Add a FusedIterator trait.

This trait can be used to avoid the overhead of a fuse wrapper when an iterator
is already well-behaved.

Conforming to: RFC 1581
Closes: #35602

9 files changed, 262 insertions, 11 deletions

diff --git a/src/libcore/char.rs b/src/libcore/char.rs
index a3440fe8aa6..98b7632a220 100644
--- a/src/libcore/char.rs
+++ b/src/libcore/char.rs
@@ -18,6 +18,7 @@
 use prelude::v1::*;
 
 use char_private::is_printable;
+use iter::FusedIterator;
 use mem::transmute;
 
 // UTF-8 ranges and tags for encoding characters
@@ -516,6 +517,9 @@ impl ExactSizeIterator for EscapeUnicode {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl FusedIterator for EscapeUnicode {}
+
 /// An iterator that yields the literal escape code of a `char`.
 ///
 /// This `struct` is created by the [`escape_default()`] method on [`char`]. See
@@ -616,6 +620,9 @@ impl ExactSizeIterator for EscapeDefault {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl FusedIterator for EscapeDefault {}
+
 /// An iterator that yields the literal escape code of a `char`.
 ///
 /// This `struct` is created by the [`escape_debug()`] method on [`char`]. See its
@@ -637,6 +644,9 @@ impl Iterator for EscapeDebug {
 #[unstable(feature = "char_escape_debug", issue = "35068")]
 impl ExactSizeIterator for EscapeDebug { }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl FusedIterator for EscapeDebug {}
+
 /// An iterator over `u8` entries represending the UTF-8 encoding of a `char`
 /// value.
 ///
@@ -675,6 +685,9 @@ impl Iterator for EncodeUtf8 {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl FusedIterator for EncodeUtf8 {}
+
 /// An iterator over `u16` entries represending the UTF-16 encoding of a `char`
 /// value.
 ///
@@ -714,6 +727,8 @@ impl Iterator for EncodeUtf16 {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl FusedIterator for EncodeUtf16 {}
 
 /// An iterator over an iterator of bytes of the characters the bytes represent
 /// as UTF-8
@@ -760,3 +775,6 @@ impl<I: Iterator<Item = u8>> Iterator for DecodeUtf8<I> {
         })
     }
 }
+
+#[unstable(feature = "fused", issue = "35602")]
+impl<I: FusedIterator<Item = u8>> FusedIterator for DecodeUtf8<I> {}
diff --git a/src/libcore/iter/mod.rs b/src/libcore/iter/mod.rs
index d2de0d46d74..cfe117c0b1d 100644
--- a/src/libcore/iter/mod.rs
+++ b/src/libcore/iter/mod.rs
@@ -330,6 +330,8 @@ pub use self::sources::{Once, once};
 pub use self::traits::{FromIterator, IntoIterator, DoubleEndedIterator, Extend};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::traits::{ExactSizeIterator, Sum, Product};
+#[unstable(feature = "fused", issue = "35602")]
+pub use self::traits::FusedIterator;
 
 mod iterator;
 mod range;
@@ -370,6 +372,10 @@ impl<I> DoubleEndedIterator for Rev<I> where I: DoubleEndedIterator {
 impl<I> ExactSizeIterator for Rev<I>
     where I: ExactSizeIterator + DoubleEndedIterator {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<I> FusedIterator for Rev<I>
+    where I: FusedIterator + DoubleEndedIterator {}
+
 /// An iterator that clones the elements of an underlying iterator.
 ///
 /// This `struct` is created by the [`cloned()`] method on [`Iterator`]. See its
@@ -413,6 +419,11 @@ impl<'a, I, T: 'a> ExactSizeIterator for Cloned<I>
     where I: ExactSizeIterator<Item=&'a T>, T: Clone
 {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, I, T: 'a> FusedIterator for Cloned<I>
+    where I: FusedIterator<Item=&'a T>, T: Clone
+{}
+
 /// An iterator that repeats endlessly.
 ///
 /// This `struct` is created by the [`cycle()`] method on [`Iterator`]. See its
@@ -451,6 +462,9 @@ impl<I> Iterator for Cycle<I> where I: Clone + Iterator {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<I> FusedIterator for Cycle<I> where I: Clone + Iterator {}
+
 /// An iterator that strings two iterators together.
 ///
 /// This `struct` is created by the [`chain()`] method on [`Iterator`]. See its
@@ -613,6 +627,13 @@ impl<A, B> DoubleEndedIterator for Chain<A, B> where
     }
 }
 
