Stabilize `Rc`, `Arc` and `Pin` as method receivers

This lets you write methods using `self: Rc<Self>`, `self: Arc<Self>`, `self: Pin<&mut Self>`, `self: Pin<Box<Self>`, and other combinations involving `Pin` and another stdlib receiver type, without needing the `arbitrary_self_types`. Other user-created receiver types can be used, but they still require the feature flag to use.

This is implemented by introducing a new trait, `Receiver`, which the method receiver's type must implement if the `arbitrary_self_types` feature is not enabled. To keep composed receiver types such as `&Arc<Self>` unstable, the receiver type is also required to implement `Deref<Target=Self>` when the feature flag is not enabled.

This lets you use `self: Rc<Self>` and `self: Arc<Self>` in stable Rust, which was not allowed previously. It was agreed that they would be stabilized in #55786. `self: Pin<&Self>` and other pinned receiver types do not require the `arbitrary_self_types` feature, but they cannot be used on stable because `Pin` still requires the `pin` feature.

4 files changed, 15 insertions, 3 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 83adcce5c74..f1581310b48 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -77,7 +77,9 @@ use core::iter::{Iterator, FromIterator, FusedIterator};
 use core::marker::{Unpin, Unsize};
 use core::mem;
 use core::pin::Pin;
-use core::ops::{CoerceUnsized, DispatchFromDyn, Deref, DerefMut, Generator, GeneratorState};
+use core::ops::{
+    CoerceUnsized, DispatchFromDyn, Deref, DerefMut, Receiver, Generator, GeneratorState
+};
 use core::ptr::{self, NonNull, Unique};
 use core::task::{LocalWaker, Poll};
 
@@ -583,6 +585,9 @@ impl<T: ?Sized> DerefMut for Box<T> {
     }
 }
 
+#[unstable(feature = "receiver_trait", issue = "0")]
+impl<T: ?Sized> Receiver for Box<T> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I: Iterator + ?Sized> Iterator for Box<I> {
     type Item = I::Item;
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index abacc62c856..e00e430fab6 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -104,6 +104,7 @@
 #![feature(ptr_internals)]
 #![feature(ptr_offset_from)]
 #![feature(rustc_attrs)]
+#![feature(receiver_trait)]
 #![feature(specialization)]
 #![feature(split_ascii_whitespace)]
 #![feature(staged_api)]
diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 6769a70ddbe..3fc70f4ac37 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -253,7 +253,7 @@ use core::intrinsics::abort;
 use core::marker;
 use core::marker::{Unpin, Unsize, PhantomData};
 use core::mem::{self, align_of_val, forget, size_of_val};
-use core::ops::Deref;
+use core::ops::{Deref, Receiver};
 use core::ops::{CoerceUnsized, DispatchFromDyn};
 use core::pin::Pin;
 use core::ptr::{self, NonNull};
@@ -813,6 +813,9 @@ impl<T: ?Sized> Deref for Rc<T> {
     }
 }
 
+#[unstable(feature = "receiver_trait", issue = "0")]
+impl<T: ?Sized> Receiver for Rc<T> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
     /// Drops the `Rc`.
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index e596694fb9d..55737016608 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -24,7 +24,7 @@ use core::fmt;
 use core::cmp::Ordering;
 use core::intrinsics::abort;
 use core::mem::{self, align_of_val, size_of_val};
-use core::ops::Deref;
+use core::ops::{Deref, Receiver};
 use core::ops::{CoerceUnsized, DispatchFromDyn};
 use core::pin::Pin;
 use core::ptr::{self, NonNull};
@@ -767,6 +767,9 @@ impl<T: ?Sized> Deref for Arc<T> {
     }
 }
 
+#[unstable(feature = "receiver_trait", issue = "0")]
+impl<T: ?Sized> Receiver for Arc<T> {}
+
 impl<T: Clone> Arc<T> {
     /// Makes a mutable reference into the given `Arc`.
     ///