Derive ToComputedValue

For now, only impls for types like in style::values::generics can be derived.

This also needed a few ToComputedValueAsSpecified impls that I would like to
replace by some #[to_computed_value(clone)] attribute, but I think it is ok
to keep it like this for now.

components/style_derive/to_computed_value.rs

@@ -0,0 +1,142 @@
+/* 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()), None));
+    }
+
+    let computed_value_type = syn::Path::from(syn::PathSegment {
+        ident: name.clone(),
+        parameters: syn::PathParameters::AngleBracketed(syn::AngleBracketedParameterData {
+            lifetimes: input.generics.lifetimes.iter().map(|l| {
+                l.lifetime.clone()
+            }).collect(),
+            types: input.generics.ty_params.iter().map(|ty| {
+                syn::Ty::Path(
+                    Some(syn::QSelf {
+                        ty: Box::new(syn::Ty::Path(None, ty.ident.clone().into())),
+                        position: 3,
+                    }),
+                    syn::Path {
+                        global: true,
+                        segments: vec![
+                            "values".into(),
+                            "computed".into(),
+                            "ToComputedValue".into(),
+                            "ComputedValue".into(),
+                        ],
+                    },
+                )
+            }).collect(),
+            .. Default::default()
+        }),
+    });
+
+    let to_body = match_body(&input, |field| {
+        quote!(::values::computed::ToComputedValue::to_computed_value(#field, context))
+    });
+    let from_body = match_body(&input, |field| {
+        quote!(::values::computed::ToComputedValue::from_computed_value(#field))
+    });
+
+    quote! {
+        impl #impl_generics ::values::computed::ToComputedValue for #name #ty_generics #where_clause {
+            type ComputedValue = #computed_value_type;
+
+            #[allow(unused_variables)]
+            #[inline]
+            fn to_computed_value(&self, context: &::values::computed::Context) -> Self::ComputedValue {
+                match *self {
+                    #to_body
+                }
+            }
+
+            #[inline]
+            fn from_computed_value(computed: &Self::ComputedValue) -> Self {
+                match *computed {
+                    #from_body
+                }
+            }
+        }
+    }
+}
+
+fn match_body<F>(input: &syn::DeriveInput, f: F) -> quote::Tokens
+    where F: Fn(&synstructure::BindingInfo) -> quote::Tokens,
+{
+    let by_ref = synstructure::BindStyle::Ref.into();
+    let by_value = synstructure::BindStyle::Move.into();
+
+    synstructure::each_variant(&input, &by_ref, |fields, variant| {
+        let name = if let syn::Body::Enum(_) = input.body {
+            format!("{}::{}", input.ident, variant.ident).into()
+        } else {
+            variant.ident.clone()
+        };
+        let (computed_value, computed_fields) = synstructure::match_pattern(&name, &variant.data, &by_value);
+        let fields_pairs = fields.iter().zip(computed_fields.iter());
+        let mut computations = quote!();
+        computations.append_all(fields_pairs.map(|(field, computed_field)| {
+            let expr = f(field);
+            quote!(let #computed_field = #expr;)
+        }));
+        Some(quote!(
+            #computations
+            #computed_value
+        ))
+    })
+}
+
+/// `#ty: ::values::computed::ToComputedValue<ComputedValue = #computed_value,>`
+fn where_predicate(ty: syn::Ty, computed_value: Option<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: trait_ref(computed_value),
+            },
+            syn::TraitBoundModifier::None
+        )],
+    })
+}
+
+/// `::values::computed::ToComputedValue<ComputedValue = #computed_value,>`
+fn trait_ref(computed_value: Option<syn::Ty>) -> syn::Path {
+    syn::Path {
+        global: true,
+        segments: vec![
+            "values".into(),
+            "computed".into(),
+            syn::PathSegment {
+                ident: "ToComputedValue".into(),
+                parameters: syn::PathParameters::AngleBracketed(
+                    syn::AngleBracketedParameterData {
+                        bindings: trait_bindings(computed_value),
+                        .. Default::default()
+                    }
+                ),
+            }
+        ],
+    }
+}
+
+/// `ComputedValue = #computed_value,`
+fn trait_bindings(computed_value: Option<syn::Ty>) -> Vec<syn::TypeBinding> {
+    computed_value.into_iter().map(|ty| {
+        syn::TypeBinding {
+            ident: "ComputedValue".into(),
+            ty: ty,
+        }
+    }).collect()
+}