Replace CoerceSized trait with DispatchFromDyn

Rename `CoerceSized` to `DispatchFromDyn`, and reverse the direction so that, for example, you write

```
impl<T: Unsize<U>, U> DispatchFromDyn<*const U> for *const T {}
```

instead of

```
impl<T: Unsize<U>, U> DispatchFromDyn<*const T> for *const U {}
```

this way the trait is really just a subset of `CoerceUnsized`.

The checks in object_safety.rs are updated for the new trait, and some documentation and method names in there are updated for the new trait name — e.g. `receiver_is_coercible` is now called `receiver_is_dispatchable`. Since the trait now works in the opposite direction, some code had to updated here for that too.

I did not update the error messages for invalid `CoerceSized` (now `DispatchFromDyn`) implementations, except to find/replace `CoerceSized` with `DispatchFromDyn`. Will ask for suggestions in the PR thread.

src/liballoc/boxed.rs

@@ -77,7 +77,7 @@ use core::iter::FusedIterator;
 use core::marker::{Unpin, Unsize};
 use core::mem;
 use core::pin::Pin;
-use core::ops::{CoerceUnsized, CoerceSized, Deref, DerefMut, Generator, GeneratorState};
+use core::ops::{CoerceUnsized, DispatchFromDyn, Deref, DerefMut, Generator, GeneratorState};
 use core::ptr::{self, NonNull, Unique};
 use core::task::{LocalWaker, Poll};
 
@@ -696,8 +696,8 @@ impl<'a, A, R> FnOnce<A> for Box<dyn FnBox<A, Output = R> + Send + 'a> {
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
 
-#[unstable(feature = "coerce_sized", issue = "0")]
-impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<Box<T>> for Box<U> {}
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
 
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl<T: Clone> Clone for Box<[T]> {

src/liballoc/lib.rs

@@ -86,7 +86,7 @@
 #![feature(box_syntax)]
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
-#![feature(coerce_sized)]
+#![feature(dispatch_from_dyn)]
 #![feature(core_intrinsics)]
 #![feature(custom_attribute)]
 #![feature(dropck_eyepatch)]

src/liballoc/rc.rs

@@ -255,7 +255,7 @@ 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::{CoerceUnsized, CoerceSized};
+use core::ops::{CoerceUnsized, DispatchFromDyn};
 use core::pin::Pin;
 use core::ptr::{self, NonNull};
 use core::convert::From;
@@ -297,8 +297,8 @@ impl<T: ?Sized> !marker::Sync for Rc<T> {}
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
 
-#[unstable(feature = "coerce_sized", issue = "0")]
-impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<Rc<T>> for Rc<U> {}
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
 
 impl<T> Rc<T> {
     /// Constructs a new `Rc<T>`.
@@ -1179,8 +1179,8 @@ impl<T: ?Sized> !marker::Sync for Weak<T> {}
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
 
-#[unstable(feature = "coerce_sized", issue = "0")]
-impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<Weak<T>> for Weak<U> {}
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
 
 impl<T> Weak<T> {
     /// Constructs a new `Weak<T>`, without allocating any memory.

src/liballoc/sync.rs

@@ -25,7 +25,7 @@ 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::{CoerceUnsized, CoerceSized};
+use core::ops::{CoerceUnsized, DispatchFromDyn};
 use core::pin::Pin;
 use core::ptr::{self, NonNull};
 use core::marker::{Unpin, Unsize, PhantomData};
@@ -214,8 +214,8 @@ unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
 
-#[unstable(feature = "coerce_sized", issue = "0")]
-impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<Arc<T>> for Arc<U> {}
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
 
 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
@@ -257,8 +257,8 @@ unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
 
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
-#[unstable(feature = "coerce_sized", issue = "0")]
-impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<Weak<T>> for Weak<U> {}
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
 
 #[stable(feature = "arc_weak", since = "1.4.0")]
 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {

src/libcore/nonzero.rs

@@ -10,7 +10,7 @@
 
 //! Exposes the NonZero lang item which provides optimization hints.
 
