extra-bounds.diff

diff --git a/servo/components/style_derive/to_css.rs b/servo/components/style_derive/to_css.rs
index a5811471cb1d..edb63b7c1ae8 100644
--- a/servo/components/style_derive/to_css.rs
+++ b/servo/components/style_derive/to_css.rs
@@ -5,24 +5,30 @@
 use cg;
 use darling::util::Override;
 use quote::{ToTokens, Tokens};
 use syn::{self, Data, Path, WhereClause};
 use synstructure::{BindingInfo, Structure, VariantInfo};
 
 pub fn derive(mut input: syn::DeriveInput) -> Tokens {
     let mut where_clause = input.generics.where_clause.take();
+    let input_attrs = cg::parse_input_attrs::<CssInputAttrs>(&input);
     for param in input.generics.type_params() {
+        if let Some(ref bounds) = input_attrs.extra_bounds {
+            cg::add_predicate(
+                &mut where_clause,
+                parse_quote!(#param: #bounds),
+            );
+        }
         cg::add_predicate(
             &mut where_clause,
             parse_quote!(#param: ::style_traits::ToCss),
         );
     }
 
-    let input_attrs = cg::parse_input_attrs::<CssInputAttrs>(&input);
     if let Data::Enum(_) = input.data {
         assert!(
             input_attrs.function.is_none(),
             "#[css(function)] is not allowed on enums"
         );
         assert!(!input_attrs.comma, "#[css(comma)] is not allowed on enums");
     }
 
@@ -144,16 +150,25 @@ fn derive_variant_fields_expr(
         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.skip_if_self {
+            expr = quote! {
+                if !#condition(self) {
+                    #expr
+                }
+            }
+        }
+
         return expr;
     }
 
     let mut expr = derive_single_field_expr(first, attrs, where_clause);
     for (binding, attrs) in iter {
         derive_single_field_expr(binding, attrs, where_clause).to_tokens(&mut expr)
     }
 
@@ -212,27 +227,36 @@ fn derive_single_field_expr(
     if let Some(condition) = attrs.skip_if {
         expr = quote! {
             if !#condition(#field) {
                 #expr
             }
         }
     }
 
+    if let Some(condition) = attrs.skip_if_self {
+        expr = quote! {
+            if !#condition(self) {
+                #expr
+            }
+        }
+    }
+
     expr
 }
 
 #[darling(attributes(css), default)]
 #[derive(Default, FromDeriveInput)]
 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 extra_bounds: Option<Path>,
 }
 
 #[darling(attributes(css), default)]
 #[derive(Default, FromVariant)]
 pub struct CssVariantAttrs {
     pub function: Option<Override<String>>,
     // Here because structs variants are also their whole type definition.
     pub derive_debug: bool,
@@ -246,9 +270,10 @@ pub struct CssVariantAttrs {
 #[derive(Default, FromField)]
 pub struct CssFieldAttrs {
     pub if_empty: Option<String>,
     pub field_bound: bool,
     pub iterable: bool,
     pub skip: bool,
     pub represents_keyword: bool,
     pub skip_if: Option<Path>,
+    pub skip_if_self: Option<Path>,
 }