Rollup merge of rust-lang#151613 - cuviper:array-windows-parity, r=Amanieu

Align `ArrayWindows` trait impls with `Windows`

With `slice::ArrayWindows` getting ready to stabilize in 1.94, I noticed that it currently has some differences in trait implementations compared to `slice::Windows`, and I think we should align these.

- Remove `derive(Copy)` -- we generally don't want `Copy` for iterators at all, as this is seen as a footgun (e.g. rust-lang#21809). This is obviously a breaking change though, so we should only remove this if we also backport the removal before it's stable. Otherwise, it should at least be replaced by a manual impl without requiring `T: Copy`.
- Manually `impl Clone`, simply to avoid requiring `T: Clone`.
- `impl FusedIterator`, because it is trivially so. The `since = "1.94.0"` assumes we'll backport this, otherwise we should change that to the "current" placeholder.
- `impl TrustedLen`, because we can trust our implementation.
- `impl TrustedRandomAccess`, because the required `__iterator_get_unchecked` method is straightforward.

r? libs-api

@rustbot label beta-nominated
(at least for the `Copy` removal, but we could be more selective about the rest).

Author: JonathanBrouwer

library/core/src/slice/iter.rs

@@ -2175,7 +2175,7 @@ unsafe impl<T> Sync for ChunksExactMut<'_, T> where T: Sync {}
 ///
 /// [`array_windows`]: slice::array_windows
 /// [slices]: slice
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug)]
 #[stable(feature = "array_windows", since = "1.94.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct ArrayWindows<'a, T: 'a, const N: usize> {
@@ -2189,6 +2189,14 @@ impl<'a, T: 'a, const N: usize> ArrayWindows<'a, T, N> {
     }
 }
 
+// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
+#[stable(feature = "array_windows", since = "1.94.0")]
+impl<T, const N: usize> Clone for ArrayWindows<'_, T, N> {
+    fn clone(&self) -> Self {
+        Self { v: self.v }
+    }
+}
+
 #[stable(feature = "array_windows", since = "1.94.0")]
 impl<'a, T, const N: usize> Iterator for ArrayWindows<'a, T, N> {
     type Item = &'a [T; N];
@@ -2224,6 +2232,14 @@ impl<'a, T, const N: usize> Iterator for ArrayWindows<'a, T, N> {
     fn last(self) -> Option<Self::Item> {
         self.v.last_chunk()
     }
+
+    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item {
+        // SAFETY: since the caller guarantees that `idx` is in bounds,
+        // which means that `idx` cannot overflow an `isize`, and the
+        // "slice" created by `cast_array` is a subslice of `self.v`
+        // thus is guaranteed to be valid for the lifetime `'a` of `self.v`.
+        unsafe { &*self.v.as_ptr().add(idx).cast_array() }
+    }
 }
 
 #[stable(feature = "array_windows", since = "1.94.0")]
@@ -2252,6 +2268,22 @@ impl<T, const N: usize> ExactSizeIterator for ArrayWindows<'_, T, N> {
     }
 }
 
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<T, const N: usize> TrustedLen for ArrayWindows<'_, T, N> {}
+
+#[stable(feature = "array_windows", since = "1.94.0")]
+impl<T, const N: usize> FusedIterator for ArrayWindows<'_, T, N> {}
+
+#[doc(hidden)]
+#[unstable(feature = "trusted_random_access", issue = "none")]
+unsafe impl<T, const N: usize> TrustedRandomAccess for ArrayWindows<'_, T, N> {}
+
+#[doc(hidden)]
+#[unstable(feature = "trusted_random_access", issue = "none")]
+unsafe impl<T, const N: usize> TrustedRandomAccessNoCoerce for ArrayWindows<'_, T, N> {
+    const MAY_HAVE_SIDE_EFFECT: bool = false;
+}
+
 /// An iterator over a slice in (non-overlapping) chunks (`chunk_size` elements at a
 /// time), starting at the end of the slice.
 ///