-use ops::{CoerceUnsized, CoerceSized};
+use ops::{CoerceUnsized, DispatchFromDyn};
 
 /// A wrapper type for raw pointers and integers that will never be
 /// NULL or 0 that might allow certain optimizations.
@@ -21,4 +21,4 @@ pub(crate) struct NonZero<T>(pub(crate) T);
 
 impl<T: CoerceUnsized<U>, U> CoerceUnsized<NonZero<U>> for NonZero<T> {}
 
-impl<T: CoerceUnsized<U>, U: CoerceSized<T>> CoerceSized<NonZero<T>> for NonZero<U> {}
+impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<NonZero<U>> for NonZero<T> {}

src/libcore/ops/mod.rs

@@ -202,5 +202,5 @@ pub use self::generator::{Generator, GeneratorState};
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 pub use self::unsize::CoerceUnsized;
 
-#[unstable(feature = "coerce_sized", issue = "0")]
-pub use self::unsize::CoerceSized;
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+pub use self::unsize::DispatchFromDyn;

src/libcore/ops/unsize.rs

@@ -79,32 +79,32 @@ impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *mut T {}
 impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *const T {}
 
 
-/// Pointers to unsized types that can be coerced to a pointer to a sized type,
-/// as long as pointee is actually a value of that sized type. This is used for
-/// object safety, to check that a method's receiver type can be coerced from the version
-///  where `Self = dyn Trait` to the version where `Self = T`, the erased, sized type
-/// of the underlying object.
+/// This is used for object safety, to check that a method's receiver type can be dispatched on.
 ///
-/// `CoerceSized` is implemented for:
-/// - `&[T]` is `CoerceSized<&[T; N]>` for any `N`
-/// - `&Trait` is `CoerceSized<&T>` for any `T: Trait`
-/// - and similarly for `&mut T`, `*const T`, `*mut T`, `Box<T>`, `Rc<T>`, `Arc<T>`
-#[unstable(feature = "coerce_sized", issue = "0")]
-#[cfg_attr(not(stage0), lang = "coerce_sized")]
-pub trait CoerceSized<T> where T: CoerceUnsized<Self> {
+/// example impl:
+///
+/// ```
+/// impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T>
+/// where
+///     T: Unsize<U>,
+/// {}
+/// ```
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+#[cfg_attr(not(stage0), lang = "dispatch_from_dyn")]
+pub trait DispatchFromDyn<T> {
     // Empty.
 }
 
-// &U -> &T
-#[unstable(feature = "coerce_sized", issue = "0")]
-impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceSized<&'a T> for &'a U {}
-// &mut U -> &mut T
-#[unstable(feature = "coerce_sized", issue = "0")]
-impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceSized<&'a mut T> for &'a mut U {}
-// *const U -> *const T
-#[unstable(feature = "coerce_sized", issue = "0")]
-impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceSized<*const T> for *const U {}
-// *mut U -> *mut T
-#[unstable(feature = "coerce_sized", issue = "0")]
-impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceSized<*mut T> for *mut U {}
+// &T -> &U
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> DispatchFromDyn<&'a U> for &'a T {}
+// &mut T -> &mut U
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> DispatchFromDyn<&'a mut U> for &'a mut T {}
+// *const T -> *const U
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized+Unsize<U>, U: ?Sized> DispatchFromDyn<*const U> for *const T {}
+// *mut T -> *mut U
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized+Unsize<U>, U: ?Sized> DispatchFromDyn<*mut U> for *mut T {}
 

src/libcore/pin.rs

@@ -91,7 +91,7 @@
 
 use fmt;
 use marker::Sized;
-use ops::{Deref, DerefMut, CoerceUnsized, CoerceSized};
+use ops::{Deref, DerefMut, CoerceUnsized, DispatchFromDyn};
 
 #[doc(inline)]
 pub use marker::Unpin;
@@ -325,10 +325,9 @@ where
 {}
 
 #[unstable(feature = "pin", issue = "49150")]
-impl<'a, P, U> CoerceSized<Pin<P>> for Pin<U>
+impl<'a, P, U> DispatchFromDyn<Pin<U>> for Pin<P>
 where
-    P: CoerceUnsized<U>,
-    U: CoerceSized<P>,
+    P: DispatchFromDyn<U>,
 {}
 
 #[unstable(feature = "pin", issue = "49150")]

src/libcore/ptr.rs

@@ -75,7 +75,7 @@
 
 use convert::From;
 use intrinsics;
-use ops::{CoerceUnsized, CoerceSized};
+use ops::{CoerceUnsized, DispatchFromDyn};
 use fmt;
 use hash;
 use marker::{PhantomData, Unsize};
@@ -2796,7 +2796,7 @@ impl<T: ?Sized> Copy for Unique<T> { }
 impl<T: ?Sized, U: ?Sized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsize<U> { }
 
