Rollup merge of #134379 - bjoernager:slice-as-array, r=dtolnay

Add `into_array` conversion destructors for `Box`, `Rc`, and `Arc`.

Tracking issue: #133508

This PR adds the `into_array` destructor for `alloc::boxed::Box<[T]>`, `alloc::rc::Rc<[T]>`, and `alloc::sync::Arc<[T]>`.

Note that this PR assumes the initial proposal of these functions returning `Option`. It is still an open question whether this should instead be `Result`. We can, however, easily change this in a follow-up PR with the necessary consensus.

1 files changed, 20 insertions, 0 deletions

diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index b7ec3af9818..e014404eff3 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -1084,6 +1084,26 @@ impl<T> Rc<[T]> {
             ))
         }
     }
+
+    /// Converts the reference-counted slice into a reference-counted array.
+    ///
+    /// This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type.
+    ///
+    /// If `N` is not exactly equal to the length of `self`, then this method returns `None`.
+    #[unstable(feature = "slice_as_array", issue = "133508")]
+    #[inline]
+    #[must_use]
+    pub fn into_array<const N: usize>(self) -> Option<Rc<[T; N]>> {
+        if self.len() == N {
+            let ptr = Self::into_raw(self) as *const [T; N];
+
+            // SAFETY: The underlying array of a slice has the exact same layout as an actual array `[T; N]` if `N` is equal to the slice's length.
+            let me = unsafe { Rc::from_raw(ptr) };
+            Some(me)
+        } else {
+            None
+        }
+    }
 }
 
 impl<T, A: Allocator> Rc<[T], A> {