Rollup merge of rust-lang#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

Author: Manishearth

src/libcollections/binary_heap.rs

@@ -655,6 +655,8 @@ impl<T: Ord> FromIterator<T> for BinaryHeap<T> {
     }
 }
 
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<T: Ord> IntoIterator for BinaryHeap<T> {
     type IntoIter = IntoIter<T>;
 
@@ -663,6 +665,18 @@ impl<T: Ord> IntoIterator for BinaryHeap<T> {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<T: Ord> IntoIterator for BinaryHeap<T> {
+    type Item = T;
+    type IntoIter = IntoIter<T>;
+
+    fn into_iter(self) -> IntoIter<T> {
+        self.into_iter()
+    }
+}
+
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<'a, T> IntoIterator for &'a BinaryHeap<T> where T: Ord {
     type IntoIter = Iter<'a, T>;
 
@@ -671,6 +685,16 @@ impl<'a, T> IntoIterator for &'a BinaryHeap<T> where T: Ord {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<'a, T> IntoIterator for &'a BinaryHeap<T> where T: Ord {
+    type Item = &'a T;
+    type IntoIter = Iter<'a, T>;
+
+    fn into_iter(self) -> Iter<'a, T> {
+        self.iter()
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Ord> Extend<T> for BinaryHeap<T> {
     fn extend<Iter: Iterator<Item=T>>(&mut self, iter: Iter) {

src/libcollections/bit.rs

@@ -1070,6 +1070,8 @@ impl<'a> RandomAccessIterator for Iter<'a> {
     }
 }
 
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<'a> IntoIterator for &'a Bitv {
     type IntoIter = Iter<'a>;
 
@@ -1078,6 +1080,16 @@ impl<'a> IntoIterator for &'a Bitv {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<'a> IntoIterator for &'a Bitv {
+    type Item = bool;
+    type IntoIter = Iter<'a>;
+
+    fn into_iter(self) -> Iter<'a> {
+        self.iter()
+    }
+}
+
 /// An implementation of a set using a bit vector as an underlying
 /// representation for holding unsigned numerical elements.
 ///
@@ -1882,6 +1894,8 @@ impl<'a> Iterator for SymmetricDifference<'a> {
     #[inline] fn size_hint(&self) -> (usize, Option<usize>) { self.0.size_hint() }
 }
 
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<'a> IntoIterator for &'a BitvSet {
     type IntoIter = SetIter<'a>;
 
@@ -1890,6 +1904,16 @@ impl<'a> IntoIterator for &'a BitvSet {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<'a> IntoIterator for &'a BitvSet {
+    type Item = usize;
+    type IntoIter = SetIter<'a>;
+
+    fn into_iter(self) -> SetIter<'a> {
+        self.iter()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use prelude::*;

src/libcollections/btree/map.rs

@@ -462,6 +462,8 @@ impl<K: Ord, V> BTreeMap<K, V> {
     }
 }
 
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<K, V> IntoIterator for BTreeMap<K, V> {
     type IntoIter = IntoIter<K, V>;
 
@@ -470,6 +472,18 @@ impl<K, V> IntoIterator for BTreeMap<K, V> {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<K, V> IntoIterator for BTreeMap<K, V> {
+    type Item = (K, V);
+    type IntoIter = IntoIter<K, V>;
+
+    fn into_iter(self) -> IntoIter<K, V> {
+        self.into_iter()
+    }
+}
+
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<'a, K, V> IntoIterator for &'a BTreeMap<K, V> {
     type IntoIter = Iter<'a, K, V>;
 
@@ -478,6 +492,18 @@ impl<'a, K, V> IntoIterator for &'a BTreeMap<K, V> {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<'a, K, V> IntoIterator for &'a BTreeMap<K, V> {
+    type Item = (&'a K, &'a V);
+    type IntoIter = Iter<'a, K, V>;
+
+    fn into_iter(self) -> Iter<'a, K, V> {
+        self.iter()
+    }
+}
+
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<'a, K, V> IntoIterator for &'a mut BTreeMap<K, V> {
     type IntoIter = IterMut<'a, K, V>;
 
@@ -486,6 +512,16 @@ impl<'a, K, V> IntoIterator for &'a mut BTreeMap<K, V> {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<'a, K, V> IntoIterator for &'a mut BTreeMap<K, V> {
+    type Item = (&'a K, &'a mut V);
+    type IntoIter = IterMut<'a, K, V>;
+
+    fn into_iter(mut self) -> IterMut<'a, K, V> {
+        self.iter_mut()
+    }
+}
+
 /// A helper enum useful for deciding whether to continue a loop since we can't
 /// return from a closure
 enum Continuation<A, B> {

src/libcollections/btree/set.rs

@@ -480,6 +480,8 @@ impl<T: Ord> FromIterator<T> for BTreeSet<T> {
     }
 }
 
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<T> IntoIterator for BTreeSet<T> {
     type IntoIter = IntoIter<T>;
 
