style: Use cbindgen for content property.

This cleans up and also allows us to keep the distinction between content: none
and content: normal, which allows us to fix the computed style we return from
getComputedStyle().

Do this last bit from the resolved value instead of StyleAdjuster, because
otherwise we need to tweak every initial struct for ::before / ::after.

Differential Revision: https://phabricator.services.mozilla.com/D58276

components/style/values/resolved/counters.rs

@@ -0,0 +1,46 @@
+/* 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/. */
+
+//! Resolved values for counter properties
+
+use super::{Context, ToResolvedValue};
+use crate::values::computed;
+
+/// https://drafts.csswg.org/css-content/#content-property
+///
+/// We implement this at resolved value time because otherwise it causes us to
+/// allocate a bunch of useless initial structs for ::before / ::after, which is
+/// a bit unfortunate.
+///
+/// Though these should be temporary, mostly, so if this causes complexity in
+/// other places, it should be fine to move to `StyleAdjuster`.
+///
+/// See https://github.com/w3c/csswg-drafts/issues/4632 for where some related
+/// issues are being discussed.
+impl ToResolvedValue for computed::Content {
+    type ResolvedValue = Self;
+
+    #[inline]
+    fn to_resolved_value(self, context: &Context) -> Self {
+        let is_before_or_after =
+            context.style.pseudo().map_or(false, |p| p.is_before_or_after());
+
+        match self {
+            Self::Normal if is_before_or_after => Self::None,
+            // For now, make `content: none` compute to `normal` on other
+            // elements, as we don't respect it.
+            //
+            // FIXME(emilio, bug 1605473): for marker this should be preserved
+            // and respected, probably.
+            Self::None if !is_before_or_after => Self::Normal,
+            other => other,
+        }
+    }
+
+    #[inline]
+    fn from_resolved_value(resolved: Self) -> Self {
+        resolved
+    }
+}
+