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]

src/libcollections/slice.rs

@@ -121,7 +121,7 @@ pub trait VectorVector<T> for Sized? {
     fn connect_vec(&self, sep: &T) -> Vec<T>;
 }
 
-impl<T: Clone, V: AsSlice<T>> VectorVector<T> for [V] {
+impl<'a, T: Clone, V: AsSlice<T>> VectorVector<T> for [V] {
     fn concat_vec(&self) -> Vec<T> {
         let size = self.iter().fold(0u, |acc, v| acc + v.as_slice().len());
         let mut result = Vec::with_capacity(size);

src/libcollections/vec.rs

@@ -20,6 +20,7 @@ use core::cmp::max;
 use core::default::Default;
 use core::fmt;
 use core::kinds::marker::{ContravariantLifetime, InvariantType};
+use core::kinds::Sized;
 use core::mem;
 use core::num::{Int, UnsignedInt};
 use core::ops;
@@ -516,7 +517,7 @@ impl<T: PartialOrd> PartialOrd for Vec<T> {
 impl<T: Eq> Eq for Vec<T> {}
 
 #[experimental]
-impl<T: PartialEq, V: AsSlice<T>> Equiv<V> for Vec<T> {
+impl<T: PartialEq, Sized? V: AsSlice<T>> Equiv<V> for Vec<T> {
     #[inline]
     fn equiv(&self, other: &V) -> bool { self.as_slice() == other.as_slice() }
 }
@@ -1181,7 +1182,7 @@ impl<T> AsSlice<T> for Vec<T> {
     }
 }
 
-impl<T: Clone, V: AsSlice<T>> Add<V, Vec<T>> for Vec<T> {
+impl<T: Clone, Sized? V: AsSlice<T>> Add<V, Vec<T>> for Vec<T> {
     #[inline]
     fn add(&self, rhs: &V) -> Vec<T> {
         let mut res = Vec::with_capacity(self.len() + rhs.as_slice().len());

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() }
 }

src/libgraphviz/maybe_owned_vec.rs

@@ -89,7 +89,7 @@ impl<'a, T: Ord> Ord for MaybeOwnedVector<'a, T> {
     }
 }
 
-impl<'a, T: PartialEq, V: AsSlice<T>> Equiv<V> for MaybeOwnedVector<'a, T> {
+impl<'a, T: PartialEq, Sized? V: AsSlice<T>> Equiv<V> for MaybeOwnedVector<'a, T> {
     fn equiv(&self, other: &V) -> bool {
         self.as_slice() == other.as_slice()
     }

src/libstd/path/posix.rs

@@ -16,6 +16,7 @@ use cmp::{PartialEq, Eq, PartialOrd, Ord, Ordering};
 use hash;
 use io::Writer;
 use iter::{DoubleEndedIterator, AdditiveIterator, Extend, Iterator, Map};
+use kinds::Sized;
 use option::{Option, None, Some};
 use str::{FromStr, Str};
 use str;
@@ -342,7 +343,7 @@ impl Path {
 
     /// Returns a normalized byte vector representation of a path, by removing all empty
     /// components, and unnecessary . and .. components.
-    fn normalize<V: AsSlice<u8>>(v: V) -> Vec<u8> {
+    fn normalize<Sized? V: AsSlice<u8>>(v: &V) -> Vec<u8> {
         // borrowck is being very picky
         let val = {
             let is_abs = !v.as_slice().is_empty() && v.as_slice()[0] == SEP_BYTE;