Add the CoerceSized trait

- add the CoerceSized trait and set it up as a lang item

Object safety still uses CoerceUnsized for now

src/liballoc/boxed.rs

@@ -63,7 +63,7 @@ use core::fmt;
 use core::future::{Future, FutureObj, LocalFutureObj, UnsafeFutureObj};
 use core::hash::{Hash, Hasher};
 use core::iter::FusedIterator;
-use core::marker::{Unpin, Unsize};
+use core::marker::{Unpin, Unsize, CoerceSized};
 use core::mem;
 use core::pin::PinMut;
 use core::ops::{CoerceUnsized, Deref, DerefMut, Generator, GeneratorState};
@@ -670,6 +670,9 @@ 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> {}
+
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl<T: Clone> Clone for Box<[T]> {
     fn clone(&self) -> Self {

src/liballoc/lib.rs

@@ -88,6 +88,7 @@
 #![feature(box_syntax)]
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
+#![feature(coerce_sized)]
 #![cfg_attr(stage0, feature(const_fn))]
 #![cfg_attr(not(stage0), feature(min_const_fn))]
 #![feature(core_intrinsics)]

src/liballoc/pin.rs

@@ -91,7 +91,7 @@ pub use core::marker::Unpin;
 use core::convert::From;
 use core::fmt;
 use core::future::{Future, FutureObj, LocalFutureObj, UnsafeFutureObj};
-use core::marker::Unsize;
+use core::marker::{Unsize, CoerceSized};
 use core::ops::{CoerceUnsized, Deref, DerefMut};
 use core::task::{Context, Poll};
 
@@ -256,6 +256,11 @@ impl<T: ?Sized> fmt::Pointer for PinBox<T> {
 #[unstable(feature = "pin", issue = "49150")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<PinBox<U>> for PinBox<T> {}
 
+#[unstable(feature = "coerce_sized", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<PinBox<T>> for PinBox<U> {}
+
+
+
 #[unstable(feature = "pin", issue = "49150")]
 impl<T: ?Sized> Unpin for PinBox<T> {}
 

src/liballoc/rc.rs

@@ -252,7 +252,7 @@ use core::fmt;
 use core::hash::{Hash, Hasher};
 use core::intrinsics::abort;
 use core::marker;
-use core::marker::{Unsize, PhantomData};
+use core::marker::{Unsize, CoerceSized, PhantomData};
 use core::mem::{self, align_of_val, forget, size_of_val};
 use core::ops::Deref;
 use core::ops::CoerceUnsized;
@@ -295,6 +295,9 @@ 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> {}
+
 impl<T> Rc<T> {
     /// Constructs a new `Rc<T>`.
     ///
@@ -1171,6 +1174,9 @@ 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> {}
+
 impl<T> Weak<T> {
     /// Constructs a new `Weak<T>`, without allocating any memory.
     /// Calling [`upgrade`] on the return value always gives [`None`].

src/liballoc/sync.rs

@@ -27,7 +27,7 @@ use core::mem::{self, align_of_val, size_of_val};
 use core::ops::Deref;
 use core::ops::CoerceUnsized;
 use core::ptr::{self, NonNull};
-use core::marker::{Unsize, PhantomData};
+use core::marker::{Unsize, CoerceSized, PhantomData};
 use core::hash::{Hash, Hasher};
 use core::{isize, usize};
 use core::convert::From;
@@ -212,6 +212,9 @@ 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> {}
+
 /// `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`
 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
@@ -252,6 +255,8 @@ 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> {}
 
 #[stable(feature = "arc_weak", since = "1.4.0")]
 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {

src/libcore/marker.rs

@@ -20,6 +20,7 @@ use cell::UnsafeCell;
 use cmp;
 use hash::Hash;
 use hash::Hasher;
+use ops::CoerceUnsized;
 
 /// Types that can be transferred across thread boundaries.
 ///
@@ -135,6 +136,34 @@ pub trait Unsize<T: ?Sized> {
     // Empty.
 }
 
+/// 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 is dyn Trait to the version where Self is the erased sized type T that implements Trait.
+///
+/// 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> {
+    // 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 {}
+
+
 /// Types whose values can be duplicated simply by copying bits.
 ///
 /// By default, variable bindings have 'move semantics.' In other

src/libcore/ops/unsize.rs

@@ -43,7 +43,7 @@ use marker::Unsize;
 /// [nomicon-coerce]: ../../nomicon/coercions.html
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 #[lang = "coerce_unsized"]
-pub trait CoerceUnsized<T> {
+pub trait CoerceUnsized<T: ?Sized> {
     // Empty.
 }
 

src/libcore/pin.rs

@@ -8,7 +8,7 @@
 
 use fmt;
 use future::{Future, UnsafeFutureObj};
-use marker::{Sized, Unpin, Unsize};
+use marker::{Sized, Unpin, Unsize, CoerceSized};
 use task::{Context, Poll};
 use ops::{Deref, DerefMut, CoerceUnsized};
 
@@ -141,6 +141,9 @@ impl<'a, T: ?Sized> fmt::Pointer for PinMut<'a, T> {
 #[unstable(feature = "pin", issue = "49150")]
 impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<PinMut<'a, U>> for PinMut<'a, T> {}
 
+#[unstable(feature = "pin", issue = "49150")]
+impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<PinMut<'a, T>> for PinMut<'a, U> {}
+
 #[unstable(feature = "pin", issue = "49150")]
 impl<'a, T: ?Sized> Unpin for PinMut<'a, T> {}
 

src/librustc/middle/lang_items.rs

@@ -252,6 +252,7 @@ language_item_table! {
     DropTraitLangItem,               "drop",                    drop_trait;
 
     CoerceUnsizedTraitLangItem,      "coerce_unsized",          coerce_unsized_trait;
+    CoerceSizedTraitLangItem,        "coerce_sized",            coerce_sized_trait;
 
     AddTraitLangItem,                "add",                     add_trait;
     SubTraitLangItem,                "sub",                     sub_trait;