From 04b3162764516aa2295d4d549969bae60b5d0cf9 Mon Sep 17 00:00:00 2001
From: fren_gor <goro@frengor.com>
Date: Sun, 20 Feb 2022 01:46:55 +0100
Subject: [PATCH 1/2] Add collect_into

---
 library/core/src/iter/traits/iterator.rs   | 71 ++++++++++++++++++++++
 library/core/tests/iter/traits/iterator.rs |  8 +++
 library/core/tests/lib.rs                  |  1 +
 3 files changed, 80 insertions(+)

diff --git a/library/core/src/iter/traits/iterator.rs b/library/core/src/iter/traits/iterator.rs
index 5a361edecd9c0..9b22023b09e3e 100644
--- a/library/core/src/iter/traits/iterator.rs
+++ b/library/core/src/iter/traits/iterator.rs
@@ -1859,6 +1859,77 @@ pub trait Iterator {
         try_process(self, |i| i.collect())
     }
 
+    /// Collects all the items from an iterator into a collection.
+    ///
+    /// This method consumes the iterator and adds all its items to the
+    /// passed collection. The collection is then returned, so the call chain
+    /// can be continued.
+    ///
+    /// This is useful when you already have a collection and wants to add
+    /// the iterator items to it.
+    ///
+    /// This method is a convenience method to call [Extend::extend](trait.Extend.html),
+    /// but instead of being called on a collection, it's called on an iterator.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(iter_collect_into)]
+    ///
+    /// let a = [1, 2, 3];
+    /// let mut vec: Vec::<i32> = vec![0, 1];
+    ///
+    /// a.iter().map(|&x| x * 2).collect_into(&mut vec);
+    /// a.iter().map(|&x| x * 10).collect_into(&mut vec);
+    ///
+    /// assert_eq!(vec![0, 1, 2, 4, 6, 10, 20, 30], vec);
+    /// ```
+    ///
+    /// `Vec` can have a manual set capacity to avoid reallocating it:
+    ///
+    /// ```
+    /// #![feature(iter_collect_into)]
+    ///
+    /// let a = [1, 2, 3];
+    /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
+    ///
+    /// a.iter().map(|&x| x * 2).collect_into(&mut vec);
+    /// a.iter().map(|&x| x * 10).collect_into(&mut vec);
+    ///
+    /// assert_eq!(6, vec.capacity());
+    /// println!("{:?}", vec);
+    /// ```
+    ///
+    /// The returned mutable reference can be used to continue the call chain:
+    ///
+    /// ```
+    /// #![feature(iter_collect_into)]
+    ///
+    /// let a = [1, 2, 3];
+    /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
+    ///
+    /// let count = a.iter().collect_into(&mut vec).iter().count();
+    ///
+    /// assert_eq!(count, vec.len());
+    /// println!("Vec len is {}", count);
+    ///
+    /// let count = a.iter().collect_into(&mut vec).iter().count();
+    ///
+    /// assert_eq!(count, vec.len());
+    /// println!("Vec len now is {}", count);
+    /// ```
+    #[inline]
+    #[unstable(feature = "iter_collect_into", reason = "new API", issue = "none")]
+    fn collect_into<E: Extend<Self::Item>>(self, collection: &mut E) -> &mut E
+    where
+        Self: Sized,
+    {
+        collection.extend(self);
+        collection
+    }
+
     /// Consumes an iterator, creating two collections from it.
     ///
     /// The predicate passed to `partition()` can return `true`, or `false`.
diff --git a/library/core/tests/iter/traits/iterator.rs b/library/core/tests/iter/traits/iterator.rs
index cf69f0a7a4d7e..32bd68e3d2554 100644
--- a/library/core/tests/iter/traits/iterator.rs
+++ b/library/core/tests/iter/traits/iterator.rs
@@ -543,6 +543,14 @@ fn test_try_collect() {
     assert_eq!(v, Continue(vec![4, 5]));
 }
 
+#[test]
+fn test_collect_into() {
+    let a = vec![1, 2, 3, 4, 5];
+    let mut b = Vec::new();
+    a.iter().cloned().collect_into(&mut b);
+    assert!(a == b);
+}
+
 // just tests by whether or not this compiles
 fn _empty_impl_all_auto_traits<T>() {
     use std::panic::{RefUnwindSafe, UnwindSafe};
diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index 06c7be054a038..39d8625e8bf6c 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -63,6 +63,7 @@
 #![feature(slice_partition_dedup)]
 #![feature(int_log)]
 #![feature(iter_advance_by)]
+#![feature(iter_collect_into)]
 #![feature(iter_partition_in_place)]
 #![feature(iter_intersperse)]
 #![feature(iter_is_partitioned)]

From 63eddb3e68355c3c6739b6fa3c3d37f106408922 Mon Sep 17 00:00:00 2001
From: fren_gor <goro@frengor.com>
Date: Wed, 9 Mar 2022 21:12:07 +0100
Subject: [PATCH 2/2] Add tracking issue

---
 library/core/src/iter/traits/iterator.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/library/core/src/iter/traits/iterator.rs b/library/core/src/iter/traits/iterator.rs
index 9b22023b09e3e..458f6903c0248 100644
--- a/library/core/src/iter/traits/iterator.rs
+++ b/library/core/src/iter/traits/iterator.rs
@@ -1921,7 +1921,7 @@ pub trait Iterator {
     /// println!("Vec len now is {}", count);
     /// ```
     #[inline]
-    #[unstable(feature = "iter_collect_into", reason = "new API", issue = "none")]
+    #[unstable(feature = "iter_collect_into", reason = "new API", issue = "94780")]
     fn collect_into<E: Extend<Self::Item>>(self, collection: &mut E) -> &mut E
     where
         Self: Sized,