Add `Shared` pointer and have `{Arc, Rc}` use it

This change has two consequences:

1. It makes `Arc<T>` and `Rc<T>` covariant in `T`.

2. It causes the compiler to reject code that was unsound with respect
to dropck. See compile-fail/issue-29106.rs for an example of code that
no longer compiles. Because of this, this is a [breaking-change].

Fixes #29037.
Fixes #29106.

1 files changed, 6 insertions, 5 deletions

diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index d8f10e6780f..33ca80ba372 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -79,9 +79,8 @@ use core::cmp::Ordering;
 use core::mem::{align_of_val, size_of_val};
 use core::intrinsics::{drop_in_place, abort};
 use core::mem;
-use core::nonzero::NonZero;
 use core::ops::{Deref, CoerceUnsized};
-use core::ptr;
+use core::ptr::{self, Shared};
 use core::marker::Unsize;
 use core::hash::{Hash, Hasher};
 use core::{usize, isize};
@@ -124,12 +123,13 @@ const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 pub struct Arc<T: ?Sized> {
     // FIXME #12808: strange name to try to avoid interfering with
     // field accesses of the contained type via Deref
-    _ptr: NonZero<*mut ArcInner<T>>,
+    _ptr: Shared<ArcInner<T>>,
 }
 
 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> { }
 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> { }
 
+#[cfg(not(stage0))] // remove cfg after new snapshot
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
 
 /// A weak pointer to an `Arc`.
@@ -141,12 +141,13 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
 pub struct Weak<T: ?Sized> {
     // FIXME #12808: strange name to try to avoid interfering with
     // field accesses of the contained type via Deref
-    _ptr: NonZero<*mut ArcInner<T>>,
+    _ptr: Shared<ArcInner<T>>,
 }
 
 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> { }
 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> { }
 
+#[cfg(not(stage0))] // remove cfg after new snapshot
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -190,7 +191,7 @@ impl<T> Arc<T> {
             weak: atomic::AtomicUsize::new(1),
             data: data,
         };
-        Arc { _ptr: unsafe { NonZero::new(Box::into_raw(x)) } }
+        Arc { _ptr: unsafe { Shared::new(Box::into_raw(x)) } }
     }
 
     /// Unwraps the contained value if the `Arc<T>` has only one strong reference.