libcore: DST-ify AsSlice

This commit changes `AsSlice` to work on unsized types, and changes the
`impl` for `&[T]` to `[T]`. Aside from making the trait more general,
this also helps some ongoing work with method resolution changes.

This is a breaking change: code that uses generics bounded by `AsSlice`
will have to change. In particular, such code previously often took
arguments of type `V` where `V: AsSlice<T>` by value. These should now
be taken by reference:

```rust
fn foo<Sized? V: AsSlice<T>>(v: &V) { .. }
```

A few std lib functions have been changed accordingly.

[breaking-change]

1 files changed, 15 insertions, 5 deletions

diff --git a/src/libcore/slice.rs b/src/libcore/slice.rs
index 48e52fab51c..7a3e06e7eb4 100644
--- a/src/libcore/slice.rs
+++ b/src/libcore/slice.rs
@@ -1008,15 +1008,25 @@ impl<T: Clone> CloneSlicePrelude<T> for [T] {
 
 /// Data that is viewable as a slice.
 #[unstable = "may merge with other traits"]
-pub trait AsSlice<T> {
+pub trait AsSlice<T> for Sized? {
     /// Work with `self` as a slice.
     fn as_slice<'a>(&'a self) -> &'a [T];
 }
 
 #[unstable = "trait is unstable"]
-impl<'a,T> AsSlice<T> for &'a [T] {
+impl<T> AsSlice<T> for [T] {
     #[inline(always)]
-    fn as_slice<'a>(&'a self) -> &'a [T] { *self }
+    fn as_slice<'a>(&'a self) -> &'a [T] { self }
+}
+
+impl<'a, T, Sized? U: AsSlice<T>> AsSlice<T> for &'a U {
+    #[inline(always)]
+    fn as_slice<'a>(&'a self) -> &'a [T] { AsSlice::as_slice(*self) }
+}
+
+impl<'a, T, Sized? U: AsSlice<T>> AsSlice<T> for &'a mut U {
+    #[inline(always)]
+    fn as_slice<'a>(&'a self) -> &'a [T] { AsSlice::as_slice(*self) }
 }
 
 #[unstable = "waiting for DST"]
@@ -1681,13 +1691,13 @@ impl<T: PartialEq> PartialEq for [T] {
 impl<T: Eq> Eq for [T] {}
 
 #[unstable = "waiting for DST"]
-impl<T: PartialEq, V: AsSlice<T>> Equiv<V> for [T] {
+impl<T: PartialEq, Sized? V: AsSlice<T>> Equiv<V> for [T] {
     #[inline]
     fn equiv(&self, other: &V) -> bool { self.as_slice() == other.as_slice() }
 }
 
 #[unstable = "waiting for DST"]
-impl<'a,T:PartialEq, V: AsSlice<T>> Equiv<V> for &'a mut [T] {
+impl<'a,T:PartialEq, Sized? V: AsSlice<T>> Equiv<V> for &'a mut [T] {
     #[inline]
     fn equiv(&self, other: &V) -> bool { self.as_slice() == other.as_slice() }
 }