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.

Author: mikeyhew

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;