diff --git a/servo/components/style/rule_tree/core.rs b/servo/components/style/rule_tree/core.rs
index 2c11868201a9..f3abef426dde 100644
--- a/servo/components/style/rule_tree/core.rs
+++ b/servo/components/style/rule_tree/core.rs
@@ -185,6 +185,9 @@ impl RuleTree {
 /// where it likely did not result from a rigorous performance analysis.)
 const RULE_TREE_GC_INTERVAL: usize = 300;
 
+/// Used for some size assertions.
+pub const RULE_NODE_SIZE: usize = std::mem::size_of::<RuleNode>();
+
 /// A node in the rule tree.
 struct RuleNode {
     /// The root node. Only the root has no root pointer, for obvious reasons.
diff --git a/servo/components/style/rule_tree/mod.rs b/servo/components/style/rule_tree/mod.rs
index ab5273db7812..e50382255ca2 100644
--- a/servo/components/style/rule_tree/mod.rs
+++ b/servo/components/style/rule_tree/mod.rs
@@ -19,7 +19,7 @@ mod map;
 mod source;
 mod unsafe_box;
 
-pub use self::core::{RuleTree, StrongRuleNode};
+pub use self::core::{RuleTree, StrongRuleNode, RULE_NODE_SIZE};
 pub use self::level::{CascadeLevel, ShadowCascadeOrder};
 pub use self::source::StyleSource;
 
diff --git a/servo/ports/geckolib/tests/size_of.rs b/servo/ports/geckolib/tests/size_of.rs
index c8f1727bd54b..d19522a2e222 100644
--- a/servo/ports/geckolib/tests/size_of.rs
+++ b/servo/ports/geckolib/tests/size_of.rs
@@ -8,7 +8,7 @@ use style::applicable_declarations::ApplicableDeclarationBlock;
 use style::data::{ElementData, ElementStyles};
 use style::gecko::selector_parser::{self, SelectorImpl};
 use style::properties::ComputedValues;
-use style::rule_tree::{RuleNode, StrongRuleNode};
+use style::rule_tree::{RULE_NODE_SIZE, StrongRuleNode};
 use style::servo_arc::Arc;
 use style::values::computed;
 use style::values::specified;
@@ -53,7 +53,10 @@ size_of_test!(
     16
 );
 
-size_of_test!(test_size_of_rule_node, RuleNode, 80);
+#[test]
+fn test_size_of_rule_node() {
+    assert_eq!(RULE_NODE_SIZE, 80, "RuleNode size changed");
+}
 
 // This is huge, but we allocate it on the stack and then never move it,
 // we only pass `&mut SourcePropertyDeclaration` references around.