Add unit test to freeze sizes of DOM structs

components/script/lib.rs

@@ -226,3 +226,6 @@ pub mod script_task;
 mod timers;
 pub mod textinput;
 mod devtools;
+
+#[cfg(test)]
+mod tests;

components/script/tests.rs

@@ -0,0 +1,48 @@
+/* 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 dom::characterdata::CharacterData;
+use dom::element::Element;
+use dom::eventtarget::EventTarget;
+use dom::htmldivelement::HTMLDivElement;
+use dom::htmlelement::HTMLElement;
+use dom::htmlspanelement::HTMLSpanElement;
+use dom::node::Node;
+use dom::text::Text;
+
+use std::mem::size_of;
+
+// Macro so that we can stringify type names
+// I'd really prefer the tests themselves to be run at plugin time,
+// however rustc::middle doesn't have access to the full type data
+macro_rules! sizeof_checker (
+    ($testname: ident, $t:ty, $known_size:expr) => (
+        #[test]
+        fn $testname() {
+            let new = size_of::<$t>();
+            let old = $known_size;
+            if new < old {
+                panic!("Your changes have decreased the stack size of commonly used DOM struct {} from {} to {}. \
+                        Good work! Please update the size in script/tests.rs",
+                        stringify!($t), old, new)
+            } else if new > old {
+                panic!("Your changes have increased the stack size of commonly used DOM struct {} from {} to {}. \
+                        These structs are present in large quantities in the DOM, and increasing the size \
+                        may dramatically affect our memory footprint. Please consider choosing a design which \
+                        avoids this increase. If you feel that the increase is necessary, \
+                        update to the new size in script/tests.rs.",
+                        stringify!($t), old, new)
+        }
+    });
+)
+
+// Update the sizes here
+sizeof_checker!(size_event_target, EventTarget, 56)
+sizeof_checker!(size_node, Node, 304)
+sizeof_checker!(size_element, Element, 448)
+sizeof_checker!(size_htmlelement, HTMLElement, 448)
+sizeof_checker!(size_div, HTMLDivElement, 448)
+sizeof_checker!(size_span, HTMLSpanElement, 448)
+sizeof_checker!(size_text, Text, 336)
+sizeof_checker!(size_characterdata, CharacterData, 336)