Auto merge of #18754 - mbrubeck:unsafe-flow, r=nox

Use raw pointers instead of transmute for UnsafeFlow

This makes the code depend a bit less on rustc implementation details.

r? pcwalton

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they are refactoring only

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components/layout/parallel.rs

@@ -8,6 +8,7 @@
 
 #![allow(unsafe_code)]
 
+use block::BlockFlow;
 use context::LayoutContext;
 use flow::{self, Flow};
 use flow_ref::FlowRef;
@@ -16,6 +17,7 @@ use rayon;
 use servo_config::opts;
 use smallvec::SmallVec;
 use std::mem;
+use std::ptr;
 use std::sync::atomic::{AtomicIsize, Ordering};
 use traversal::{AssignBSizes, AssignISizes, BubbleISizes};
 use traversal::{PostorderFlowTraversal, PreorderFlowTraversal};
@@ -26,22 +28,18 @@ const CHUNK_SIZE: usize = 16;
 pub type FlowList = SmallVec<[UnsafeFlow; CHUNK_SIZE]>;
 
 /// Vtable + pointer representation of a Flow trait object.
-pub type UnsafeFlow = (usize, usize);
+#[derive(Clone, Copy, Eq, PartialEq)]
+pub struct UnsafeFlow(*const Flow);
+
+unsafe impl Sync for UnsafeFlow {}
+unsafe impl Send for UnsafeFlow {}
 
 fn null_unsafe_flow() -> UnsafeFlow {
-    (0, 0)
+    UnsafeFlow(ptr::null::<BlockFlow>())
 }
 
 pub fn mut_owned_flow_to_unsafe_flow(flow: *mut FlowRef) -> UnsafeFlow {
-    unsafe {
-        mem::transmute::<&Flow, UnsafeFlow>(&**flow)
-    }
-}
-
-pub fn borrowed_flow_to_unsafe_flow(flow: &Flow) -> UnsafeFlow {
-    unsafe {
-        mem::transmute::<&Flow, UnsafeFlow>(flow)
-    }
+    unsafe { UnsafeFlow(&**flow) }
 }
 
 /// Information that we need stored in each flow.
@@ -103,7 +101,7 @@ fn bottom_up_flow(mut unsafe_flow: UnsafeFlow,
         // of our parent to finish processing? If so, we can continue
         // on with our parent; otherwise, we've gotta wait.
         let parent: &mut Flow = unsafe {
-            mem::transmute(unsafe_parent)
+            &mut *(unsafe_parent.0 as *mut Flow)
         };
         let parent_base = flow::mut_base(parent);
         if parent_base.parallel.children_count.fetch_sub(1, Ordering::Relaxed) == 1 {
@@ -140,7 +138,7 @@ fn top_down_flow<'scope>(unsafe_flows: &[UnsafeFlow],
             // Possibly enqueue the children.
             for kid in flow::child_iter_mut(flow) {
                 had_children = true;
-                discovered_child_flows.push(borrowed_flow_to_unsafe_flow(kid));
+                discovered_child_flows.push(UnsafeFlow(kid));
             }
         }
 
@@ -191,7 +189,7 @@ pub fn reflow(
 
     let assign_isize_traversal = &AssignISizes { layout_context: &context };
     let assign_bsize_traversal = &AssignBSizes { layout_context: &context };
-    let nodes = [borrowed_flow_to_unsafe_flow(root)];
+    let nodes = [UnsafeFlow(root)];
 
     queue.install(move || {
         rayon::scope(move |scope| {