Rollup merge of #22313 - japaric:iter, r=aturon

 `IntoIterator` now has an extra associated item:

``` rust
trait IntoIterator {
    type Item;
    type IntoIter: Iterator<Self=Self::Item>;
}
```

This lets you bind the iterator \"`Item`\" directly when writing generic functions:

``` rust
// hypothetical change, not included in this PR
impl Extend<T> for Vec<T> {
    // you can now write
    fn extend<I>(&mut self, it: I) where I: IntoIterator<Item=T> { .. }
    // instead of
    fn extend<I: IntoIterator>(&mut self, it: I) where I::IntoIter: Iterator<Item=T> { .. }
}
```

The downside is that now you have to write an extra associated type in your `IntoIterator` implementations:

``` diff
 impl<T> IntoIterator for Vec<T> {
+    type Item = T;
     type IntoIter = IntoIter<T>;

     fn into_iter(self) -> IntoIter<T> { .. }
 }
```

Because this breaks all downstream implementations of `IntoIterator`, this is a [breaking-change]

---

r? @aturon

1 files changed, 12 insertions, 0 deletions

diff --git a/src/libcollections/enum_set.rs b/src/libcollections/enum_set.rs
index da533d34703..5c37be188fe 100644
--- a/src/libcollections/enum_set.rs
+++ b/src/libcollections/enum_set.rs
@@ -257,6 +257,8 @@ impl<E:CLike> FromIterator<E> for EnumSet<E> {
     }
 }
 
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<'a, E> IntoIterator for &'a EnumSet<E> where E: CLike {
     type IntoIter = Iter<E>;
 
@@ -265,6 +267,16 @@ impl<'a, E> IntoIterator for &'a EnumSet<E> where E: CLike {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<'a, E> IntoIterator for &'a EnumSet<E> where E: CLike {
+    type Item = E;
+    type IntoIter = Iter<E>;
+
+    fn into_iter(self) -> Iter<E> {
+        self.iter()
+    }
+}
+
 impl<E:CLike> Extend<E> for EnumSet<E> {
     fn extend<I: Iterator<Item=E>>(&mut self, iterator: I) {
         for element in iterator {