Rollup merge of #62459 - timvermeulen:result_sum_internal_iteration, r=scottmcm

Use internal iteration in the Sum and Product impls of Result and Option

This PR adds internal iteration to the `ResultShunt` iterator type underlying the `Sum` and `Product` impls of `Result`. I had to change `ResultShunt` to hold a mutable reference to an error instead, similar to `itertools::ProcessResults`, in order to be able to pass the `ResultShunt` itself by value (which is necessary for internal iteration).

`ResultShunt::process` can unfortunately no longer be an associated function because that would make it generic over the lifetime of the error reference, which wouldn't work, so I turned it into the free function `process_results`.

I removed the `OptionShunt` type and forwarded the `Sum` and `Product` impls of `Option` to their respective impls of `Result` instead, to avoid having to repeat the internal iteration logic.

1 files changed, 17 insertions, 0 deletions

diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs
index e27e1605607..3615fab7915 100644
--- a/src/libcore/tests/iter.rs
+++ b/src/libcore/tests/iter.rs
@@ -1203,6 +1203,23 @@ fn test_iterator_sum_result() {
     assert_eq!(v.iter().cloned().sum::<Result<i32, _>>(), Ok(10));
     let v: &[Result<i32, ()>] = &[Ok(1), Err(()), Ok(3), Ok(4)];
     assert_eq!(v.iter().cloned().sum::<Result<i32, _>>(), Err(()));
+
+    #[derive(PartialEq, Debug)]
+    struct S(Result<i32, ()>);
+
+    impl Sum<Result<i32, ()>> for S {
+        fn sum<I: Iterator<Item = Result<i32, ()>>>(mut iter: I) -> Self {
+            // takes the sum by repeatedly calling `next` on `iter`,
+            // thus testing that repeated calls to `ResultShunt::try_fold`
+            // produce the expected results
+            Self(iter.by_ref().sum())
+        }
+    }
+
+    let v: &[Result<i32, ()>] = &[Ok(1), Ok(2), Ok(3), Ok(4)];
+    assert_eq!(v.iter().cloned().sum::<S>(), S(Ok(10)));
+    let v: &[Result<i32, ()>] = &[Ok(1), Err(()), Ok(3), Ok(4)];
+    assert_eq!(v.iter().cloned().sum::<S>(), S(Err(())));
 }
 
 #[test]