1 files changed, 99 insertions, 10 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 4341b0b2975..6f125cdba81 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -151,7 +151,7 @@ impl<T> Place<T> for IntermediateBox<T> {
 unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
     let p = b.ptr as *mut T;
     mem::forget(b);
-    mem::transmute(p)
+    Box::from_raw(p)
 }
 
 fn make_place<T>() -> IntermediateBox<T> {
@@ -269,7 +269,38 @@ impl<T: ?Sized> Box<T> {
     #[stable(feature = "box_raw", since = "1.4.0")]
     #[inline]
     pub unsafe fn from_raw(raw: *mut T) -> Self {
-        mem::transmute(raw)
+        Box::from_unique(Unique::new_unchecked(raw))
+    }
+
+    /// Constructs a `Box` from a `Unique<T>` pointer.
+    ///
+    /// After calling this function, the memory is owned by a `Box` and `T` can
+    /// then be destroyed and released upon drop.
+    ///
+    /// # Safety
+    ///
+    /// A `Unique<T>` can be safely created via [`Unique::new`] and thus doesn't
+    /// necessarily own the data pointed to nor is the data guaranteed to live
+    /// as long as the pointer.
+    ///
+    /// [`Unique::new`]: ../../core/ptr/struct.Unique.html#method.new
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(unique)]
+    ///
+    /// fn main() {
+    ///     let x = Box::new(5);
+    ///     let ptr = Box::into_unique(x);
+    ///     let x = unsafe { Box::from_unique(ptr) };
+    /// }
+    /// ```
+    #[unstable(feature = "unique", reason = "needs an RFC to flesh out design",
+               issue = "27730")]
+    #[inline]
+    pub unsafe fn from_unique(u: Unique<T>) -> Self {
+        Box(u)
     }
 
     /// Consumes the `Box`, returning the wrapped raw pointer.
@@ -295,7 +326,7 @@ impl<T: ?Sized> Box<T> {
     #[stable(feature = "box_raw", since = "1.4.0")]
     #[inline]
     pub fn into_raw(b: Box<T>) -> *mut T {
-        unsafe { mem::transmute(b) }
+        Box::into_unique(b).as_ptr()
     }
 
     /// Consumes the `Box`, returning the wrapped pointer as `Unique<T>`.
@@ -303,13 +334,18 @@ impl<T: ?Sized> Box<T> {
     /// After calling this function, the caller is responsible for the
     /// memory previously managed by the `Box`. In particular, the
     /// caller should properly destroy `T` and release the memory. The
-    /// proper way to do so is to convert the raw pointer back into a
-    /// `Box` with the [`Box::from_raw`] function.
+    /// proper way to do so is to either convert the `Unique<T>` pointer:
+    ///
+    /// - Into a `Box` with the [`Box::from_unique`] function.
+    ///
+    /// - Into a raw pointer and back into a `Box` with the [`Box::from_raw`]
+    ///   function.
     ///
     /// Note: this is an associated function, which means that you have
     /// to call it as `Box::into_unique(b)` instead of `b.into_unique()`. This
     /// is so that there is no conflict with a method on the inner type.
     ///
+    /// [`Box::from_unique`]: struct.Box.html#method.from_unique
     /// [`Box::from_raw`]: struct.Box.html#method.from_raw
     ///
     /// # Examples
@@ -326,7 +362,62 @@ impl<T: ?Sized> Box<T> {
                issue = "27730")]
     #[inline]
     pub fn into_unique(b: Box<T>) -> Unique<T> {
-        unsafe { mem::transmute(b) }
+        let unique = b.0;
+        mem::forget(b);
+        unique
+    }
+
+    /// Consumes and leaks the `Box`, returning a mutable reference,
+    /// `&'a mut T`. Here, the lifetime `'a` may be chosen to be `'static`.
+    ///
+    /// This function is mainly useful for data that lives for the remainder of
+    /// the program's life. Dropping the returned reference will cause a memory
+    /// leak. If this is not acceptable, the reference should first be wrapped
+    /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
+    /// then be dropped which will properly destroy `T` and release the
+    /// allocated memory.
+    ///
+    /// Note: this is an associated function, which means that you have
+    /// to call it as `Box::leak(b)` instead of `b.leak()`. This
+    /// is so that there is no conflict with a method on the inner type.
+    ///
+    /// [`Box::from_raw`]: struct.Box.html#method.from_raw
+    ///
+    /// # Examples
+    ///
+    /// Simple usage:
+    ///
+    /// ```
+    /// #![feature(box_leak)]
+    ///
+    /// fn main() {
+    ///     let x = Box::new(41);
+    ///     let static_ref: &'static mut usize = Box::leak(x);
+    ///     *static_ref += 1;
+    ///     assert_eq!(*static_ref, 42);
+    /// }
+    /// ```
+    ///
+    /// 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")]
+    #[inline]
+    pub fn leak<'a>(b: Box<T>) -> &'a mut T
+    where
+        T: 'a // Technically not needed, but kept to be explicit.
+    {
+        unsafe { &mut *Box::into_raw(b) }
     }
 }
 
@@ -528,9 +619,7 @@ impl<'a> From<&'a str> for Box<str> {
 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
 impl From<Box<str>> for Box<[u8]> {
     fn from(s: Box<str>) -> Self {
-        unsafe {
-            mem::transmute(s)
-        }
+        unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
     }
 }
 
@@ -593,7 +682,7 @@ impl Box<Any + Send> {
     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
         <Box<Any>>::downcast(self).map_err(|s| unsafe {
             // reapply the Send marker
-            mem::transmute::<Box<Any>, Box<Any + Send>>(s)
+            Box::from_raw(Box::into_raw(s) as *mut (Any + Send))
         })
     }
 }