Use pointer casts instead of tramsutes to raw::TraitObject

Casting `*const T` to `*const U` with `U: Sized` is allowed even if `T: ?Sized`.
This safely extracts the data pointer out of a trait object,
without relying on the memory representation of trait objects.

components/layout/flow.rs

@@ -44,7 +44,7 @@ use multicol::MulticolFlow;
 use parallel::FlowParallelInfo;
 use serde::ser::{Serialize, SerializeStruct, Serializer};
 use servo_geometry::{au_rect_to_f32_rect, f32_rect_to_au_rect, max_rect};
-use std::{fmt, mem};
+use std::fmt;
 use std::iter::Zip;
 use std::slice::IterMut;
 use std::sync::Arc;
@@ -452,10 +452,9 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
 #[inline(always)]
 #[allow(unsafe_code)]
 pub fn base<T: ?Sized + Flow>(this: &T) -> &BaseFlow {
-    unsafe {
-        let obj = mem::transmute::<&&T, &::TraitObject>(&this);
-        mem::transmute::<*mut (), &BaseFlow>(obj.data)
-    }
+    let ptr: *const T = this;
+    let ptr = ptr as *const BaseFlow;
+    unsafe { &*ptr }
 }
 
 /// Iterates over the children of this immutable flow.
@@ -466,10 +465,9 @@ pub fn child_iter<'a>(flow: &'a Flow) -> FlowListIterator {
 #[inline(always)]
 #[allow(unsafe_code)]
 pub fn mut_base<T: ?Sized + Flow>(this: &mut T) -> &mut BaseFlow {
-    unsafe {
-        let obj = mem::transmute::<&&mut T, &::TraitObject>(&this);
-        mem::transmute::<*mut (), &mut BaseFlow>(obj.data)
-    }
+    let ptr: *mut T = this;
+    let ptr = ptr as *mut BaseFlow;
+    unsafe { &mut *ptr }
 }
 
 /// Iterates over the children of this flow.
@@ -1419,11 +1417,9 @@ impl ContainingBlockLink {
 pub struct OpaqueFlow(pub usize);
 
 impl OpaqueFlow {
-    #[allow(unsafe_code)]
     pub fn from_flow(flow: &Flow) -> OpaqueFlow {
-        unsafe {
-            let object = mem::transmute::<&Flow, ::TraitObject>(flow);
-            OpaqueFlow(object.data as usize)
-        }
+        let object_ptr: *const Flow = flow;
+        let data_ptr = object_ptr as *const ();
+        OpaqueFlow(data_ptr as usize)
     }
 }

components/layout/lib.rs

@@ -90,12 +90,3 @@ pub use self::data::LayoutData;
 // We can't use servo_arc for everything in layout, because the Flow stuff uses
 // weak references.
 use servo_arc::Arc as ServoArc;
-
-/// Stable copy of std::raw::TraitObject
-/// test/unit/layout/lib.rs asserts that the memory layout matches.
-#[repr(C)]
-#[derive(Clone, Copy)]
-pub struct TraitObject {
-    pub data: *mut (),
-    pub vtable: *mut (),
-}

tests/unit/layout/lib.rs

@@ -2,36 +2,7 @@
  * 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));
-}