Add method .as_mut_slice() to MutableVector

This method is primarily intended to allow for converting a [T, ..N] to
a &mut [T].

src/libstd/vec.rs

@@ -2069,6 +2069,10 @@ unsafe fn step<T>(ptr: &mut *mut T) -> *mut T {
 /// Extension methods for vectors such that their elements are
 /// mutable.
 pub trait MutableVector<'a, T> {
+    /// Work with `self` as a mut slice.
+    /// Primarily intended for getting a &mut [T] from a [T, ..N].
+    fn as_mut_slice(self) -> &'a mut [T];
+
     /// Return a slice that points into another slice.
     fn mut_slice(self, start: uint, end: uint) -> &'a mut [T];
 
@@ -2302,6 +2306,8 @@ fn mut_split_at(self, mid: uint) -> (&'a mut [T],
 
 impl<'a,T> MutableVector<'a, T> for &'a mut [T] {
     #[inline]
+    fn as_mut_slice(self) -> &'a mut [T] { self }
+
     fn mut_slice(self, start: uint, end: uint) -> &'a mut [T] {
         assert!(start <= end);
         assert!(end <= self.len());