Mark Rc, Arc, and their Weaks as #[repr(transparent)].

Author: jeremyBanks

library/alloc/src/rc.rs

@@ -306,6 +306,7 @@ struct RcBox<T: ?Sized> {
 #[cfg_attr(not(test), rustc_diagnostic_item = "Rc")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_insignificant_dtor]
+#[repr(transparent)]
 pub struct Rc<T: ?Sized> {
     ptr: NonNull<RcBox<T>>,
     phantom: PhantomData<RcBox<T>>,
@@ -2143,6 +2144,7 @@ impl<T, I: iter::TrustedLen<Item = T>> ToRcSlice<T> for I {
 ///
 /// [`upgrade`]: Weak::upgrade
 #[stable(feature = "rc_weak", since = "1.4.0")]
+#[repr(transparent)]
 pub struct Weak<T: ?Sized> {
     // This is a `NonNull` to allow optimizing the size of this type in enums,
     // but it is not necessarily a valid pointer.

library/alloc/src/sync.rs

@@ -232,6 +232,7 @@ macro_rules! acquire {
 /// [rc_examples]: crate::rc#examples
 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[repr(transparent)]
 pub struct Arc<T: ?Sized> {
     ptr: NonNull<ArcInner<T>>,
     phantom: PhantomData<ArcInner<T>>,
@@ -282,6 +283,7 @@ impl<T: ?Sized> Arc<T> {
 ///
 /// [`upgrade`]: Weak::upgrade
 #[stable(feature = "arc_weak", since = "1.4.0")]
+#[repr(transparent)]
 pub struct Weak<T: ?Sized> {
     // This is a `NonNull` to allow optimizing the size of this type in enums,
     // but it is not necessarily a valid pointer.