auto merge of #18698 : japaric/rust/index, r=alexcrichton

Closes #16529

r? @aturon 

src/libcore/ops.rs

@@ -638,7 +638,7 @@ fn shr(&self, other: &uint) -> $t { (*self) >> (*other) }
  * ```
  */
 #[lang="index"]
-pub trait Index<Index, Sized? Result> {
+pub trait Index<Index, Sized? Result> for Sized? {
     /// The method for the indexing (`Foo[Bar]`) operation
     fn index<'a>(&'a self, index: &Index) -> &'a Result;
 }
@@ -669,7 +669,7 @@ pub trait Index<Index, Sized? Result> {
  * ```
  */
 #[lang="index_mut"]
-pub trait IndexMut<Index, Result> {
+pub trait IndexMut<Index, Result> for Sized? {
     /// The method for the indexing (`Foo[Bar]`) operation
     fn index_mut<'a>(&'a mut self, index: &Index) -> &'a mut Result;
 }

src/libcore/slice.rs

@@ -256,7 +256,6 @@ pub trait SlicePrelude<T> for Sized? {
     #[inline]
     #[experimental = "not triaged yet"]
     fn is_empty(&self) -> bool { self.len() == 0 }
-
     /// Returns a mutable reference to the element at the given index,
     /// or `None` if the index is out of bounds
     #[unstable = "waiting on final error conventions"]
@@ -698,6 +697,22 @@ fn as_mut_ptr(&mut self) -> *mut T {
     }
 }
 
+impl<T> ops::Index<uint, T> for [T] {
+    fn index(&self, &index: &uint) -> &T {
+        assert!(index < self.len());
+
+        unsafe { mem::transmute(self.repr().data.offset(index as int)) }
+    }
+}
+
+impl<T> ops::IndexMut<uint, T> for [T] {
+    fn index_mut(&mut self, &index: &uint) -> &mut T {
+        assert!(index < self.len());
+
+        unsafe { mem::transmute(self.repr().data.offset(index as int)) }
+    }
+}
+
 impl<T> ops::Slice<uint, [T]> for [T] {
     #[inline]
     fn as_slice_<'a>(&'a self) -> &'a [T] {