+    }
+
+    let computed_value_type = cg::fmap_trait_output(&input, &trait_path, &output_type_name);
+
+    let mut add_field_bound = |binding: &BindingInfo| {
+        let ty = &binding.ast().ty;
+
+        let output_type = cg::map_type_params(
+            ty,
+            &params,
+            &computed_value_type,
+            &mut |ident| parse_quote!(<#ident as #trait_path>::#output_type_name),
+        );
+
+        cg::add_predicate(
+            &mut where_clause,
+            parse_quote!(
+                #ty: #trait_path<#output_type_name = #output_type>
+            ),
+        );
+    };
+
+    let (to_body, from_body) = if params.is_empty() {
+        let mut s = synstructure::Structure::new(&input);
+        s.variants_mut().iter_mut().for_each(|v| {
+            v.bind_with(|_| bind_style);
+        });
+
+        for variant in s.variants() {
+            for binding in variant.bindings() {
+                let attrs = binding_attrs(&binding);
+                assert!(
+                    !attrs.field_bound,
+                    "It is default on a non-generic implementation",
+                );
+                if !attrs.no_field_bound {
+                    // Add field bounds to all bindings except the manually
+                    // excluded. This ensures the correctness of the clone() /
+                    // move based implementation.
+                    add_field_bound(binding);
+                }
+            }
+        }
+
+        let to_body = if moves {
+            quote! { self }
+        } else {
+            quote! { std::clone::Clone::clone(self) }
+        };
+
+        let from_body = if moves {
+            quote! { from }
+        } else {
+            quote! { std::clone::Clone::clone(from) }
+        };
+
+        (to_body, from_body)
+    } else {
+        let to_body = cg::fmap_match(&input, bind_style, |binding| {
+            let attrs = binding_attrs(&binding);
+            assert!(
+                !attrs.no_field_bound,
+                "It doesn't make sense on a generic implementation"
+            );
+            if attrs.field_bound {
+                add_field_bound(&binding);
+            }
+            call_to(&binding)
+        });
+
+        let from_body = cg::fmap_match(&input, bind_style, |binding| call_from(&binding));
+
+        let self_ = if moves {
+            quote! { self }
+        } else {
+            quote! { *self }
+        };
+        let from_ = if moves {
+            quote! { from }
+        } else {
+            quote! { *from }
+        };
+
+        let to_body = quote! {
+            match #self_ {
+                #to_body
+            }
+        };
+
+        let from_body = quote! {
+            match #from_ {
+                #from_body
+            }
+        };
+
+        (to_body, from_body)
+    };
+
+    input.generics.where_clause = where_clause;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    let impl_ = trait_impl(from_body, to_body);
+
+    quote! {
+        impl #impl_generics #trait_path for #name #ty_generics #where_clause {
+            type #output_type_name = #computed_value_type;
+
+            #impl_
+        }
+    }
+}
+
+pub fn derive(input: DeriveInput) -> TokenStream {
+    let trait_impl = |from_body, to_body| {
+        quote! {
+             #[inline]
+             fn from_computed_value(from: &Self::ComputedValue) -> Self {
+                 #from_body
+             }
+
+             #[allow(unused_variables)]
+             #[inline]
+             fn to_computed_value(&self, context: &crate::values::computed::Context) -> Self::ComputedValue {
+                 #to_body
+             }
+        }
+    };
+
+    derive_to_value(
+        input,
+        parse_quote!(crate::values::computed::ToComputedValue),
+        parse_quote!(ComputedValue),
+        BindStyle::Ref,
+        |binding| {
+            let attrs = cg::parse_field_attrs::<ComputedValueAttrs>(&binding.ast());
+            ToValueAttrs {
+                field_bound: attrs.field_bound,
+                no_field_bound: attrs.no_field_bound,
+            }
+        },
+        |binding| quote!(crate::values::computed::ToComputedValue::from_computed_value(#binding)),
+        |binding| quote!(crate::values::computed::ToComputedValue::to_computed_value(#binding, context)),
+        trait_impl,
+    )
+}
+
+#[derive(Default)]
+pub struct ToValueAttrs {
+    pub field_bound: bool,
+    pub no_field_bound: bool,
+}
+
+#[derive(Default, FromField)]
+#[darling(attributes(compute), default)]
+struct ComputedValueAttrs {
+    field_bound: bool,
+    no_field_bound: bool,
+}
diff --git a/servo/components/style_derive/to_css.rs b/servo/components/style_derive/to_css.rs
new file mode 100644
index 0000000000..aa33536648
--- /dev/null
+++ b/servo/components/style_derive/to_css.rs
@@ -0,0 +1,396 @@
+/* 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::Override;
+use derive_common::cg;
+use proc_macro2::{Span, TokenStream};