+// Note: *both* must be fused to handle double-ended iterators.
+#[unstable(feature = "fused", issue = "35602")]
+impl<A, B> FusedIterator for Chain<A, B>
+    where A: FusedIterator,
+          B: FusedIterator<Item=A::Item>,
+{}
+
 /// An iterator that iterates two other iterators simultaneously.
 ///
 /// This `struct` is created by the [`zip()`] method on [`Iterator`]. See its
@@ -823,6 +844,10 @@ unsafe impl<A, B> TrustedRandomAccess for Zip<A, B>
 
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<A, B> FusedIterator for Zip<A, B>
+    where A: FusedIterator, B: FusedIterator, {}
+
 /// An iterator that maps the values of `iter` with `f`.
 ///
 /// This `struct` is created by the [`map()`] method on [`Iterator`]. See its
@@ -919,6 +944,10 @@ impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for Map<I, F> where
 impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F>
     where F: FnMut(I::Item) -> B {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<B, I: FusedIterator, F> FusedIterator for Map<I, F>
+    where F: FnMut(I::Item) -> B {}
+
 /// An iterator that filters the elements of `iter` with `predicate`.
 ///
 /// This `struct` is created by the [`filter()`] method on [`Iterator`]. See its
@@ -979,6 +1008,10 @@ impl<I: DoubleEndedIterator, P> DoubleEndedIterator for Filter<I, P>
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<I: FusedIterator, P> FusedIterator for Filter<I, P>
+    where P: FnMut(&I::Item) -> bool {}
+
 /// An iterator that uses `f` to both filter and map elements from `iter`.
 ///
 /// This `struct` is created by the [`filter_map()`] method on [`Iterator`]. See its
@@ -1041,6 +1074,10 @@ impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for FilterMap<I, F>
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<B, I: FusedIterator, F> FusedIterator for FilterMap<I, F>
+    where F: FnMut(I::Item) -> Option<B> {}
+
 /// An iterator that yields the current count and the element during iteration.
 ///
 /// This `struct` is created by the [`enumerate()`] method on [`Iterator`]. See its
@@ -1128,6 +1165,9 @@ unsafe impl<I> TrustedRandomAccess for Enumerate<I>
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<I> FusedIterator for Enumerate<I> where I: FusedIterator {}
+
 /// An iterator with a `peek()` that returns an optional reference to the next
 /// element.
 ///
@@ -1195,6 +1235,9 @@ impl<I: Iterator> Iterator for Peekable<I> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<I: FusedIterator> FusedIterator for Peekable<I> {}
+
 impl<I: Iterator> Peekable<I> {
     /// Returns a reference to the next() value without advancing the iterator.
     ///
@@ -1296,6 +1339,10 @@ impl<I: Iterator, P> Iterator for SkipWhile<I, P>
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<I, P> FusedIterator for SkipWhile<I, P>
+    where I: FusedIterator, P: FnMut(&I::Item) -> bool {}
+
 /// An iterator that only accepts elements while `predicate` is true.
 ///
 /// This `struct` is created by the [`take_while()`] method on [`Iterator`]. See its
@@ -1351,6 +1398,10 @@ impl<I: Iterator, P> Iterator for TakeWhile<I, P>
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<I, P> FusedIterator for TakeWhile<I, P>
+    where I: FusedIterator, P: FnMut(&I::Item) -> bool {}
+
 /// An iterator that skips over `n` elements of `iter`.
 ///
 /// This `struct` is created by the [`skip()`] method on [`Iterator`]. See its
@@ -1442,6 +1493,9 @@ impl<I> DoubleEndedIterator for Skip<I> where I: DoubleEndedIterator + ExactSize
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<I> FusedIterator for Skip<I> where I: FusedIterator {}
+
 /// An iterator that only iterates over the first `n` iterations of `iter`.
 ///
 /// This `struct` is created by the [`take()`] method on [`Iterator`]. See its
@@ -1503,6 +1557,8 @@ impl<I> Iterator for Take<I> where I: Iterator{
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I> ExactSizeIterator for Take<I> where I: ExactSizeIterator {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<I> FusedIterator for Take<I> where I: FusedIterator {}
 
