Auto merge of #18854 - servo:servo-unstable-feature, r=nox

Make optional the usage of some unstable features

With `--no-default-features --features default-except-unstable`, more crates can now be compiled on stable Rust. This will help integrate them in rustc’s regression testing and compiler performance benchmarking.

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tests/unit/layout/lib.rs

@@ -2,7 +2,36 @@
  * 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/. */
 
+#![feature(raw)]
+
 extern crate layout;
 #[macro_use] extern crate size_of_test;
 
 #[cfg(all(test, target_pointer_width = "64"))] mod size_of;
+
+use std::mem;
+use std::ptr;
+use std::raw;
+
+#[test]
+fn test_trait_object_layout() {
+    assert_eq!(mem::size_of::<raw::TraitObject>(), mem::size_of::<layout::TraitObject>());
+    let null: *mut () = ptr::null_mut();
+    let a = raw::TraitObject {
+        data: null,
+        vtable: null,
+    };
+    let b = layout::TraitObject {
+        data: null,
+        vtable: null,
+    };
+
+    fn offset<T, U>(struct_: &T, field: &U) -> usize {
+        let addr_struct = struct_ as *const T as usize;
+        let addr_field = field as *const U as usize;
+        addr_field - addr_struct
+    }
+
+    assert_eq!(offset(&a, &a.data), offset(&b, &b.data));
+    assert_eq!(offset(&a, &a.vtable), offset(&b, &b.vtable));
+}