Rollup merge of rust-lang#48110 - Centril:stabilize/box_leak, r=alexcrichton

Stabilize Box::leak

Stabilizes the following:
+ `Box::leak` (`box_leak`, in nightly since 2017-11-23)

cc rust-lang#46179

 r? @rust-lang/libs

Author: Manishearth

src/liballoc/boxed.rs

@@ -359,8 +359,6 @@ impl<T: ?Sized> Box<T> {
     /// Simple usage:
     ///
     /// ```
-    /// #![feature(box_leak)]
-    ///
     /// fn main() {
     ///     let x = Box::new(41);
     ///     let static_ref: &'static mut usize = Box::leak(x);
@@ -372,17 +370,14 @@ impl<T: ?Sized> Box<T> {
     /// Unsized data:
     ///
     /// ```
-    /// #![feature(box_leak)]
-    ///
     /// fn main() {
     ///     let x = vec![1, 2, 3].into_boxed_slice();
     ///     let static_ref = Box::leak(x);
     ///     static_ref[0] = 4;
     ///     assert_eq!(*static_ref, [4, 2, 3]);
     /// }
     /// ```
-    #[unstable(feature = "box_leak", reason = "needs an FCP to stabilize",
-               issue = "46179")]
+    #[stable(feature = "box_leak", since = "1.26.0")]
     #[inline]
     pub fn leak<'a>(b: Box<T>) -> &'a mut T
     where