 /// An iterator to maintain state while iterating another iterator.
 ///
@@ -1549,6 +1605,10 @@ impl<B, I, St, F> Iterator for Scan<I, St, F> where
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<B, I, St, F> FusedIterator for Scan<I, St, F>
+    where I: FusedIterator, F: FnMut(&mut St, I::Item) -> Option<B> {}
+
 /// An iterator that maps each element to an iterator, and yields the elements
 /// of the produced iterators.
 ///
@@ -1635,6 +1695,10 @@ impl<I: DoubleEndedIterator, U, F> DoubleEndedIterator for FlatMap<I, U, F> wher
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<I, U, F> FusedIterator for FlatMap<I, U, F>
+    where I: FusedIterator, U: IntoIterator, F: FnMut(I::Item) -> U {}
+
 /// An iterator that yields `None` forever after the underlying iterator
 /// yields `None` once.
 ///
@@ -1651,12 +1715,15 @@ pub struct Fuse<I> {
     done: bool
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<I> FusedIterator for Fuse<I> where I: Iterator {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I> Iterator for Fuse<I> where I: Iterator {
     type Item = <I as Iterator>::Item;
 
     #[inline]
-    fn next(&mut self) -> Option<<I as Iterator>::Item> {
+    default fn next(&mut self) -> Option<<I as Iterator>::Item> {
         if self.done {
             None
         } else {
@@ -1667,7 +1734,7 @@ impl<I> Iterator for Fuse<I> where I: Iterator {
     }
 
     #[inline]
-    fn nth(&mut self, n: usize) -> Option<I::Item> {
+    default fn nth(&mut self, n: usize) -> Option<I::Item> {
         if self.done {
             None
         } else {
@@ -1678,7 +1745,7 @@ impl<I> Iterator for Fuse<I> where I: Iterator {
     }
 
     #[inline]
-    fn last(self) -> Option<I::Item> {
+    default fn last(self) -> Option<I::Item> {
         if self.done {
             None
         } else {
@@ -1687,7 +1754,7 @@ impl<I> Iterator for Fuse<I> where I: Iterator {
     }
 
     #[inline]
-    fn count(self) -> usize {
+    default fn count(self) -> usize {
         if self.done {
             0
         } else {
@@ -1696,7 +1763,7 @@ impl<I> Iterator for Fuse<I> where I: Iterator {
     }
 
     #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
+    default fn size_hint(&self) -> (usize, Option<usize>) {
         if self.done {
             (0, Some(0))
         } else {
@@ -1708,7 +1775,7 @@ impl<I> Iterator for Fuse<I> where I: Iterator {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I> DoubleEndedIterator for Fuse<I> where I: DoubleEndedIterator {
     #[inline]
-    fn next_back(&mut self) -> Option<<I as Iterator>::Item> {
+    default fn next_back(&mut self) -> Option<<I as Iterator>::Item> {
         if self.done {
             None
         } else {
@@ -1719,6 +1786,53 @@ impl<I> DoubleEndedIterator for Fuse<I> where I: DoubleEndedIterator {
     }
 }
 
+unsafe impl<I> TrustedRandomAccess for Fuse<I>
+    where I: TrustedRandomAccess,
+{
+    unsafe fn get_unchecked(&mut self, i: usize) -> I::Item {
+        self.iter.get_unchecked(i)
+    }
+}
+
+#[unstable(feature = "fused", issue = "35602")]
+impl<I> Iterator for Fuse<I> where I: FusedIterator {
+    #[inline]
+    fn next(&mut self) -> Option<<I as Iterator>::Item> {
+        self.iter.next()
+    }
+
+    #[inline]
+    fn nth(&mut self, n: usize) -> Option<I::Item> {
+        self.iter.nth(n)
+    }
+
+    #[inline]
+    fn last(self) -> Option<I::Item> {
+        self.iter.last()
+    }
+
+    #[inline]
+    fn count(self) -> usize {
+        self.iter.count()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
+
+#[unstable(feature = "fused", reason = "recently added", issue = "35602")]
+impl<I> DoubleEndedIterator for Fuse<I>
+    where I: DoubleEndedIterator + FusedIterator
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<<I as Iterator>::Item> {
+        self.iter.next_back()
+    }
+}
+
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I> ExactSizeIterator for Fuse<I> where I: ExactSizeIterator {}
 