+use quote::{ToTokens, TokenStreamExt};
+use syn::{self, Data, Ident, Path, WhereClause};
+use synstructure::{BindingInfo, Structure, VariantInfo};
+
+fn derive_bitflags(input: &syn::DeriveInput, bitflags: &CssBitflagAttrs) -> TokenStream {
+    let name = &input.ident;
+    let mut body = TokenStream::new();
+    for (rust_name, css_name) in bitflags.single_flags() {
+        let rust_ident = Ident::new(&rust_name, Span::call_site());
+        body.append_all(quote! {
+            if *self == Self::#rust_ident {
+                return dest.write_str(#css_name);
+            }
+        });
+    }
+
+    body.append_all(quote! {
+        let mut has_any = false;
+    });
+
+    if bitflags.overlapping_bits {
+        body.append_all(quote! {
+            let mut serialized = Self::empty();
+        });
+    }
+
+    for (rust_name, css_name) in bitflags.mixed_flags() {
+        let rust_ident = Ident::new(&rust_name, Span::call_site());
+        let serialize = quote! {
+            if has_any {
+                dest.write_char(' ')?;
+            }
+            has_any = true;
+            dest.write_str(#css_name)?;
+        };
+        if bitflags.overlapping_bits {
+            body.append_all(quote! {
+                if self.contains(Self::#rust_ident) && !serialized.intersects(Self::#rust_ident) {
+                    #serialize
+                    serialized.insert(Self::#rust_ident);
+                }
+            });
+        } else {
+            body.append_all(quote! {
+                if self.intersects(Self::#rust_ident) {
+                    #serialize
+                }
+            });
+        }
+    }
+
+    body.append_all(quote! {
+        Ok(())
+    });
+
+    quote! {
+        impl style_traits::ToCss for #name {
+            #[allow(unused_variables)]
+            #[inline]
+            fn to_css<W>(
+                &self,
+                dest: &mut style_traits::CssWriter<W>,
+            ) -> std::fmt::Result
+            where
+                W: std::fmt::Write,
+            {
+                #body
+            }
+        }
+    }
+}
+
+pub fn derive(mut input: syn::DeriveInput) -> TokenStream {
+    let mut where_clause = input.generics.where_clause.take();
+    for param in input.generics.type_params() {
+        cg::add_predicate(&mut where_clause, parse_quote!(#param: style_traits::ToCss));
+    }
+
+    let input_attrs = cg::parse_input_attrs::<CssInputAttrs>(&input);
+    if matches!(input.data, Data::Enum(..)) || input_attrs.bitflags.is_some() {
+        assert!(
+            input_attrs.function.is_none(),
+            "#[css(function)] is not allowed on enums or bitflags"
+        );
+        assert!(
+            !input_attrs.comma,
+            "#[css(comma)] is not allowed on enums or bitflags"
+        );
+    }
+
+    if let Some(ref bitflags) = input_attrs.bitflags {
+        assert!(
+            !input_attrs.derive_debug,
+            "Bitflags can derive debug on their own"
+        );
+        assert!(where_clause.is_none(), "Generic bitflags?");
+        return derive_bitflags(&input, bitflags);
+    }
+
+    let match_body = {
+        let s = Structure::new(&input);
+        s.each_variant(|variant| derive_variant_arm(variant, &mut where_clause))
+    };
+    input.generics.where_clause = where_clause;
+
+    let name = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    let mut impls = quote! {
+        impl #impl_generics style_traits::ToCss for #name #ty_generics #where_clause {
+            #[allow(unused_variables)]
+            #[inline]
+            fn to_css<W>(
+                &self,
+                dest: &mut style_traits::CssWriter<W>,
+            ) -> std::fmt::Result
+            where
+                W: std::fmt::Write,
+            {
+                match *self {
+                    #match_body
+                }
+            }
+        }
+    };
+
+    if input_attrs.derive_debug {
+        impls.append_all(quote! {
+            impl #impl_generics std::fmt::Debug for #name #ty_generics #where_clause {
+                fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+                    style_traits::ToCss::to_css(
+                        self,
+                        &mut style_traits::CssWriter::new(f),
+                    )
+                }
+            }
+        });
+    }
+
+    impls
+}
+
+fn derive_variant_arm(variant: &VariantInfo, generics: &mut Option<WhereClause>) -> TokenStream {
+    let bindings = variant.bindings();
+    let identifier = cg::to_css_identifier(&variant.ast().ident.to_string());
+    let ast = variant.ast();
+    let variant_attrs = cg::parse_variant_attrs_from_ast::<CssVariantAttrs>(&ast);
+    let separator = if variant_attrs.comma { ", " } else { " " };
+
+    if variant_attrs.skip {
+        return quote!(Ok(()));
+    }
+    if variant_attrs.dimension {
+        assert_eq!(bindings.len(), 1);
+        assert!(
+            variant_attrs.function.is_none() && variant_attrs.keyword.is_none(),
