Rollup merge of rust-lang#102023 - SUPERCILEX:maybeuninit-transpose, …

…r=scottmcm

Add MaybeUninit array transpose From impls

See discussion in rust-lang#101179 and rust-lang#96097. I believe this solution offers the simplest implementation with minimal future API regret.

`@RalfJung` mind doing a correctness review?

library/core/src/mem/maybe_uninit.rs

@@ -1284,3 +1284,40 @@ impl<T> MaybeUninit<T> {
         }
     }
 }
+
+impl<T, const N: usize> MaybeUninit<[T; N]> {
+    /// Transposes a `MaybeUninit<[T; N]>` into a `[MaybeUninit<T>; N]`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(maybe_uninit_uninit_array_transpose)]
+    /// # use std::mem::MaybeUninit;
+    ///
+    /// let data: [MaybeUninit<u8>; 1000] = MaybeUninit::uninit().transpose();
+    /// ```
+    #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
+    pub fn transpose(self) -> [MaybeUninit<T>; N] {
+        // SAFETY: T and MaybeUninit<T> have the same layout
+        unsafe { super::transmute_copy(&ManuallyDrop::new(self)) }
+    }
+}
+
+impl<T, const N: usize> [MaybeUninit<T>; N] {
+    /// Transposes a `[MaybeUninit<T>; N]` into a `MaybeUninit<[T; N]>`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(maybe_uninit_uninit_array_transpose)]
+    /// # use std::mem::MaybeUninit;
+    ///
+    /// let data = [MaybeUninit::<u8>::uninit(); 1000];
+    /// let data: MaybeUninit<[u8; 1000]> = data.transpose();
+    /// ```
+    #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
+    pub fn transpose(self) -> MaybeUninit<[T; N]> {
+        // SAFETY: T and MaybeUninit<T> have the same layout
+        unsafe { super::transmute_copy(&ManuallyDrop::new(self)) }
+    }
+}