@@ -1788,3 +1902,7 @@ impl<I: DoubleEndedIterator, F> DoubleEndedIterator for Inspect<I, F>
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I: ExactSizeIterator, F> ExactSizeIterator for Inspect<I, F>
     where F: FnMut(&I::Item) {}
+
+#[unstable(feature = "fused", issue = "35602")]
+impl<I: FusedIterator, F> FusedIterator for Inspect<I, F>
+    where F: FnMut(&I::Item) {}
diff --git a/src/libcore/iter/range.rs b/src/libcore/iter/range.rs
index 079dfe2a81f..48816bf66bb 100644
--- a/src/libcore/iter/range.rs
+++ b/src/libcore/iter/range.rs
@@ -16,7 +16,7 @@ use option::Option::{self, Some, None};
 use marker::Sized;
 use usize;
 
-use super::{DoubleEndedIterator, ExactSizeIterator, Iterator};
+use super::{DoubleEndedIterator, ExactSizeIterator, Iterator, FusedIterator};
 
 /// Objects that can be stepped over in both directions.
 ///
@@ -364,6 +364,10 @@ impl<A> Iterator for StepBy<A, ops::RangeFrom<A>> where
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<A> FusedIterator for StepBy<A, ops::RangeFrom<A>>
+    where A: Clone, for<'a> &'a A: Add<&'a A, Output = A> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A: Step + Clone> Iterator for StepBy<A, ops::Range<A>> {
     type Item = A;
@@ -401,6 +405,9 @@ impl<A: Step + Clone> Iterator for StepBy<A, ops::Range<A>> {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<A: Step + Clone> FusedIterator for StepBy<A, ops::Range<A>> {}
+
 #[unstable(feature = "inclusive_range",
            reason = "recently added, follows RFC",
            issue = "28237")]
@@ -468,6 +475,9 @@ impl<A: Step + Clone> Iterator for StepBy<A, ops::RangeInclusive<A>> {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<A: Step + Clone> FusedIterator for StepBy<A, ops::RangeInclusive<A>> {}
+
 macro_rules! range_exact_iter_impl {
     ($($t:ty)*) => ($(
         #[stable(feature = "rust1", since = "1.0.0")]
@@ -526,6 +536,10 @@ impl<A: Step + Clone> DoubleEndedIterator for ops::Range<A> where
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<A> FusedIterator for ops::Range<A>
+    where A: Step, for<'a> &'a A: Add<&'a A, Output = A> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A: Step> Iterator for ops::RangeFrom<A> where
     for<'a> &'a A: Add<&'a A, Output = A>
@@ -540,6 +554,10 @@ impl<A: Step> Iterator for ops::RangeFrom<A> where
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<A> FusedIterator for ops::RangeFrom<A>
+    where A: Step, for<'a> &'a A: Add<&'a A, Output = A> {}
+
 #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
 impl<A: Step> Iterator for ops::RangeInclusive<A> where
     for<'a> &'a A: Add<&'a A, Output = A>
@@ -638,3 +656,7 @@ impl<A: Step> DoubleEndedIterator for ops::RangeInclusive<A> where
         n
     }
 }
+
+#[unstable(feature = "fused", issue = "35602")]
+impl<A> FusedIterator for ops::RangeInclusive<A>
+    where A: Step, for<'a> &'a A: Add<&'a A, Output = A> {}
diff --git a/src/libcore/iter/sources.rs b/src/libcore/iter/sources.rs
index ecd4a78b9e7..a2a019a07dc 100644
--- a/src/libcore/iter/sources.rs
+++ b/src/libcore/iter/sources.rs
@@ -15,7 +15,7 @@ use marker;
 use option::Option::{self, Some, None};
 use usize;
 
-use super::{DoubleEndedIterator, IntoIterator, Iterator, ExactSizeIterator};
+use super::{DoubleEndedIterator, IntoIterator, Iterator, ExactSizeIterator, FusedIterator};
 
