Correct ExactSizeIterator impl for RepeatedIter

- size_hint must be implemented as described in the docs
- len/size_hint must return the number of items remaining in the iterator, not the size of the container

PiperOrigin-RevId: 604704172

rust/repeated.rs

@@ -381,7 +381,6 @@ where
     }
 }
 
-// TODO: impl ExactSizeIterator
 impl<'msg, T> iter::Iterator for RepeatedIter<'msg, T>
 where
     T: ProxiedInRepeated + ?Sized + 'msg,
@@ -395,14 +394,20 @@ where
         }
         val
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.len();
+        (len, Some(len))
+    }
 }
 
 impl<'msg, T: ?Sized + ProxiedInRepeated> ExactSizeIterator for RepeatedIter<'msg, T> {
     fn len(&self) -> usize {
-        self.view.len()
+        self.view.len() - self.current_index
     }
 }
 
+// TODO: impl DoubleEndedIterator
 impl<'msg, T: ?Sized + ProxiedInRepeated> FusedIterator for RepeatedIter<'msg, T> {}
 
 impl<'msg, T> iter::IntoIterator for RepeatedView<'msg, T>

rust/test/shared/accessors_repeated_test.rs

@@ -68,6 +68,28 @@ macro_rules! generate_repeated_numeric_test {
                   eq(mutator2.iter().collect::<Vec<_>>())
               );
           }
+
+          #[test]
+          fn [< test_repeated_ $field _exact_size_iterator >]() {
+            let mut msg = TestAllTypes::new();
+            let mut mutator = msg.[<repeated_ $field _mut>]();
+            for i in 0..5 {
+                mutator.push(i as $t);
+            }
+            let mut iter = mutator.iter();
+            // size_hint/len must indicate the _remaining_ items in the iterator.
+            for i in 0..5 {
+                assert_that!(iter.len(), eq(5 - i));
+                assert_that!(iter.size_hint(), eq((5 - i, Some(5 - i))));
+                assert_that!(iter.next(), eq(Some(i as $t)));
+            }
+            assert_that!(iter.size_hint(), eq((0, Some(0))));
+            assert_that!(iter.len(), eq(0));
+            assert_that!(iter.next(), eq(None));
+
+            // Also check FusedIterator - calling `next` multiple times should return `None`.
+            assert_that!(iter.next(), eq(None));
+          }
       )* }
   };
 }