Add size_of_test crate

components/size_of_test/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "size_of_test"
+version = "0.0.1"
+authors = ["The Servo Project Developers"]
+license = "MPL-2.0"
+publish = false
+
+[lib]
+path = "lib.rs"

components/size_of_test/lib.rs

@@ -0,0 +1,28 @@
+/* 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/. */
+
+#[macro_export]
+macro_rules! size_of_test {
+    ($testname: ident, $t: ty, $expected_size: expr) => {
+        #[test]
+        fn $testname() {
+            let new = ::std::mem::size_of::<$t>();
+            let old = $expected_size;
+            if new < old {
+                panic!(
+                    "Your changes have decreased the stack size of {} from {} to {}. \
+                     Good work! Please update the expected size in {}.",
+                    stringify!($t), old, new, file!()
+                )
+            } else if new > old {
+                panic!(
+                    "Your changes have increased the stack size of {} from {} to {}. \
+                     Please consider choosing a design which avoids this increase. \
+                     If you feel that the increase is necessary, update the size in {}.",
+                    stringify!($t), old, new, file!()
+                )
+            }
+        }
+    }
+}