 #[unstable(feature = "ptr_internals", issue = "0")]
-impl<T: ?Sized, U: ?Sized> CoerceSized<Unique<T>> for Unique<U> where T: Unsize<U> { }
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Unique<U>> for Unique<T> where T: Unsize<U> { }
 
 #[unstable(feature = "ptr_internals", issue = "0")]
 impl<T: ?Sized> fmt::Pointer for Unique<T> {
@@ -2954,8 +2954,8 @@ impl<T: ?Sized> Copy for NonNull<T> { }
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized, U: ?Sized> CoerceUnsized<NonNull<U>> for NonNull<T> where T: Unsize<U> { }
 
-#[unstable(feature = "coerce_sized", issue = "0")]
-impl<T: ?Sized, U: ?Sized> CoerceSized<NonNull<T>> for NonNull<U> where T: Unsize<U> { }
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<NonNull<U>> for NonNull<T> where T: Unsize<U> { }
 
 #[stable(feature = "nonnull", since = "1.25.0")]
 impl<T: ?Sized> fmt::Debug for NonNull<T> {

src/librustc/middle/lang_items.rs

@@ -271,7 +271,7 @@ language_item_table! {
     DropTraitLangItem,           "drop",               drop_trait,              Target::Trait;
 
     CoerceUnsizedTraitLangItem,  "coerce_unsized",     coerce_unsized_trait,    Target::Trait;
-    CoerceSizedTraitLangItem,    "coerce_sized",       coerce_sized_trait,      Target::Trait;
+    DispatchFromDynTraitLangItem,"dispatch_from_dyn",  dispatch_from_dyn_trait, Target::Trait;
 
     AddTraitLangItem,            "add",                add_trait,               Target::Trait;
     SubTraitLangItem,            "sub",                sub_trait,               Target::Trait;

src/librustc/traits/object_safety.rs

@@ -319,41 +319,42 @@ impl<'a, 'tcx> TyCtxt<'a, 'tcx, 'tcx> {
             &sig.map_bound(|sig| sig.inputs()[0]),
         );
 
-        // until `unsized_locals` is fully implemented, `self: Self` can't be coerced from
-        // `Self=dyn Trait` to `Self=T`. However, this is already considered object-safe. We allow
-        // it as a special case here.
-        // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_coercible` allows
+        // until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
+        // However, this is already considered object-safe. We allow it as a special case here.
+        // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
         // `Receiver: Unsize<Receiver[Self => dyn Trait]>`
         if receiver_ty != self.mk_self_type() {
-            if !self.receiver_is_coercible(method, receiver_ty) {
+            if !self.receiver_is_dispatchable(method, receiver_ty) {
                 return Some(MethodViolationCode::UncoercibleReceiver);
             }
         }
 
