Make Rc and Arc #[fundamental]

src/liballoc/rc.rs

@@ -283,6 +283,7 @@ struct RcBox<T: ?Sized> {
 /// [get_mut]: #method.get_mut
 #[cfg_attr(not(test), lang = "rc")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[fundamental]
 pub struct Rc<T: ?Sized> {
     ptr: NonNull<RcBox<T>>,
     phantom: PhantomData<T>,

src/liballoc/sync.rs

@@ -200,6 +200,7 @@ const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 /// [rc_examples]: ../../std/rc/index.html#examples
 #[cfg_attr(not(test), lang = "arc")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[fundamental]
 pub struct Arc<T: ?Sized> {
     ptr: NonNull<ArcInner<T>>,
     phantom: PhantomData<T>,

src/libserialize/serialize.rs

@@ -921,4 +921,7 @@ impl<T: UseSpecializedDecodable> Decodable for T {
 impl<'a, T: ?Sized + Encodable> UseSpecializedEncodable for &'a T {}
 impl<T: ?Sized + Encodable> UseSpecializedEncodable for Box<T> {}
 impl<T: Decodable> UseSpecializedDecodable for Box<T> {}
-
+impl<T: ?Sized + Encodable> UseSpecializedEncodable for Rc<T> {}
+impl<T: Decodable> UseSpecializedDecodable for Rc<T> {}
+impl<T: ?Sized + Encodable> UseSpecializedEncodable for Arc<T> {}
+impl<T: Decodable> UseSpecializedDecodable for Arc<T> {}

src/test/ui/issues/issue-41974.stderr

@@ -1,13 +1,13 @@
-error[E0119]: conflicting implementations of trait `std::ops::Drop` for type `std::boxed::Box<_>`:
+error[E0119]: conflicting implementations of trait `std::ops::Drop` for type `std::sync::Arc<_>`:
   --> $DIR/issue-41974.rs:17:1
    |
 LL | impl<T> Drop for T where T: A { //~ ERROR E0119
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: conflicting implementation in crate `alloc`:
-           - impl<T> std::ops::Drop for std::boxed::Box<T>
+           - impl<T> std::ops::Drop for std::sync::Arc<T>
              where T: ?Sized;
-   = note: downstream crates may implement trait `A` for type `std::boxed::Box<_>`
+   = note: downstream crates may implement trait `A` for type `std::sync::Arc<_>`
 
 error[E0120]: the Drop trait may only be implemented on structures
   --> $DIR/issue-41974.rs:17:18