Fix layout differences of Header<Atomic> v.s. Header<NonAtomic>

This fixes #58, which shows UB
when converting between non-atomic and atomic `Tendril`.
This conversion is exposed without `unsafe` through `SendTendril`
and implemented with `transmute`, so matching layout is essential.

src/tendril.rs

@@ -69,29 +69,25 @@ pub unsafe trait Atomicity: 'static {
 ///
 /// This is akin to using `Rc` for reference counting.
 #[repr(C)]
-pub struct NonAtomic(Cell<PackedUsize>);
-
-#[repr(C, packed)]
-#[derive(Copy, Clone)]
-struct PackedUsize(usize);
+pub struct NonAtomic(Cell<usize>);
 
 unsafe impl Atomicity for NonAtomic {
     #[inline]
     fn new() -> Self {
-        NonAtomic(Cell::new(PackedUsize(1)))
+        NonAtomic(Cell::new(1))
     }
 
     #[inline]
     fn increment(&self) -> usize {
-        let value = self.0.get().0;
-        self.0.set(PackedUsize(value.checked_add(1).expect(OFLOW)));
+        let value = self.0.get();
+        self.0.set(value.checked_add(1).expect(OFLOW));
         value
     }
 
     #[inline]
     fn decrement(&self) -> usize {
-        let value = self.0.get().0;
-        self.0.set(PackedUsize(value - 1));
+        let value = self.0.get();
+        self.0.set(value - 1);
         value
     }
 
@@ -1712,7 +1708,7 @@ mod test {
             mem::size_of::<Header<Atomic>>(),
         );
         assert_eq!(
-            mem::size_of::<*const ()>() + 4,
+            mem::size_of::<Header<Atomic>>(),
             mem::size_of::<Header<NonAtomic>>(),
         );
     }
@@ -2448,6 +2444,16 @@ mod test {
         assert_eq!("this is a string", &*t);
     }
 
+    /// https://github.com/servo/tendril/issues/58
+    #[test]
+    fn issue_58() {
+        let data = "<p><i>Hello!</p>, World!</i>";
+        let s: Tendril<fmt::UTF8, NonAtomic> = data.into();
+        assert_eq!(&*s, data);
+        let s: Tendril<fmt::UTF8, Atomic> = s.into_send().into();
+        assert_eq!(&*s, data);
+    }
+
     #[test]
     fn inline_send() {
         let s = "x".to_tendril();