 /// An iterator that repeats an element endlessly.
 ///
@@ -44,6 +44,9 @@ impl<A: Clone> DoubleEndedIterator for Repeat<A> {
     fn next_back(&mut self) -> Option<A> { Some(self.element.clone()) }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<A: Clone> FusedIterator for Repeat<A> {}
+
 /// Creates a new iterator that endlessly repeats a single element.
 ///
 /// The `repeat()` function repeats a single value over and over and over and
@@ -138,6 +141,9 @@ impl<T> ExactSizeIterator for Empty<T> {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<T> FusedIterator for Empty<T> {}
+
 // not #[derive] because that adds a Clone bound on T,
 // which isn't necessary.
 #[stable(feature = "iter_empty", since = "1.2.0")]
@@ -213,6 +219,9 @@ impl<T> ExactSizeIterator for Once<T> {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<T> FusedIterator for Once<T> {}
+
 /// Creates an iterator that yields an element exactly once.
 ///
 /// This is commonly used to adapt a single value into a [`chain()`] of other
diff --git a/src/libcore/iter/traits.rs b/src/libcore/iter/traits.rs
index 4cbabe3f5ed..0b6ab5a4bbb 100644
--- a/src/libcore/iter/traits.rs
+++ b/src/libcore/iter/traits.rs
@@ -658,3 +658,19 @@ macro_rules! float_sum_product {
 
 integer_sum_product! { i8 i16 i32 i64 isize u8 u16 u32 u64 usize }
 float_sum_product! { f32 f64 }
+
+/// An iterator that always continues to yield `None` when exhausted.
+///
+/// Calling next on a fused iterator that has returned `None` once is guaranteed
+/// to return `None` again. This trait is should be implemented by all iterators
+/// that behave this way because it allows for some significant optimizations.
+///
+/// Note: In general, you should not use `FusedIterator` in generic bounds if
+/// you need a fused iterator. Instead, you should just call `Iterator::fused()`
+/// on the iterator. If the iterator is already fused, the additional `Fuse`
+/// wrapper will be a no-op with no performance penalty.
+#[unstable(feature = "fused", issue = "35602")]
+pub trait FusedIterator: Iterator {}
+
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, I: FusedIterator + ?Sized> FusedIterator for &'a mut I {}
diff --git a/src/libcore/option.rs b/src/libcore/option.rs
index fe508adb713..51bbad085fb 100644
--- a/src/libcore/option.rs
+++ b/src/libcore/option.rs
@@ -144,8 +144,8 @@ use self::Option::*;
 use clone::Clone;
 use convert::From;
 use default::Default;
-use iter::ExactSizeIterator;
-use iter::{Iterator, DoubleEndedIterator, FromIterator, IntoIterator};
+use iter::{Iterator, FromIterator, IntoIterator, ExactSizeIterator, DoubleEndedIterator};
+use iter::FusedIterator;
 use mem;
 use ops::FnOnce;
 use result::Result::{Ok, Err};
@@ -796,6 +796,7 @@ impl<A> DoubleEndedIterator for Item<A> {
 }
 
 impl<A> ExactSizeIterator for Item<A> {}
+impl<A> FusedIterator for Item<A> {}
 
 /// An iterator over a reference of the contained item in an Option.
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -821,6 +822,9 @@ impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, A> ExactSizeIterator for Iter<'a, A> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, A> FusedIterator for Iter<'a, A> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, A> Clone for Iter<'a, A> {
     fn clone(&self) -> Iter<'a, A> {
@@ -852,6 +856,9 @@ impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, A> ExactSizeIterator for IterMut<'a, A> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, A> FusedIterator for IterMut<'a, A> {}
+
 /// An iterator over the item contained inside an Option.
 #[derive(Clone, Debug)]
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -876,6 +883,9 @@ impl<A> DoubleEndedIterator for IntoIter<A> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A> ExactSizeIterator for IntoIter<A> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<A> FusedIterator for IntoIter<A> {}
+
 /////////////////////////////////////////////////////////////////////////////
 // FromIterator
 /////////////////////////////////////////////////////////////////////////////
diff --git a/src/libcore/result.rs b/src/libcore/result.rs
index c7ca70fc162..718fdf865a9 100644
--- a/src/libcore/result.rs
+++ b/src/libcore/result.rs
@@ -241,6 +241,7 @@ use self::Result::{Ok, Err};
 use clone::Clone;
 use fmt;
 use iter::{Iterator, DoubleEndedIterator, FromIterator, ExactSizeIterator, IntoIterator};
+use iter::FusedIterator;
 use ops::FnOnce;
 use option::Option::{self, None, Some};
 
