Bug 1506760 - Store variable references with a boxed slice rather than a PrecomputedHashSet r=emilio

Once we've parsed the variable references, there is no need to keep an entire HashSet
object around, as all we do is iterate over the values.

Differential Revision: https://phabricator.services.mozilla.com/D11735

--HG--
extra : moz-landing-system : lando

servo/components/style/custom_properties.rs

@@ -109,7 +109,7 @@ pub struct VariableValue {
     references_environment: bool,
 
     /// Custom property names in var() functions.
-    references: PrecomputedHashSet<Name>,
+    references: Box<[Name]>,
 }
 
 impl ToCss for SpecifiedValue {
@@ -278,7 +278,7 @@ impl VariableValue {
             css: String::new(),
             last_token_type: TokenSerializationType::nothing(),
             first_token_type: TokenSerializationType::nothing(),
-            references: PrecomputedHashSet::default(),
+            references: Default::default(),
             references_environment: false,
         }
     }
@@ -335,11 +335,16 @@ impl VariableValue {
         let (first_token_type, css, last_token_type) =
             parse_self_contained_declaration_value(input, Some(&mut references))?;
 
+        let custom_property_references = references.custom_property_references
+            .into_iter()
+            .collect::<Vec<_>>()
+            .into_boxed_slice();
+
         Ok(Arc::new(VariableValue {
             css: css.into_owned(),
             first_token_type,
             last_token_type,
-            references: references.custom_property_references,
+            references: custom_property_references,
             references_environment: references.references_environment,
         }))
     }