@@ -488,6 +490,18 @@ impl<T> IntoIterator for BTreeSet<T> {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<T> IntoIterator for BTreeSet<T> {
+    type Item = T;
+    type IntoIter = IntoIter<T>;
+
+    fn into_iter(self) -> IntoIter<T> {
+        self.into_iter()
+    }
+}
+
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<'a, T> IntoIterator for &'a BTreeSet<T> {
     type IntoIter = Iter<'a, T>;
 
@@ -496,6 +510,16 @@ impl<'a, T> IntoIterator for &'a BTreeSet<T> {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<'a, T> IntoIterator for &'a BTreeSet<T> {
+    type Item = &'a T;
+    type IntoIter = Iter<'a, T>;
+
+    fn into_iter(self) -> Iter<'a, T> {
+        self.iter()
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Ord> Extend<T> for BTreeSet<T> {
     #[inline]

src/libcollections/dlist.rs

@@ -837,6 +837,8 @@ impl<A> FromIterator<A> for DList<A> {
     }
 }
 
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<T> IntoIterator for DList<T> {
     type IntoIter = IntoIter<T>;
 
@@ -845,15 +847,49 @@ impl<T> IntoIterator for DList<T> {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<T> IntoIterator for DList<T> {
+    type Item = T;
+    type IntoIter = IntoIter<T>;
+
+    fn into_iter(self) -> IntoIter<T> {
+        self.into_iter()
+    }
+}
+
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
+impl<'a, T> IntoIterator for &'a DList<T> {
+    type IntoIter = Iter<'a, T>;
+
+    fn into_iter(self) -> Iter<'a, T> {
+        self.iter()
+    }
+}
+
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
 impl<'a, T> IntoIterator for &'a DList<T> {
+    type Item = &'a T;
     type IntoIter = Iter<'a, T>;
 
     fn into_iter(self) -> Iter<'a, T> {
         self.iter()
     }
 }
 
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
+impl<'a, T> IntoIterator for &'a mut DList<T> {
+    type IntoIter = IterMut<'a, T>;
+
+    fn into_iter(mut self) -> IterMut<'a, T> {
+        self.iter_mut()
+    }
+}
+
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
 impl<'a, T> IntoIterator for &'a mut DList<T> {
+    type Item = &'a mut T;
     type IntoIter = IterMut<'a, T>;
 
     fn into_iter(mut self) -> IterMut<'a, T> {

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 {

src/libcollections/ring_buf.rs

@@ -1699,6 +1699,8 @@ impl<A> FromIterator<A> for RingBuf<A> {
     }
 }
 
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<T> IntoIterator for RingBuf<T> {
     type IntoIter = IntoIter<T>;
 
@@ -1707,6 +1709,18 @@ impl<T> IntoIterator for RingBuf<T> {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<T> IntoIterator for RingBuf<T> {
+    type Item = T;
+    type IntoIter = IntoIter<T>;
+
+    fn into_iter(self) -> IntoIter<T> {
+        self.into_iter()
+    }
+}
+
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<'a, T> IntoIterator for &'a RingBuf<T> {
     type IntoIter = Iter<'a, T>;
 
@@ -1715,6 +1729,18 @@ impl<'a, T> IntoIterator for &'a RingBuf<T> {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<'a, T> IntoIterator for &'a RingBuf<T> {
+    type Item = &'a T;
+    type IntoIter = Iter<'a, T>;
+
+    fn into_iter(self) -> Iter<'a, T> {
+        self.iter()
+    }
+}
+
+// NOTE(stage0): remove impl after a snapshot
+#[cfg(stage0)]
 impl<'a, T> IntoIterator for &'a mut RingBuf<T> {
     type IntoIter = IterMut<'a, T>;
 
@@ -1723,6 +1749,16 @@ impl<'a, T> IntoIterator for &'a mut RingBuf<T> {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
+impl<'a, T> IntoIterator for &'a mut RingBuf<T> {
+    type Item = &'a mut T;
+    type IntoIter = IterMut<'a, T>;
+
+    fn into_iter(mut self) -> IterMut<'a, T> {
+        self.iter_mut()
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A> Extend<A> for RingBuf<A> {
     fn extend<T: Iterator<Item=A>>(&mut self, iterator: T) {