From bf392503761eb356a4f6b8f463f35eb956345c88 Mon Sep 17 00:00:00 2001
From: Kevin Svetlitski <kevin_svetlitski@berkeley.edu>
Date: Sat, 10 Jul 2021 18:18:24 -0500
Subject: [PATCH] Add impl to collect an iterator of Result<T,E> into
 (Collection<T>, Collection<E>)

Partitioning an iterator of Results into two collections containing
the unwrapped values of the Ok and Err cases respectively requires a
significant amount of boilerplate [1]. The feature introduced by this
commit provides a more ergonomic method of accomplishing this via a
single call to Iterator::collect.

[1]: https://github.com/rust-lang/rust-by-example/blob/9a81505a26f199e7d8b0f21c6cd56a4211c2e6b1/src/error/iter_result.md#collect-all-valid-values-and-failures-with-partition
---
 library/core/src/result.rs   | 19 +++++++++++++++++++
 library/core/tests/result.rs | 18 ++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/library/core/src/result.rs b/library/core/src/result.rs
index 325efe721e30c..8eb1ed641cfae 100644
--- a/library/core/src/result.rs
+++ b/library/core/src/result.rs
@@ -1626,6 +1626,25 @@ impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
     }
 }
 
+//FIXME: Figure out how to mark this as unstable without x.py refusing to run any tests
+#[stable(feature = "tuple_from_result_iter", since = "1.53.0")]
+impl<T, E, U, V> FromIterator<Result<T, E>> for (U, V)
+where
+    U: Default + Extend<T>,
+    V: Default + Extend<E>,
+{
+    fn from_iter<I: IntoIterator<Item = Result<T, E>>>(iter: I) -> (U, V) {
+        let (mut oks, mut errs) = (U::default(), V::default());
+        for result in iter {
+            match result {
+                Ok(ok) => oks.extend_one(ok),
+                Err(err) => errs.extend_one(err),
+            }
+        }
+        (oks, errs)
+    }
+}
+
 #[unstable(feature = "try_trait_v2", issue = "84277")]
 impl<T, E> ops::TryV2 for Result<T, E> {
     type Output = T;
diff --git a/library/core/tests/result.rs b/library/core/tests/result.rs
index 612f083a5c178..415e2e43cdd82 100644
--- a/library/core/tests/result.rs
+++ b/library/core/tests/result.rs
@@ -402,3 +402,21 @@ fn result_try_trait_v2_branch() {
     assert_eq!(Ok::<NonZeroU32, ()>(one).branch(), Continue(one));
     assert_eq!(Err::<NonZeroU32, ()>(()).branch(), Break(Err(())));
 }
+
+#[test]
+fn tuple_from_result_iter() {
+    let results = [Ok(1), Err(false), Ok(3), Ok(4), Err(true)];
+    let (oks, errs) = IntoIterator::into_iter(results).collect::<(Vec<_>, Vec<bool>)>();
+    assert_eq!(oks, [1, 3, 4]);
+    assert_eq!(errs, [false, true]);
+    // All `Ok`s
+    let results = [Ok(5), Ok(6), Ok(7)];
+    let (oks, errs) = IntoIterator::into_iter(results).collect::<(Vec<_>, Vec<String>)>();
+    assert_eq!(oks, [5, 6, 7]);
+    assert_eq!(errs, [] as [String; 0]);
+    // All `Errs`s
+    let results: [Result<i32, _>; 2] = [Err("hello"), Err("world")];
+    let (oks, errs) = IntoIterator::into_iter(results).collect::<(Vec<i32>, Vec<_>)>();
+    assert_eq!(oks, [] as [i32; 0]);
+    assert_eq!(errs, ["hello", "world"]);
+}