style: Don't panic when creating empty ThinArcs.

The change from [T; 1] to [T; 0] shouldn't change behavior since they have the
right alignment and we never poke at that particular array, but feels more
correct to avoid creating types that point to uninitialized data or outside of
their allocation, now that we allow empty slices.

Differential Revision: https://phabricator.services.mozilla.com/D30352

components/servo_arc/lib.rs

@@ -168,7 +168,7 @@ impl<T> Arc<T> {
     pub fn new(data: T) -> Self {
         let x = Box::new(ArcInner {
             count: atomic::AtomicUsize::new(1),
-            data: data,
+            data,
         });
         unsafe {
             Arc {
@@ -610,7 +610,7 @@ impl<H, T> Arc<HeaderSlice<H, [T]>> {
         let size = {
             // First, determine the alignment of a hypothetical pointer to a
             // HeaderSlice.
-            let fake_slice_ptr_align: usize = mem::align_of::<ArcInner<HeaderSlice<H, [T; 1]>>>();
+            let fake_slice_ptr_align: usize = mem::align_of::<ArcInner<HeaderSlice<H, [T; 0]>>>();
 
             // Next, synthesize a totally garbage (but properly aligned) pointer
             // to a sequence of T.
@@ -667,23 +667,24 @@ impl<H, T> Arc<HeaderSlice<H, [T]>> {
             };
             ptr::write(&mut ((*ptr).count), count);
             ptr::write(&mut ((*ptr).data.header), header);
-            let mut current: *mut T = &mut (*ptr).data.slice[0];
-            for _ in 0..num_items {
-                ptr::write(
-                    current,
-                    items
-                        .next()
-                        .expect("ExactSizeIterator over-reported length"),
-                );
-                current = current.offset(1);
+            if num_items != 0 {
+                let mut current: *mut T = &mut (*ptr).data.slice[0];
+                for _ in 0..num_items {
+                    ptr::write(
+                        current,
+                        items
+                            .next()
+                            .expect("ExactSizeIterator over-reported length"),
+                    );
+                    current = current.offset(1);
+                }
+                // We should have consumed the buffer exactly.
+                debug_assert_eq!(current as *mut u8, buffer.offset(size as isize));
             }
             assert!(
                 items.next().is_none(),
                 "ExactSizeIterator under-reported length"
             );
-
-            // We should have consumed the buffer exactly.
-            debug_assert_eq!(current as *mut u8, buffer.offset(size as isize));
         }
 
         // Return the fat Arc.
@@ -772,11 +773,13 @@ type HeaderSliceWithLength<H, T> = HeaderSlice<HeaderWithLength<H>, T>;
 /// have a thin pointer instead, perhaps for FFI compatibility
 /// or space efficiency.
 ///
+/// Note that we use `[T; 0]` in order to have the right alignment for `T`.
+///
 /// `ThinArc` solves this by storing the length in the allocation itself,
 /// via `HeaderSliceWithLength`.
 #[repr(C)]
 pub struct ThinArc<H, T> {
-    ptr: ptr::NonNull<ArcInner<HeaderSliceWithLength<H, [T; 1]>>>,
+    ptr: ptr::NonNull<ArcInner<HeaderSliceWithLength<H, [T; 0]>>>,
     phantom: PhantomData<(H, T)>,
 }
 
@@ -787,7 +790,7 @@ unsafe impl<H: Sync + Send, T: Sync + Send> Sync for ThinArc<H, T> {}
 //
 // See the comment around the analogous operation in from_header_and_iter.
 fn thin_to_thick<H, T>(
-    thin: *mut ArcInner<HeaderSliceWithLength<H, [T; 1]>>,
+    thin: *mut ArcInner<HeaderSliceWithLength<H, [T; 0]>>,
 ) -> *mut ArcInner<HeaderSliceWithLength<H, [T]>> {
     let len = unsafe { (*thin).data.header.length };
     let fake_slice: *mut [T] = unsafe { slice::from_raw_parts_mut(thin as *mut T, len) };
@@ -905,7 +908,7 @@ impl<H, T> Arc<HeaderSliceWithLength<H, [T]>> {
         let thin_ptr = fat_ptr as *mut [usize] as *mut usize;
         ThinArc {
             ptr: unsafe {
-                ptr::NonNull::new_unchecked(thin_ptr as *mut ArcInner<HeaderSliceWithLength<H, [T; 1]>>)
+                ptr::NonNull::new_unchecked(thin_ptr as *mut ArcInner<HeaderSliceWithLength<H, [T; 0]>>)
             },
             phantom: PhantomData,
         }
@@ -1320,6 +1323,17 @@ mod tests {
         }
     }
 
+    #[test]
+    fn empty_thin() {
+        let header = HeaderWithLength::new(100u32, 0);
+        let x = Arc::from_header_and_iter(header, std::iter::empty::<i32>());
+        let y = Arc::into_thin(x.clone());
+        assert_eq!(y.header.header, 100);
+        assert!(y.slice.is_empty());
+        assert_eq!(x.header.header, 100);
+        assert!(x.slice.is_empty());
+    }
+
     #[test]
     fn slices_and_thin() {
         let mut canary = atomic::AtomicUsize::new(0);