+            "That makes no sense"
+        );
+    }
+
+    let mut expr = if let Some(keyword) = variant_attrs.keyword {
+        assert!(bindings.is_empty());
+        quote! {
+            std::fmt::Write::write_str(dest, #keyword)
+        }
+    } else if !bindings.is_empty() {
+        derive_variant_fields_expr(bindings, generics, separator)
+    } else {
+        quote! {
+            std::fmt::Write::write_str(dest, #identifier)
+        }
+    };
+
+    if variant_attrs.dimension {
+        expr = quote! {
+            #expr?;
+            std::fmt::Write::write_str(dest, #identifier)
+        }
+    } else if let Some(function) = variant_attrs.function {
+        let mut identifier = function.explicit().map_or(identifier, |name| name);
+        identifier.push('(');
+        expr = quote! {
+            std::fmt::Write::write_str(dest, #identifier)?;
+            #expr?;
+            std::fmt::Write::write_str(dest, ")")
+        }
+    }
+    expr
+}
+
+fn derive_variant_fields_expr(
+    bindings: &[BindingInfo],
+    where_clause: &mut Option<WhereClause>,
+    separator: &str,
+) -> TokenStream {
+    let mut iter = bindings
+        .iter()
+        .filter_map(|binding| {
+            let attrs = cg::parse_field_attrs::<CssFieldAttrs>(&binding.ast());
+            if attrs.skip {
+                return None;
+            }
+            Some((binding, attrs))
+        })
+        .peekable();
+
+    let (first, attrs) = match iter.next() {
+        Some(pair) => pair,
+        None => return quote! { Ok(()) },
+    };
+    if attrs.field_bound {
+        let ty = &first.ast().ty;
+        // TODO(emilio): IntoIterator might not be enough for every type of
+        // iterable thing (like ArcSlice<> or what not). We might want to expose
+        // an `item = "T"` attribute to handle that in the future.
+        let predicate = if attrs.iterable {
+            parse_quote!(<#ty as IntoIterator>::Item: style_traits::ToCss)
+        } else {
+            parse_quote!(#ty: style_traits::ToCss)
+        };
+        cg::add_predicate(where_clause, predicate);
+    }
+    if !attrs.iterable && iter.peek().is_none() {
+        let mut expr = quote! { style_traits::ToCss::to_css(#first, dest) };
+        if let Some(condition) = attrs.skip_if {
+            expr = quote! {
+                if !#condition(#first) {
+                    #expr
+                }
+            }
+        }
+
+        if let Some(condition) = attrs.contextual_skip_if {
+            expr = quote! {
+                if !#condition(#(#bindings), *) {
+                    #expr
+                }
+            }
+        }
+        return expr;
+    }
+
+    let mut expr = derive_single_field_expr(first, attrs, where_clause, bindings);
+    for (binding, attrs) in iter {
+        derive_single_field_expr(binding, attrs, where_clause, bindings).to_tokens(&mut expr)
+    }
+
+    quote! {{
+        let mut writer = style_traits::values::SequenceWriter::new(dest, #separator);
+        #expr
+        Ok(())
+    }}
+}
+
+fn derive_single_field_expr(
+    field: &BindingInfo,
+    attrs: CssFieldAttrs,
+    where_clause: &mut Option<WhereClause>,
+    bindings: &[BindingInfo],
+) -> TokenStream {
+    let mut expr = if attrs.iterable {
+        if let Some(if_empty) = attrs.if_empty {
+            return quote! {
+                {
+                    let mut iter = #field.iter().peekable();
+                    if iter.peek().is_none() {
+                        writer.raw_item(#if_empty)?;
+                    } else {
+                        for item in iter {
+                            writer.item(&item)?;
+                        }
+                    }
+                }
+            };
+        }
+        quote! {
+            for item in #field.iter() {
+                writer.item(&item)?;
+            }
+        }
+    } else if attrs.represents_keyword {
+        let ident = field
+            .ast()
+            .ident
+            .as_ref()
+            .expect("Unnamed field with represents_keyword?");
+        let ident = cg::to_css_identifier(&ident.to_string()).replace("_", "-");
+        quote! {
+            if *#field {
+                writer.raw_item(#ident)?;
+            }
+        }
+    } else {
+        if attrs.field_bound {
+            let ty = &field.ast().ty;
+            cg::add_predicate(where_clause, parse_quote!(#ty: style_traits::ToCss));
+        }
+        quote! { writer.item(#field)?; }
+    };
+
+    if let Some(condition) = attrs.skip_if {
+        expr = quote! {
+            if !#condition(#field) {
+                #expr
+            }
+        }
+    }
+
+    if let Some(condition) = attrs.contextual_skip_if {
+        expr = quote! {
+            if !#condition(#(#bindings), *) {
+                #expr
+            }
+        }
+    }
+
+    expr
+}
+
+#[derive(Default, FromMeta)]
+#[darling(default)]
+pub struct CssBitflagAttrs {
+    /// Flags that can only go on their own, comma-separated.
