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.

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;

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)]

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`.

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`.
     ///

src/libcore/ops/deref.rs

@@ -177,3 +177,19 @@ pub trait DerefMut: Deref {
 impl<T: ?Sized> DerefMut for &mut T {
     fn deref_mut(&mut self) -> &mut T { *self }
 }
+
+/// 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>`.
+#[cfg_attr(not(stage0), lang = "receiver")]
+#[unstable(feature = "receiver_trait", issue = "0")]
+#[doc(hidden)]
+pub trait Receiver {
+    // Empty.
+}
+
+#[unstable(feature = "receiver_trait", issue = "0")]
+impl<T: ?Sized> Receiver for &T {}
+
+#[unstable(feature = "receiver_trait", issue = "0")]
+impl<T: ?Sized> Receiver for &mut T {}

src/libcore/ops/mod.rs

@@ -178,6 +178,9 @@ pub use self::bit::{BitAndAssign, BitOrAssign, BitXorAssign, ShlAssign, ShrAssig
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::deref::{Deref, DerefMut};
 
+#[unstable(feature = "receiver_trait", issue = "0")]
+pub use self::deref::Receiver;
+
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::drop::Drop;
 

src/libcore/pin.rs

@@ -101,7 +101,7 @@
 
 use fmt;
 use marker::Sized;
-use ops::{Deref, DerefMut, CoerceUnsized, DispatchFromDyn};
+use ops::{Deref, DerefMut, Receiver, CoerceUnsized, DispatchFromDyn};
 
 #[doc(inline)]
 pub use marker::Unpin;
@@ -302,6 +302,9 @@ where
     }
 }
 
+#[unstable(feature = "receiver_trait", issue = "0")]
+impl<P: Receiver> Receiver for Pin<P> {}
+
 #[unstable(feature = "pin", issue = "49150")]
 impl<P: fmt::Debug> fmt::Debug for Pin<P> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

src/librustc/middle/lang_items.rs

@@ -301,6 +301,7 @@ language_item_table! {
 
     DerefTraitLangItem,          "deref",              deref_trait,             Target::Trait;
     DerefMutTraitLangItem,       "deref_mut",          deref_mut_trait,         Target::Trait;
+    ReceiverTraitLangItem,       "receiver",           receiver_trait,          Target::Trait;
 
     FnTraitLangItem,             "fn",                 fn_trait,                Target::Trait;
     FnMutTraitLangItem,          "fn_mut",             fn_mut_trait,            Target::Trait;

src/librustc_typeck/check/wfcheck.rs

@@ -13,9 +13,8 @@ use constrained_type_params::{identify_constrained_type_params, Parameter};
 
 use hir::def_id::DefId;
 use rustc::traits::{self, ObligationCauseCode};
-use rustc::ty::{self, Lift, Ty, TyCtxt, TyKind, GenericParamDefKind, TypeFoldable};
+use rustc::ty::{self, Lift, Ty, TyCtxt, TyKind, GenericParamDefKind, TypeFoldable, ToPredicate};
 use rustc::ty::subst::{Subst, Substs};
-use rustc::ty::util::ExplicitSelf;
 use rustc::util::nodemap::{FxHashSet, FxHashMap};
 use rustc::middle::lang_items;
 use rustc::infer::opaque_types::may_define_existential_type;
@@ -749,72 +748,164 @@ fn check_method_receiver<'fcx, 'gcx, 'tcx>(fcx: &FnCtxt<'fcx, 'gcx, 'tcx>,
         &ty::Binder::bind(self_ty)
     );
 
-    let self_arg_ty = sig.inputs()[0];
+    let receiver_ty = sig.inputs()[0];
 
-    let cause = fcx.cause(span, ObligationCauseCode::MethodReceiver);
-    let self_arg_ty = fcx.normalize_associated_types_in(span, &self_arg_ty);
-    let self_arg_ty = fcx.tcx.liberate_late_bound_regions(
+    let receiver_ty = fcx.normalize_associated_types_in(span, &receiver_ty);
+    let receiver_ty = fcx.tcx.liberate_late_bound_regions(
         method.def_id,
-        &ty::Binder::bind(self_arg_ty)
+        &ty::Binder::bind(receiver_ty)
     );
 
