Derive the most trivial ToCss implementations 🌋

For now, all variants get serialised as the space-separated serialisations
of their fields. Unit variants are not supported.

components/style_derive/to_css.rs

@@ -0,0 +1,70 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+use quote;
+use syn;
+use synstructure;
+
+pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
+    let name = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+    let mut where_clause = where_clause.clone();
+    for param in &input.generics.ty_params {
+        where_clause.predicates.push(where_predicate(syn::Ty::Path(None, param.ident.clone().into())))
+    }
+
+    let style = synstructure::BindStyle::Ref.into();
+    let match_body = synstructure::each_variant(&input, &style, |bindings, _| {
+        if bindings.is_empty() {
+            panic!("unit variants are not yet supported");
+        }
+        let (first, rest) = bindings.split_first().expect("unit variants are not yet supported");
+        where_clause.predicates.push(where_predicate(first.field.ty.clone()));
+        let mut expr = quote! {
+            ::style_traits::ToCss::to_css(#first, dest)
+        };
+        for binding in rest {
+            where_clause.predicates.push(where_predicate(binding.field.ty.clone()));
+            expr = quote! {
+                #expr?;
+                dest.write_str(" ")?;
+                ::style_traits::ToCss::to_css(#binding, dest)
+            };
+        }
+        Some(expr)
+    });
+
+    quote! {
+        impl #impl_generics ::style_traits::ToCss for #name #ty_generics #where_clause {
+            #[allow(unused_variables, unused_imports)]
+            #[inline]
+            fn to_css<W>(&self, dest: &mut W) -> ::std::fmt::Result
+            where
+                W: ::std::fmt::Write
+            {
+                match *self {
+                    #match_body
+                }
+            }
+        }
+    }
+}
+
+/// `#ty: ::style_traits::ToCss`
+fn where_predicate(ty: syn::Ty) -> syn::WherePredicate {
+    syn::WherePredicate::BoundPredicate(syn::WhereBoundPredicate {
+        bound_lifetimes: vec![],
+        bounded_ty: ty,
+        bounds: vec![syn::TyParamBound::Trait(
+            syn::PolyTraitRef {
+                bound_lifetimes: vec![],
+                trait_ref: syn::Path {
+                    global: true,
+                    segments: vec!["style_traits".into(), "ToCss".into()],
+                },
+            },
+            syn::TraitBoundModifier::None
+        )],
+    })
+}