Bug 1978217 - Add support for custom style use counters. r=firefox-style-system-reviewers,layout-reviewers,dshin, a=dsmith

In the future if we need to we can hook these up to telemetry (in fact it's not
hard, we just need to do that in Document::ReportUseCounters).

For now I want to start using them to record whether we might have a data URI.

Differential Revision: https://phabricator.services.mozilla.com/D258291

servo/components/style/use_counters/mod.rs

@@ -26,7 +26,29 @@ pub struct NonCustomPropertyUseCounters {
     storage: [Cell<usize>; (property_counts::NON_CUSTOM - 1 + BITS_PER_ENTRY) / BITS_PER_ENTRY],
 }
 
-macro_rules! property_use_counters_methods {
+/// A custom style use counter that we may want to record.
+#[derive(Copy, Clone, Debug)]
+#[repr(u32)]
+pub enum CustomUseCounter {
+    /// Dummy value, used for indexing purposes.
+    Last,
+}
+
+impl CustomUseCounter {
+    #[inline]
+    fn bit(self) -> usize {
+        self as usize
+    }
+}
+
+/// One bit for each custom use counter.
+#[derive(Debug, Default, Clone)]
+pub struct CustomUseCounters {
+    storage:
+        [Cell<usize>; ((CustomUseCounter::Last as usize) - 1 + BITS_PER_ENTRY) / BITS_PER_ENTRY],
+}
+
+macro_rules! use_counters_methods {
     ($id: ident) => {
         /// Returns the bucket a given property belongs in, and the bitmask for that
         /// property.
@@ -65,11 +87,15 @@ macro_rules! property_use_counters_methods {
 }
 
 impl CountedUnknownPropertyUseCounters {
-    property_use_counters_methods!(CountedUnknownProperty);
+    use_counters_methods!(CountedUnknownProperty);
 }
 
 impl NonCustomPropertyUseCounters {
-    property_use_counters_methods!(NonCustomPropertyId);
+    use_counters_methods!(NonCustomPropertyId);
+}
+
+impl CustomUseCounters {
+    use_counters_methods!(CustomUseCounter);
 }
 
 /// The use-counter data related to a given document we want to store.
@@ -80,6 +106,8 @@ pub struct UseCounters {
     pub non_custom_properties: NonCustomPropertyUseCounters,
     /// The counters for css properties which we haven't implemented yet.
     pub counted_unknown_properties: CountedUnknownPropertyUseCounters,
+    /// Custom counters for virtually everything else.
+    pub custom: CustomUseCounters,
 }
 
 impl UseCounters {
@@ -92,5 +120,6 @@ impl UseCounters {
             .merge(&other.non_custom_properties);
         self.counted_unknown_properties
             .merge(&other.counted_unknown_properties);
+        self.custom.merge(&other.custom);
     }
 }