         None
     }
 
-    /// checks the method's receiver (the `self` argument) can be coerced from
-    /// a fat pointer, including the trait object vtable, to a thin pointer.
-    /// e.g. from `Rc<dyn Trait>` to `Rc<T>`, where `T` is the erased type of the underlying object.
-    /// More formally:
+    /// checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
+    /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
+    /// in the following way:
     /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`
     /// - require the following bound:
     ///       forall(T: Trait) {
-    ///           Receiver[Self => dyn Trait]: CoerceSized<Receiver[Self => T]>
+    ///           Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
     ///       }
     ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
     ///   (substitution notation).
     ///
     /// some examples of receiver types and their required obligation
-    /// - `&'a mut self` requires `&'a mut dyn Trait: CoerceSized<&'a mut T>`
-    /// - `self: Rc<Self>` requires `Rc<dyn Trait>: CoerceSized<Rc<T>>`
+    /// - `&'a mut self` requires `&'a mut T: DispatchFromDyn<&'a mut dyn Trait>`
+    /// - `self: Rc<Self>` requires `Rc<T>: DispatchFromDyn<Rc<dyn Trait>>`
+    /// - `self: Pin<Box<Self>>` requires `Pin<Box<T>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`
     ///
-    /// The only case where the receiver is not coercible, but is still a valid receiver
+    /// The only case where the receiver is not dispatchable, but is still a valid receiver
     /// type (just not object-safe), is when there is more than one level of pointer indirection.
     /// e.g. `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
-    /// is no way, or at least no inexpensive way, to coerce the receiver, because the object that
-    /// needs to be coerced is behind a pointer.
+    /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
+    /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
+    /// contained by the trait object, because the object that needs to be coerced is behind
+    /// a pointer.
     ///
     /// In practice, there are issues with the above bound: `where` clauses that apply to `Self`
     /// would have to apply to `T`, trait object types have a lot of parameters that need to
@@ -364,37 +365,38 @@ impl<'a, 'tcx> TyCtxt<'a, 'tcx, 'tcx> {
     ///
     ///     forall (U: ?Sized) {
     ///         if (Self: Unsize<U>) {
-    ///             Receiver[Self => U]: CoerceSized<Receiver>
+    ///             Receiver: DispatchFromDyn<Receiver[Self => U]>
     ///         }
     ///     }
     ///
-    /// for `self: &'a mut Self`, this means `&'a mut U: CoerceSized<&'a mut Self>`
-    /// for `self: Rc<Self>`, this means `Rc<U>: CoerceSized<Rc<Self>>`
+    /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
+    /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
+    /// for `self: Pin<Box<Self>>, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
     //
     // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
     // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
     // `self: Wrapper<Self>`.
     #[allow(dead_code)]
-    fn receiver_is_coercible(
+    fn receiver_is_dispatchable(
         self,
         method: &ty::AssociatedItem,
         receiver_ty: Ty<'tcx>,
     ) -> bool {
-        debug!("receiver_is_coercible: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
+        debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 
         let traits = (self.lang_items().unsize_trait(),
-                      self.lang_items().coerce_sized_trait());
-        let (unsize_did, coerce_sized_did) = if let (Some(u), Some(cu)) = traits {
+                      self.lang_items().dispatch_from_dyn_trait());
+        let (unsize_did, dispatch_from_dyn_did) = if let (Some(u), Some(cu)) = traits {
             (u, cu)
         } else {
-            debug!("receiver_is_coercible: Missing Unsize or CoerceSized traits");
+            debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
             return false;
         };
 
         // use a bogus type parameter to mimick a forall(U) query using u32::MAX for now.
         // FIXME(mikeyhew) this is a total hack, and we should replace it when real forall queries
         // are implemented
-        let target_self_ty: Ty<'tcx> = self.mk_ty_param(
+        let unsized_self_ty: Ty<'tcx> = self.mk_ty_param(
             ::std::u32::MAX,
             Name::intern("RustaceansAreAwesome").as_interned_str(),
         );
@@ -405,7 +407,7 @@ impl<'a, 'tcx> TyCtxt<'a, 'tcx, 'tcx> {
 
             let predicate = ty::TraitRef {
                 def_id: unsize_did,
-                substs: self.mk_substs_trait(self.mk_self_type(), &[target_self_ty.into()]),
+                substs: self.mk_substs_trait(self.mk_self_type(), &[unsized_self_ty.into()]),
             }.to_predicate();
 
             let caller_bounds: Vec<Predicate<'tcx>> = param_env.caller_bounds.iter().cloned()
@@ -419,19 +421,19 @@ impl<'a, 'tcx> TyCtxt<'a, 'tcx, 'tcx> {
 
         let receiver_substs = Substs::for_item(self, method.def_id, |param, _| {
             if param.index == 0 {
-                target_self_ty.into()
+                unsized_self_ty.into()
             } else {
                 self.mk_param_from_def(param)
             }
         });
         // the type `Receiver[Self => U]` in the query
         let unsized_receiver_ty = receiver_ty.subst(self, receiver_substs);
 
-        // Receiver[Self => U]: CoerceSized<Receiver>
+        // Receiver: DispatchFromDyn<Receiver[Self => U]>
         let obligation = {
             let predicate = ty::TraitRef {
-                def_id: coerce_sized_did,
-                substs: self.mk_substs_trait(unsized_receiver_ty, &[receiver_ty.into()]),
+                def_id: dispatch_from_dyn_did,
+                substs: self.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
             }.to_predicate();
 
             Obligation::new(
@@ -442,7 +444,7 @@ impl<'a, 'tcx> TyCtxt<'a, 'tcx, 'tcx> {
         };
 
         self.infer_ctxt().enter(|ref infcx| {
-            // the receiver is coercible iff the obligation holds
+            // the receiver is dispatchable iff the obligation holds
             infcx.predicate_must_hold(&obligation)
         })
     }