Rename Receiver -> LegacyReceiver

As part of the "arbitrary self types v2" project, we are going to
replace the current `Receiver` trait with a new mechanism based on a
new, different `Receiver` trait.

This PR renames the old trait to get it out the way. Naming is hard.
Options considered included:
* HardCodedReceiver (because it should only be used for things in the
  standard library, and hence is sort-of hard coded)
* LegacyReceiver
* TargetLessReceiver
* OldReceiver

These are all bad names, but fortunately this will be temporary.
Assuming the new mechanism proceeds to stabilization as intended, the
legacy trait will be removed altogether.

Although we expect this trait to be used only in the standard library,
we suspect it may be in use elsehwere, so we're landing this change
separately to identify any surprising breakages.

It's known that this trait is used within the Rust for Linux project; a
patch is in progress to remove their dependency.

This is a part of the arbitrary self types v2 project,
https://github.com/rust-lang/rfcs/pull/3519
https://github.com/rust-lang/rust/issues/44874

r? @wesleywiser

library/core/src/ops/deref.rs

@@ -297,15 +297,21 @@ unsafe impl<T: ?Sized> DerefPure for &mut T {}
 /// Indicates that a struct can be used as a method receiver, without the
 /// `arbitrary_self_types` feature. This is implemented by stdlib pointer types like `Box<T>`,
 /// `Rc<T>`, `&T`, and `Pin<P>`.
-#[lang = "receiver"]
-#[unstable(feature = "receiver_trait", issue = "none")]
+///
+/// This trait will shortly be removed and replaced with a more generic
+/// facility based around the current "arbitrary self types" unstable feature.
+/// That new facility will use a replacement trait called `Receiver` which is
+/// why this is now named `LegacyReceiver`.
+#[cfg_attr(bootstrap, lang = "receiver")]
+#[cfg_attr(not(bootstrap), lang = "legacy_receiver")]
+#[unstable(feature = "legacy_receiver_trait", issue = "none")]
 #[doc(hidden)]
-pub trait Receiver {
+pub trait LegacyReceiver {
     // Empty.
 }
 
-#[unstable(feature = "receiver_trait", issue = "none")]
-impl<T: ?Sized> Receiver for &T {}
+#[unstable(feature = "legacy_receiver_trait", issue = "none")]
+impl<T: ?Sized> LegacyReceiver for &T {}
 
-#[unstable(feature = "receiver_trait", issue = "none")]
-impl<T: ?Sized> Receiver for &mut T {}
+#[unstable(feature = "legacy_receiver_trait", issue = "none")]
+impl<T: ?Sized> LegacyReceiver for &mut T {}

library/core/src/ops/mod.rs

@@ -168,8 +168,8 @@ pub use self::control_flow::ControlFlow;
 pub use self::coroutine::{Coroutine, CoroutineState};
 #[unstable(feature = "deref_pure_trait", issue = "87121")]
 pub use self::deref::DerefPure;
-#[unstable(feature = "receiver_trait", issue = "none")]
-pub use self::deref::Receiver;
+#[unstable(feature = "legacy_receiver_trait", issue = "none")]
+pub use self::deref::LegacyReceiver;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::deref::{Deref, DerefMut};
 #[stable(feature = "rust1", since = "1.0.0")]

library/core/src/pin.rs

@@ -921,7 +921,7 @@
 #![stable(feature = "pin", since = "1.33.0")]
 
 use crate::hash::{Hash, Hasher};
-use crate::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn, Receiver};
+use crate::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn, LegacyReceiver};
 #[allow(unused_imports)]
 use crate::{
     cell::{RefCell, UnsafeCell},
@@ -1692,8 +1692,8 @@ impl<Ptr: DerefMut<Target: Unpin>> DerefMut for Pin<Ptr> {
 #[unstable(feature = "deref_pure_trait", issue = "87121")]
 unsafe impl<Ptr: DerefPure> DerefPure for Pin<Ptr> {}
 
-#[unstable(feature = "receiver_trait", issue = "none")]
-impl<Ptr: Receiver> Receiver for Pin<Ptr> {}
+#[unstable(feature = "legacy_receiver_trait", issue = "none")]
+impl<Ptr: LegacyReceiver> LegacyReceiver for Pin<Ptr> {}
 
 #[stable(feature = "pin", since = "1.33.0")]
 impl<Ptr: fmt::Debug> fmt::Debug for Pin<Ptr> {