Rollup merge of #23462 - alexcrichton:stabilize-cloned, r=aturon

 This commit stabilizes the `cloned` iterator after tweaking the signature to
require that the iterator is over `&T` instead of `U: Deref<T>`. This method has
had time to bake for awhile now and it's not clear whether the `Deref` bound is
worth it. Additionally, there aren't clear conventions on when to bound and/or
implement the `Deref` trait, so for now the conservative route is to require
references instead of `U: Deref<T>`.

To change this signature to using `Deref` would technically be a
backwards-incompatible change, but it is doubtful that any code will actually
break in practice.

src/libcollectionstest/vec_deque.rs

@@ -360,7 +360,7 @@ fn test_mut_rev_iter_wrap() {
     assert_eq!(d.pop_front(), Some(1));
     d.push_back(4);
 
-    assert_eq!(d.iter_mut().rev().cloned().collect::<Vec<_>>(),
+    assert_eq!(d.iter_mut().rev().map(|x| *x).collect::<Vec<_>>(),
                vec![4, 3, 2]);
 }
 

src/libcore/iter.rs

@@ -65,7 +65,7 @@
 use marker;
 use mem;
 use num::{ToPrimitive, Int};
-use ops::{Add, Deref, FnMut, RangeFrom};
+use ops::{Add, FnMut, RangeFrom};
 use option::Option;
 use option::Option::{Some, None};
 use marker::Sized;
@@ -976,12 +976,11 @@ fn size_hint(&self) -> (usize, Option<usize>) {
         (ts, us)
     }
 
-    /// Creates an iterator that clones the elements it yields. Useful for converting an
-    /// Iterator<&T> to an Iterator<T>.
-    #[unstable(feature = "core", reason = "recent addition")]
-    fn cloned(self) -> Cloned<Self> where
-        Self::Item: Deref,
-        <Self::Item as Deref>::Target: Clone,
+    /// Creates an iterator that clones the elements it yields. Useful for
+    /// converting an Iterator<&T> to an Iterator<T>.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn cloned<'a, T: 'a>(self) -> Cloned<Self>
+        where Self: Iterator<Item=&'a T>, T: Clone
     {
         Cloned { it: self }
     }
@@ -1279,14 +1278,12 @@ pub struct Cloned<I> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<I> Iterator for Cloned<I> where
-    I: Iterator,
-    I::Item: Deref,
-    <I::Item as Deref>::Target: Clone
+impl<'a, I, T: 'a> Iterator for Cloned<I>
+    where I: Iterator<Item=&'a T>, T: Clone
 {
-    type Item = <I::Item as Deref>::Target;
+    type Item = T;
 
-    fn next(&mut self) -> Option<<Self as Iterator>::Item> {
+    fn next(&mut self) -> Option<T> {
         self.it.next().cloned()
     }
 
@@ -1296,28 +1293,22 @@ fn size_hint(&self) -> (usize, Option<usize>) {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<I> DoubleEndedIterator for Cloned<I> where
-    I: DoubleEndedIterator,
-    I::Item: Deref,
-    <I::Item as Deref>::Target: Clone
+impl<'a, I, T: 'a> DoubleEndedIterator for Cloned<I>
+    where I: DoubleEndedIterator<Item=&'a T>, T: Clone
 {
-    fn next_back(&mut self) -> Option<<Self as Iterator>::Item> {
+    fn next_back(&mut self) -> Option<T> {
         self.it.next_back().cloned()
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<I> ExactSizeIterator for Cloned<I> where
-    I: ExactSizeIterator,
-    I::Item: Deref,
-    <I::Item as Deref>::Target: Clone
+impl<'a, I, T: 'a> ExactSizeIterator for Cloned<I>
+    where I: ExactSizeIterator<Item=&'a T>, T: Clone
 {}
 
 #[unstable(feature = "core", reason = "trait is experimental")]
-impl<I> RandomAccessIterator for Cloned<I> where
-    I: RandomAccessIterator,
-    I::Item: Deref,
-    <I::Item as Deref>::Target: Clone
+impl<'a, I, T: 'a> RandomAccessIterator for Cloned<I>
+    where I: RandomAccessIterator<Item=&'a T>, T: Clone
 {
     #[inline]
     fn indexable(&self) -> usize {
@@ -1325,7 +1316,7 @@ fn indexable(&self) -> usize {
     }
 
     #[inline]
-    fn idx(&mut self, index: usize) -> Option<<Self as Iterator>::Item> {
+    fn idx(&mut self, index: usize) -> Option<T> {
         self.it.idx(index).cloned()
     }
 }