Add Iterator::collect_into and impl Extend for &mut Extend

src/libcore/iter/iterator.rs

@@ -2463,6 +2463,47 @@ pub trait Iterator {
             }
         }
     }
+
+    /// Collects all items from iterator into a collection.
+    ///
+    /// This method consumes the iterator and includes all its items to the
+    /// passed collection. The collection is then returned, so the call chain
+    /// can be continued. Collections can be passed and returned either by
+    /// value or by mutable reference.
+    ///
+    /// This method is a counter-part of [Extend::extend](trait.Extend.html),
+    /// but instead of being called on collection, it's called on iterator.
+    ///
+    /// # Examples
+    /// Basic usage on collection passed by value
+    ///
+    /// ```
+    /// let result = (3..5).collect_into(vec![1, 2]);
+    /// assert_eq!(vec![1, 2, 3, 4], result);
+    /// ```
+    /// More complex usage on collection passed by mutable reference
+    ///
+    /// ```
+    /// let primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43];
+    /// let mut vec_3 = vec![33, 53];
+    /// let vec_103 = primes.into_iter()
+    ///     .cloned()
+    ///     .filter(|p| p % 10 == 3)
+    ///     .collect_into(&mut vec_3)
+    ///     .iter()
+    ///     .map(|i| i + 100)
+    ///     .collect::<Vec<_>>();
+    /// assert_eq!(vec![33, 53, 3, 13, 23, 43], vec_3);
+    /// assert_eq!(vec![133, 153, 103, 113, 123, 143], vec_103);
+    /// ```
+    #[unstable(feature = "collect_into", issue = "0")]
+    fn collect_into<E>(self, mut collection: E) -> E where
+        E: Extend<Self::Item>,
+        Self: Sized
+    {
+        collection.extend(self);
+        collection
+    }
 }
 
 /// Select an element from an iterator based on the given "projection"

src/libcore/iter/traits.rs

@@ -267,6 +267,11 @@ impl<I: Iterator> IntoIterator for I {
 /// or, in the case of collections that permit multiple entries with equal
 /// keys, that entry is inserted.
 ///
+/// When extending with items from [Iterator](trait.Iterator.html), it's often
+/// more convenient to use
+/// [Iterator::collect_into](trait.Iterator.html#method.collect_into). It works
+/// exactly the same way, but is called on series instead of collection.
+///
 /// # Examples
 ///
 /// Basic usage:
@@ -352,6 +357,13 @@ pub trait Extend<A> {
     fn extend<T: IntoIterator<Item=A>>(&mut self, iter: T);
 }
 
+#[unstable(feature = "collect_into", issue = "0")]
+impl <'a, A, E: Extend<A>> Extend<A> for &'a mut E {
+    fn extend<T: IntoIterator<Item=A>>(&mut self, iter: T) {
+        (*self).extend(iter)
+    }
+}
+
 /// An iterator able to yield elements from both ends.
 ///
 /// Something that implements `DoubleEndedIterator` has one extra capability

src/libcore/tests/iter.rs

@@ -2084,3 +2084,31 @@ fn test_monad_laws_associativity() {
     assert_eq!((0..10).flat_map(f).flat_map(g).sum::<usize>(),
                 (0..10).flat_map(|x| f(x).flat_map(g)).sum::<usize>());
 }
+
+#[test]
+fn test_collect_into_passed_by_value() {
+    let collection = vec![1, 2];
+
+    let result = (3..5).collect_into(collection);
+
+    assert_eq!(vec![1, 2, 3, 4], result);
+}
+
+#[test]
+fn test_collect_into_passed_by_mut_ref() {
+    let mut collection = vec![1, 2];
+
+    let result = (3..5).collect_into(&mut collection) as * const _;
+
+    assert_eq!(&collection as *const _, result);
+    assert_eq!(vec![1, 2, 3, 4], collection);
+}
+
+#[test]
+fn test_extend_impl_for_mut_ref_extend() {
+    let mut collection = vec![1, 2];
+
+    (&mut collection).extend(3..5);
+
+    assert_eq!(vec![1, 2, 3, 4], collection);
+}