-    let mut autoderef = fcx.autoderef(span, self_arg_ty).include_raw_pointers();
-
-    loop {
-        if let Some((potential_self_ty, _)) = autoderef.next() {
-            debug!("check_method_receiver: potential self type `{:?}` to match `{:?}`",
-                potential_self_ty, self_ty);
-
-            if fcx.infcx.can_eq(fcx.param_env, self_ty, potential_self_ty).is_ok() {
-                autoderef.finalize(fcx);
-                if let Some(mut err) = fcx.demand_eqtype_with_origin(
-                    &cause, self_ty, potential_self_ty) {
-                    err.emit();
-                }
-                break
-            }
-        } else {
+    if fcx.tcx.features().arbitrary_self_types {
+        if !receiver_is_valid(fcx, span, receiver_ty, self_ty, true) {
+            // report error, arbitrary_self_types was enabled
             fcx.tcx.sess.diagnostic().mut_span_err(
-                span, &format!("invalid `self` type: {:?}", self_arg_ty))
-            .note(&format!("type must be `{:?}` or a type that dereferences to it", self_ty))
+                span, &format!("invalid `self` type: {:?}", receiver_ty)
+            ).note(&format!("type must be `{:?}` or a type that dereferences to it", self_ty))
             .help("consider changing to `self`, `&self`, `&mut self`, or `self: Box<Self>`")
             .code(DiagnosticId::Error("E0307".into()))
             .emit();
-            return
         }
-    }
-
-    let is_self_ty = |ty| fcx.infcx.can_eq(fcx.param_env, self_ty, ty).is_ok();
-    let self_kind = ExplicitSelf::determine(self_arg_ty, is_self_ty);
-
-    if !fcx.tcx.features().arbitrary_self_types {
-        match self_kind {
-            ExplicitSelf::ByValue |
-            ExplicitSelf::ByReference(_, _) |
-            ExplicitSelf::ByBox => (),
-
-            ExplicitSelf::ByRawPointer(_) => {
+    } else {
+        if !receiver_is_valid(fcx, span, receiver_ty, self_ty, false) {
+            if receiver_is_valid(fcx, span, receiver_ty, self_ty, true) {
+                // report error, would have worked with arbitrary_self_types
                 feature_gate::feature_err(
                     &fcx.tcx.sess.parse_sess,
                     "arbitrary_self_types",
                     span,
                     GateIssue::Language,
-                    "raw pointer `self` is unstable")
+                    &format!(
+                        "`{}` cannot be used as the type of `self` without \
+                            the `arbitrary_self_types` feature",
+                        receiver_ty,
+                    ),
+                ).help("consider changing to `self`, `&self`, `&mut self`, or `self: Box<Self>`")
+                .emit();
+            } else {
+                // report error, would not have worked with arbitrary_self_types
+                fcx.tcx.sess.diagnostic().mut_span_err(
+                    span, &format!("invalid `self` type: {:?}", receiver_ty)
+                ).note(&format!("type must be `{:?}` or a type that dereferences to it", self_ty))
                 .help("consider changing to `self`, `&self`, `&mut self`, or `self: Box<Self>`")
+                .code(DiagnosticId::Error("E0307".into()))
                 .emit();
             }
+        }
+    }
+}
 