+    pub single: Option<String>,
+    /// Flags that can go mixed with each other, comma-separated.
+    pub mixed: Option<String>,
+    /// Extra validation of the resulting mixed flags.
+    pub validate_mixed: Option<Path>,
+    /// Whether there are overlapping bits we need to take care of when
+    /// serializing.
+    pub overlapping_bits: bool,
+}
+
+impl CssBitflagAttrs {
+    /// Returns a vector of (rust_name, css_name) of a given flag list.
+    fn names(s: &Option<String>) -> Vec<(String, String)> {
+        let s = match s {
+            Some(s) => s,
+            None => return vec![],
+        };
+        s.split(',')
+            .map(|css_name| (cg::to_scream_case(css_name), css_name.to_owned()))
+            .collect()
+    }
+
+    pub fn single_flags(&self) -> Vec<(String, String)> {
+        Self::names(&self.single)
+    }
+
+    pub fn mixed_flags(&self) -> Vec<(String, String)> {
+        Self::names(&self.mixed)
+    }
+}
+
+#[derive(Default, FromDeriveInput)]
+#[darling(attributes(css), default)]
+pub struct CssInputAttrs {
+    pub derive_debug: bool,
+    // Here because structs variants are also their whole type definition.
+    pub function: Option<Override<String>>,
+    // Here because structs variants are also their whole type definition.
+    pub comma: bool,
+    pub bitflags: Option<CssBitflagAttrs>,
+}
+
+#[derive(Default, FromVariant)]
+#[darling(attributes(css), default)]
+pub struct CssVariantAttrs {
+    pub function: Option<Override<String>>,
+    // Here because structs variants are also their whole type definition.
+    pub derive_debug: bool,
+    pub comma: bool,
+    pub bitflags: Option<CssBitflagAttrs>,
+    pub dimension: bool,
+    pub keyword: Option<String>,
+    pub skip: bool,
+}
+
+#[derive(Default, FromField)]
+#[darling(attributes(css), default)]
+pub struct CssFieldAttrs {
+    pub if_empty: Option<String>,
+    pub field_bound: bool,
+    pub iterable: bool,
+    pub skip: bool,
+    pub represents_keyword: bool,
+    pub contextual_skip_if: Option<Path>,
+    pub skip_if: Option<Path>,
+}
diff --git a/servo/components/style_derive/to_resolved_value.rs b/servo/components/style_derive/to_resolved_value.rs
new file mode 100644
index 0000000000..e049f91152
--- /dev/null
+++ b/servo/components/style_derive/to_resolved_value.rs
@@ -0,0 +1,52 @@
+/* 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 derive_common::cg;
+use proc_macro2::TokenStream;
+use syn::DeriveInput;
+use synstructure::BindStyle;
+use to_computed_value;
+
+pub fn derive(input: DeriveInput) -> TokenStream {
+    let trait_impl = |from_body, to_body| {
+        quote! {
+             #[inline]
+             fn from_resolved_value(from: Self::ResolvedValue) -> Self {
+                 #from_body
+             }
+
+             #[inline]
+             fn to_resolved_value(
+                 self,
+                 context: &crate::values::resolved::Context,
+             ) -> Self::ResolvedValue {
+                 #to_body
+             }
+        }
+    };
+
+    to_computed_value::derive_to_value(
+        input,
+        parse_quote!(crate::values::resolved::ToResolvedValue),
+        parse_quote!(ResolvedValue),
+        BindStyle::Move,
+        |binding| {
+            let attrs = cg::parse_field_attrs::<ResolvedValueAttrs>(&binding.ast());
+            to_computed_value::ToValueAttrs {
+                field_bound: attrs.field_bound,
+                no_field_bound: attrs.no_field_bound,
+            }
+        },
+        |binding| quote!(crate::values::resolved::ToResolvedValue::from_resolved_value(#binding)),
+        |binding| quote!(crate::values::resolved::ToResolvedValue::to_resolved_value(#binding, context)),
+        trait_impl,
+    )
+}
+
+#[derive(Default, FromField)]
+#[darling(attributes(resolve), default)]
+struct ResolvedValueAttrs {
+    field_bound: bool,
+    no_field_bound: bool,
+}