style: Add derive code for a new ToResolvedValue trait, and a few trivial implementations.

Differential Revision: https://phabricator.services.mozilla.com/D26782

components/style_derive/to_resolved_value.rs

@@ -0,0 +1,69 @@
+/* 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(resolved: Self::ResolvedValue) -> Self {
+                match resolved {
+                    #from_body
+                }
+            }
+
+            #[inline]
+            fn to_resolved_value(
+                self,
+                context: &crate::values::resolved::Context,
+            ) -> Self::ResolvedValue {
+                match self {
+                    #to_body
+                }
+            }
+       }
+    };
+
+    let non_generic_implementation = || {
+        Some(quote! {
+            type ResolvedValue = Self;
+
+            #[inline]
+            fn from_resolved_value(resolved: Self::ResolvedValue) -> Self {
+                resolved
+            }
+
+            #[inline]
+            fn to_resolved_value(
+                self,
+                context: &crate::values::resolved::Context,
+            ) -> Self {
+                self
+            }
+        })
+    };
+
+    to_computed_value::derive_to_value(
+        input,
+        parse_quote!(crate::values::resolved::ToResolvedValue),
+        parse_quote!(ResolvedValue),
+        BindStyle::Move,
+        |binding| cg::parse_field_attrs::<ResolvedValueAttrs>(&binding.ast()).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,
+        non_generic_implementation,
+    )
+}
+
+#[darling(attributes(resolve), default)]
+#[derive(Default, FromField)]
+struct ResolvedValueAttrs {
+    field_bound: bool,
+}