-            ExplicitSelf::Other => {
-                feature_gate::feature_err(
-                    &fcx.tcx.sess.parse_sess,
-                    "arbitrary_self_types",
-                    span,
-                    GateIssue::Language,"arbitrary `self` types are unstable")
-                .help("consider changing to `self`, `&self`, `&mut self`, or `self: Box<Self>`")
-                .emit();
+fn receiver_is_valid<'fcx, 'tcx, 'gcx>(
+    fcx: &FnCtxt<'fcx, 'gcx, 'tcx>,
+    span: Span,
+    receiver_ty: Ty<'tcx>,
+    self_ty: Ty<'tcx>,
+    arbitrary_self_types_enabled: bool,
+) -> bool {
+    let cause = fcx.cause(span, traits::ObligationCauseCode::MethodReceiver);
+
+    let can_eq_self = |ty| fcx.infcx.can_eq(fcx.param_env, self_ty, ty).is_ok();
+
+    if can_eq_self(receiver_ty) {
+        if let Some(mut err) = fcx.demand_eqtype_with_origin(&cause, self_ty, receiver_ty) {
+            err.emit();
+        }
+        return true
+    }
+
+    let mut autoderef = fcx.autoderef(span, receiver_ty);
+
+    if arbitrary_self_types_enabled {
+        autoderef = autoderef.include_raw_pointers();
+    }
+
+    // skip the first type, we know its not equal to `self_ty`
+    autoderef.next();
+
+    let potential_self_ty = loop {
+        if let Some((potential_self_ty, _)) = autoderef.next() {
+            debug!("receiver_is_valid: potential self type `{:?}` to match `{:?}`",
+                potential_self_ty, self_ty);
+
+            if can_eq_self(potential_self_ty) {
+                break potential_self_ty
+            }
+        } else {
+            debug!("receiver_is_valid: type `{:?}` does not deref to `{:?}`",
+                receiver_ty, self_ty);
+            return false
+        }
+    };
+
+    if !arbitrary_self_types_enabled {
+        // check that receiver_ty: Receiver<Target=self_ty>
+
+        let receiver_trait_def_id = match fcx.tcx.lang_items().receiver_trait() {
+            Some(did) => did,
+            None => {
+                debug!("receiver_is_valid: missing Receiver trait");
+                return false
             }
+        };
+
+        let receiver_trait_ref = ty::TraitRef{
+            def_id: receiver_trait_def_id,
+            substs: fcx.tcx.mk_substs_trait(receiver_ty, &[]),
+        };
+
+        let receiver_obligation = traits::Obligation::new(
+            cause.clone(),
+            fcx.param_env,
+            receiver_trait_ref.to_predicate()
+        );
+
+        if !fcx.predicate_must_hold(&receiver_obligation) {
+            debug!("receiver_is_valid: type `{:?}` does not implement `Receiver` trait",
+                receiver_ty);
+            return false
+        }
+
+        let deref_trait_def_id = match fcx.tcx.lang_items().deref_trait() {
+            Some(did) => did,
+            None => {
+                debug!("receiver_is_valid: missing Deref trait");
+                return false
+            }
+        };
+
+        let deref_trait_ref = ty::TraitRef {
+            def_id: deref_trait_def_id,
+            substs: fcx.tcx.mk_substs_trait(receiver_ty, &[]),
+        };
+
+        let projection_ty = ty::ProjectionTy::from_ref_and_name(
+            fcx.tcx, deref_trait_ref, ast::Ident::from_str("Target")
+        );
+
+        let projection_predicate = ty::Binder::dummy(ty::ProjectionPredicate {
+            projection_ty, ty: self_ty
+        }).to_predicate();
+
+        let deref_obligation = traits::Obligation::new(
+            cause.clone(),
+            fcx.param_env,
+            projection_predicate,
+        );
+
+        if !fcx.predicate_must_hold(&deref_obligation) {
+            debug!("receiver_is_valid: type `{:?}` does not directly deref to `{:?}`",
+                receiver_ty, self_ty);
+            return false
         }
     }
+
+    if let Some(mut err) = fcx.demand_eqtype_with_origin(&cause, self_ty, potential_self_ty) {
+        err.emit();
+    }
+
+    autoderef.finalize(fcx);
+
+    true
 }
 
 fn check_variances_for_type_defn<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,

src/test/run-pass/arbitrary_self_types_stdlib_pointers.rs

@@ -8,7 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(arbitrary_self_types)]
 #![feature(pin)]
 #![feature(rustc_attrs)]
 
@@ -23,6 +22,7 @@ trait Trait {
     fn by_arc(self: Arc<Self>) -> i64;
     fn by_pin_mut(self: Pin<&mut Self>) -> i64;
     fn by_pin_box(self: Pin<Box<Self>>) -> i64;
+    fn by_pin_pin_pin_ref(self: Pin<Pin<Pin<&Self>>>) -> i64;
 }
 
 impl Trait for i64 {
@@ -38,6 +38,9 @@ impl Trait for i64 {
     fn by_pin_box(self: Pin<Box<Self>>) -> i64 {
         *self
     }
+    fn by_pin_pin_pin_ref(self: Pin<Pin<Pin<&Self>>>) -> i64 {
+        *self
+    }
 }
 
 fn main() {
@@ -53,4 +56,8 @@ fn main() {
 
     let pin_box = Into::<Pin<Box<i64>>>::into(Box::new(4i64)) as Pin<Box<dyn Trait>>;
     assert_eq!(4, pin_box.by_pin_box());
+
+    let value = 5i64;
+    let pin_pin_pin_ref = Pin::new(Pin::new(Pin::new(&value))) as Pin<Pin<Pin<&dyn Trait>>>;
+    assert_eq!(5, pin_pin_pin_ref.by_pin_pin_pin_ref());
 }