Implement PeekableIterator for IntoIter

Author: wmstack

library/core/src/array/iter.rs

@@ -1,7 +1,7 @@
 //! Defines the `IntoIter` owned iterator for arrays.
 
 use crate::intrinsics::transmute_unchecked;
-use crate::iter::{FusedIterator, TrustedLen, TrustedRandomAccessNoCoerce};
+use crate::iter::{FusedIterator, PeekableIterator, TrustedLen, TrustedRandomAccessNoCoerce};
 use crate::mem::MaybeUninit;
 use crate::num::NonZero;
 use crate::ops::{IndexRange, Range, Try};
@@ -350,6 +350,13 @@ impl<T, const N: usize> FusedIterator for IntoIter<T, N> {}
 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
 unsafe impl<T, const N: usize> TrustedLen for IntoIter<T, N> {}
 
+#[unstable(feature = "peekable_iterator", issue = "132973")]
+impl<T, const N: usize> PeekableIterator for IntoIter<T, N> {
+    fn peek_with<U>(&mut self, func: impl for<'b> FnOnce(Option<&'b Self::Item>) -> U) -> U {
+        self.unsize_mut().peek_with(func)
+    }
+}
+
 #[doc(hidden)]
 #[unstable(issue = "none", feature = "std_internals")]
 #[rustc_unsafe_specialization_marker]

library/core/src/array/iter/iter_inner.rs

@@ -175,6 +175,16 @@ impl<T> PolymorphicIter<[MaybeUninit<T>]> {
         })
     }
 
+    #[inline]
+    pub(super) fn peek_with<U>(&mut self, func: impl for<'b> FnOnce(Option<&'b T>) -> U) -> U {
+        let tmp = self.alive.clone().next().map(|idx| {
+            // SAFETY: `idx` is in self.alive range
+            unsafe { self.data.get_unchecked(idx).assume_init_read() }
+        });
+
+        func(tmp.as_ref())
+    }
+
     #[inline]
     pub(super) fn size_hint(&self) -> (usize, Option<usize>) {
         let len = self.len();