Implement ExactSizeIterator for Zip<A, Repeat<B>>

Where:
    A: ExactSizeIterator,
    B: Clone,

And the symmetrical instance.

The only other instance for `ExactSizeIterator<Zip<_, _>>` requires each
zipped iterator to be `ExactSizeIterator`, and since `Repeat` does not
implement `ExactSizeIterator`, these instances don't overlap with
anything.

Author: 414owen

library/core/src/iter/adapters/zip.rs

@@ -1,7 +1,7 @@
 use crate::cmp;
 use crate::fmt::{self, Debug};
 use crate::iter::{
-    FusedIterator, InPlaceIterable, SourceIter, TrustedFused, TrustedLen, UncheckedIterator,
+    FusedIterator, InPlaceIterable, Repeat, SourceIter, TrustedFused, TrustedLen, UncheckedIterator,
 };
 use crate::num::NonZero;
 
@@ -413,6 +413,22 @@ where
 {
 }
 
+#[stable(feature = "exact_size_zip_repeat", since = "1.91.0")]
+impl<A, B> ExactSizeIterator for Zip<A, Repeat<B>>
+where
+    A: ExactSizeIterator,
+    B: Clone,
+{
+}
+
+#[stable(feature = "exact_size_zip_repeat", since = "1.91.0")]
+impl<A, B> ExactSizeIterator for Zip<Repeat<A>, B>
+where
+    A: Clone,
+    B: ExactSizeIterator,
+{
+}
+
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<A, B> TrustedRandomAccess for Zip<A, B>

library/coretests/tests/iter/adapters/zip.rs

@@ -370,3 +370,18 @@ fn test_issue_82291() {
     zip.next();
     assert_eq!(called.get(), 1);
 }
+
+#[test]
+fn zip_repeat_bounds() {
+    let iter = [1, 2, 3].into_iter().zip(repeat(2));
+    let (lower, upper) = iter.size_hint();
+    assert_eq!(lower, 3);
+    assert_eq!(upper, Some(3));
+    assert_eq!(iter.len(), 3);
+
+    let iter = repeat(2).zip([1, 2, 3].into_iter());
+    let (lower, upper) = iter.size_hint();
+    assert_eq!(lower, 3);
+    assert_eq!(upper, Some(3));
+    assert_eq!(iter.len(), 3);
+}