Add a test to check the size of PropertyDeclaration

tests/unit/style/size_of.rs

@@ -0,0 +1,24 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+use std::mem::size_of;
+use style::properties::PropertyDeclaration;
+
+#[test]
+fn size_of_property_declaration() {
+    let old = 240;
+    let new = size_of::<PropertyDeclaration>();
+    if new < old {
+        panic!("Your changes have decreased the stack size of PropertyDeclaration enum from {} to {}. \
+                Good work! Please update the size in tests/unit/style/size_of.rs.",
+                old, new)
+    } else if new > old {
+        panic!("Your changes have increased the stack size of PropertyDeclaration enum from {} to {}. \
+                These enum is present in large quantities in the style, and increasing the size \
+                may dramatically affect our memory footprint. Please consider using `boxed=\"True\"` in \
+                the longhand If you feel that the increase is necessary, update to the new size in \
+                tests/unit/style/size_of.rs.",
+                old, new)
+    }
+}