Address Alex's PR comments

Author: Stjepan Glavina

src/libcollections/lib.rs

@@ -52,7 +52,7 @@
 #![feature(shared)]
 #![feature(slice_get_slice)]
 #![feature(slice_patterns)]
-#![feature(sort_unstable)]
+#![cfg_attr(not(test), feature(sort_unstable))]
 #![feature(specialization)]
 #![feature(staged_api)]
 #![feature(str_internals)]

src/libcollections/slice.rs

@@ -1173,6 +1173,8 @@ impl<T> [T] {
     /// # Examples
     ///
     /// ```
+    /// #![feature(sort_unstable)]
+    ///
     /// let mut v = [-5, 4, 1, -3, 2];
     ///
     /// v.sort_unstable();
@@ -1208,6 +1210,8 @@ impl<T> [T] {
     /// # Examples
     ///
     /// ```
+    /// #![feature(sort_unstable)]
+    ///
     /// let mut v = [5, 4, 1, 3, 2];
     /// v.sort_unstable_by(|a, b| a.cmp(b));
     /// assert!(v == [1, 2, 3, 4, 5]);
@@ -1246,6 +1250,8 @@ impl<T> [T] {
     /// # Examples
     ///
     /// ```
+    /// #![feature(sort_unstable)]
+    ///
     /// let mut v = [-5i32, 4, 1, -3, 2];
     ///
     /// v.sort_unstable_by_key(|k| k.abs());

src/libcore/slice/sort.rs

@@ -16,8 +16,6 @@
 //! Unstable sorting is compatible with libcore because it doesn't allocate memory, unlike our
 //! stable sorting implementation.
 
-#![unstable(feature = "sort_unstable", issue = "40585")]
-
 use cmp;
 use mem;
 use ptr;