Add a unit test for Fragment size

tests/unit/layout/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "layout_tests"
+version = "0.0.1"
+authors = ["The Servo Project Developers"]
+
+[lib]
+name = "layout_tests"
+path = "lib.rs"
+doctest = false
+
+[dependencies.layout]
+path = "../../../components/layout"

tests/unit/layout/lib.rs

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+/* 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/. */
+
+extern crate layout;
+
+#[cfg(all(test, target_pointer_width = "64"))] mod size_of;

tests/unit/layout/size_of.rs

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+/* 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 layout::Fragment;
+use std::mem::size_of;
+
+#[test]
+fn test_size_of_fragment() {
+    let expected = 184;
+    let actual = size_of::<Fragment>();
+
+    if actual < expected {
+        panic!("Your changes have decreased the stack size of layout::fragment::Fragment \
+                from {} to {}. Good work! Please update the size in tests/layout/size_of.rs",
+                expected, actual);
+    }
+
+    if actual > expected {
+        panic!("Your changes have increased the stack size of layout::fragment::Fragment \
+                from {} to {}.  Please consider choosing a design which avoids this increase. \
+                If you feel that the increase is necessary, update the size in \
+                tests/layout/size_of.rs.",
+                expected, actual);
+    }
+}