Rollup merge of #71421 - elichai:2020-04-boxed-slice, r=sfackler

Add a function to turn Box<T> into Box<[T]>

Hi,
I think this is very useful, as currently it's not possible in safe rust to do this without re-allocating.
an alternative implementation of the same function can be:
```rust
pub fn into_boxed_slice<T>(boxed: Box<T>) -> Box<[T]> {
    unsafe {
        let slice = slice::from_raw_parts_mut(Box::into_raw(boxed), 1);
        Box::from_raw(slice)
    }
}
```

The only thing that makes me a little uncomfortable is this line :
> The alignment of array types is greater or equal to the alignment of its element type

from https://rust-lang.github.io/unsafe-code-guidelines/layout/arrays-and-slices.html

But then I see:
> The alignment of &T, &mut T, *const T and *mut T are the same, and are at least the word size.
> The alignment of &[T] is the word size.

from https://rust-lang.github.io/unsafe-code-guidelines/layout/pointers.html#representation

So I do believe this is valid(FWIW it also passes in miri https://play.rust-lang.org/?gist=c002b99364ee6b29862aeb3565a91c19)

1 files changed, 10 insertions, 0 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 3d657396a9f..b3a771a721d 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -239,6 +239,16 @@ impl<T> Box<T> {
     pub fn pin(x: T) -> Pin<Box<T>> {
         (box x).into()
     }
+
+    /// Converts a `Box<T>` into a `Box<[T]>`
+    ///
+    /// This conversion does not allocate on the heap and happens in place.
+    ///
+    #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
+    pub fn into_boxed_slice(boxed: Box<T>) -> Box<[T]> {
+        // *mut T and *mut [T; 1] have the same size and alignment
+        unsafe { Box::from_raw(Box::into_raw(boxed) as *mut [T; 1] as *mut [T]) }
+    }
 }
 
 impl<T> Box<[T]> {