Rollup merge of #103163 - SUPERCILEX:uninit-array-assume2, r=scottmcm

Remove all uses of array_assume_init

See https://github.com/rust-lang/rust/pull/103134#discussion_r997462733

r? `@scottmcm`

library/alloc/src/lib.rs

@@ -125,9 +125,9 @@
 #![feature(iter_advance_by)]
 #![feature(iter_next_chunk)]
 #![feature(layout_for_ptr)]
-#![feature(maybe_uninit_array_assume_init)]
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array)]
+#![feature(maybe_uninit_uninit_array_transpose)]
 #![cfg_attr(test, feature(new_uninit))]
 #![feature(nonnull_slice_from_raw_parts)]
 #![feature(pattern)]

library/alloc/src/vec/into_iter.rs

@@ -223,7 +223,7 @@ fn count(self) -> usize {
 
             self.ptr = self.ptr.wrapping_byte_add(N);
             // Safety: ditto
-            return Ok(unsafe { MaybeUninit::array_assume_init(raw_ary) });
+            return Ok(unsafe { raw_ary.transpose().assume_init() });
         }
 
         if len < N {
@@ -241,7 +241,7 @@ fn count(self) -> usize {
         return unsafe {
             ptr::copy_nonoverlapping(self.ptr, raw_ary.as_mut_ptr() as *mut T, N);
             self.ptr = self.ptr.add(N);
-            Ok(MaybeUninit::array_assume_init(raw_ary))
+            Ok(raw_ary.transpose().assume_init())
         };
     }
 

library/core/src/array/iter.rs

@@ -104,8 +104,7 @@ impl<T, const N: usize> IntoIter<T, N> {
     ///
     /// ```
     /// #![feature(array_into_iter_constructors)]
-    ///
-    /// #![feature(maybe_uninit_array_assume_init)]
+    /// #![feature(maybe_uninit_uninit_array_transpose)]
     /// #![feature(maybe_uninit_uninit_array)]
     /// use std::array::IntoIter;
     /// use std::mem::MaybeUninit;
@@ -134,7 +133,7 @@ impl<T, const N: usize> IntoIter<T, N> {
     ///     }
     ///
     ///     // SAFETY: We've initialized all N items
-    ///     unsafe { Ok(MaybeUninit::array_assume_init(buffer)) }
+    ///     unsafe { Ok(buffer.transpose().assume_init()) }
     /// }
     ///
     /// let r: [_; 4] = next_chunk(&mut (10..16)).unwrap();

library/core/src/array/mod.rs

@@ -912,7 +912,7 @@ fn drop(&mut self) {
 
     mem::forget(guard);
     // SAFETY: All elements of the array were populated in the loop above.
-    let output = unsafe { MaybeUninit::array_assume_init(array) };
+    let output = unsafe { array.transpose().assume_init() };
     Ok(Try::from_output(output))
 }
 

library/core/tests/lib.rs

@@ -49,8 +49,8 @@
 #![feature(slice_from_ptr_range)]
 #![feature(split_as_slice)]
 #![feature(maybe_uninit_uninit_array)]
-#![feature(maybe_uninit_array_assume_init)]
 #![feature(maybe_uninit_write_slice)]
+#![feature(maybe_uninit_uninit_array_transpose)]
 #![feature(min_specialization)]
 #![feature(numfmt)]
 #![feature(step_trait)]

library/core/tests/mem.rs

@@ -163,18 +163,18 @@ fn assume_init_good() {
 
 #[test]
 fn uninit_array_assume_init() {
-    let mut array: [MaybeUninit<i16>; 5] = MaybeUninit::uninit_array();
+    let mut array = [MaybeUninit::<i16>::uninit(); 5];
     array[0].write(3);
     array[1].write(1);
     array[2].write(4);
     array[3].write(1);
     array[4].write(5);
 
-    let array = unsafe { MaybeUninit::array_assume_init(array) };
+    let array = unsafe { array.transpose().assume_init() };
 
     assert_eq!(array, [3, 1, 4, 1, 5]);
 
-    let [] = unsafe { MaybeUninit::<!>::array_assume_init([]) };
+    let [] = unsafe { [MaybeUninit::<!>::uninit(); 0].transpose().assume_init() };
 }
 
 #[test]