Add repr(align(2)) to RcInner and ArcInner

Author: taiki-e

library/alloc/src/rc.rs

@@ -277,7 +277,9 @@ use crate::vec::Vec;
 // This is repr(C) to future-proof against possible field-reordering, which
 // would interfere with otherwise safe [into|from]_raw() of transmutable
 // inner types.
-#[repr(C)]
+// repr(align(2)) (forcing alignment to at least 2) is required because usize
+// has 1-byte alignment on AVR.
+#[repr(C, align(2))]
 struct RcInner<T: ?Sized> {
  strong: Cell<usize>,
  weak: Cell<usize>,

library/alloc/src/sync.rs

@@ -341,7 +341,7 @@ pub struct Weak<
  // but it is not necessarily a valid pointer.
  // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
  // to allocate space on the heap. That's not a value a real pointer
- // will ever have because RcInner has alignment at least 2.
+ // will ever have because ArcInner has alignment at least 2.
  ptr: NonNull<ArcInner<T>>,
  alloc: A,
 }
@@ -366,7 +366,9 @@ impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> {
 // This is repr(C) to future-proof against possible field-reordering, which
 // would interfere with otherwise safe [into|from]_raw() of transmutable
 // inner types.
-#[repr(C)]
+// Unlike RcInner, repr(align(2)) is not strictly required because atomic types
+// have the alignment same as its size, but we use it for consistency and clarity.
+#[repr(C, align(2))]
 struct ArcInner<T: ?Sized> {
  strong: Atomic<usize>,
 
@@ -1622,9 +1624,9 @@ impl<T: ?Sized, A: Allocator> Arc<T, A> {
  pub fn as_ptr(this: &Self) -> *const T {
  let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
 
- // SAFETY: This cannot go through Deref::deref or RcInnerPtr::inner because
+ // SAFETY: This cannot go through Deref::deref or ArcInnerPtr::inner because
  // this is required to retain raw/mut provenance such that e.g. `get_mut` can
- // write through the pointer after the Rc is recovered through `from_raw`.
+ // write through the pointer after the Arc is recovered through `from_raw`.
  unsafe { &raw mut (*ptr).data }
  }
 
@@ -2459,7 +2461,7 @@ impl<T: ?Sized, A: Allocator> Arc<T, A> {
  /// If any other `Arc` or [`Weak`] pointers to the same allocation exist, then
  /// they must not be dereferenced or have active borrows for the duration
  /// of the returned borrow, and their inner type must be exactly the same as the
- /// inner type of this Rc (including lifetimes). This is trivially the case if no
+ /// inner type of this Arc (including lifetimes). This is trivially the case if no
  /// such pointers exist, for example immediately after `Arc::new`.
  ///
  /// # Examples
@@ -3031,7 +3033,7 @@ impl<T: ?Sized, A: Allocator> Weak<T, A> {
  // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
  // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
  let offset = unsafe { data_offset(ptr) };
- // Thus, we reverse the offset to get the whole RcInner.
+ // Thus, we reverse the offset to get the whole ArcInner.
  // SAFETY: the pointer originated from a Weak, so this offset is safe.
  unsafe { ptr.byte_sub(offset) as *mut ArcInner<T> }
  };
@@ -4024,7 +4026,7 @@ impl<T: ?Sized, A: Allocator> Unpin for Arc<T, A> {}
 /// valid instance of T, but the T is allowed to be dropped.
 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> usize {
  // Align the unsized value to the end of the ArcInner.
- // Because RcInner is repr(C), it will always be the last field in memory.
+ // Because ArcInner is repr(C), it will always be the last field in memory.
  // SAFETY: since the only unsized types possible are slices, trait objects,
  // and extern types, the input safety requirement is currently enough to
  // satisfy the requirements of align_of_val_raw; this is an implementation