@@ -869,6 +870,9 @@ impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> ExactSizeIterator for Iter<'a, T> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, T> FusedIterator for Iter<'a, T> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> Clone for Iter<'a, T> {
     fn clone(&self) -> Iter<'a, T> { Iter { inner: self.inner } }
@@ -901,6 +905,9 @@ impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> ExactSizeIterator for IterMut<'a, T> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, T> FusedIterator for IterMut<'a, T> {}
+
 /// An iterator over the value in a `Ok` variant of a `Result`.
 #[derive(Debug)]
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -928,6 +935,9 @@ impl<T> DoubleEndedIterator for IntoIter<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> ExactSizeIterator for IntoIter<T> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<T> FusedIterator for IntoIter<T> {}
+
 /////////////////////////////////////////////////////////////////////////////
 // FromIterator
 /////////////////////////////////////////////////////////////////////////////
diff --git a/src/libcore/slice.rs b/src/libcore/slice.rs
index 3a820a14f12..603f55a6e10 100644
--- a/src/libcore/slice.rs
+++ b/src/libcore/slice.rs
@@ -992,6 +992,9 @@ iterator!{struct Iter -> *const T, &'a T}
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> ExactSizeIterator for Iter<'a, T> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, T> FusedIterator for Iter<'a, T> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> Clone for Iter<'a, T> {
     fn clone(&self) -> Iter<'a, T> { Iter { ptr: self.ptr, end: self.end, _marker: self._marker } }
@@ -1110,6 +1113,9 @@ iterator!{struct IterMut -> *mut T, &'a mut T}
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> ExactSizeIterator for IterMut<'a, T> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, T> FusedIterator for IterMut<'a, T> {}
+
 /// An internal abstraction over the splitting iterators, so that
 /// splitn, splitn_mut etc can be implemented once.
 #[doc(hidden)]
@@ -1202,6 +1208,9 @@ impl<'a, T, P> SplitIter for Split<'a, T, P> where P: FnMut(&T) -> bool {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, T, P> FusedIterator for Split<'a, T, P> where P: FnMut(&T) -> bool {}
+
 /// An iterator over the subslices of the vector which are separated
 /// by elements that match `pred`.
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1292,6 +1301,9 @@ impl<'a, T, P> DoubleEndedIterator for SplitMut<'a, T, P> where
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, T, P> FusedIterator for SplitMut<'a, T, P> where P: FnMut(&T) -> bool {}
+
 /// An private iterator over subslices separated by elements that
 /// match a predicate function, splitting at most a fixed number of
 /// times.
@@ -1408,6 +1420,10 @@ macro_rules! forward_iterator {
                 self.inner.size_hint()
             }
         }
+
+        #[unstable(feature = "fused", issue = "35602")]
+        impl<'a, $elem, P> FusedIterator for $name<'a, $elem, P>
+            where P: FnMut(&T) -> bool {}
     }
 }
 
@@ -1506,6 +1522,9 @@ impl<'a, T> DoubleEndedIterator for Windows<'a, T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> ExactSizeIterator for Windows<'a, T> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, T> FusedIterator for Windows<'a, T> {}
+
 /// An iterator over a slice in (non-overlapping) chunks (`size` elements at a
 /// time).
 ///
@@ -1609,6 +1628,9 @@ impl<'a, T> DoubleEndedIterator for Chunks<'a, T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> ExactSizeIterator for Chunks<'a, T> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, T> FusedIterator for Chunks<'a, T> {}
+
 /// An iterator over a slice in (non-overlapping) mutable chunks (`size`
 /// elements at a time). When the slice len is not evenly divided by the chunk
 /// size, the last slice of the iteration will be the remainder.
@@ -1704,6 +1726,9 @@ impl<'a, T> DoubleEndedIterator for ChunksMut<'a, T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> ExactSizeIterator for ChunksMut<'a, T> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a, T> FusedIterator for ChunksMut<'a, T> {}
+
 //
 // Free functions
 //
diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index fdcadd43a0f..e18aa07b75d 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -23,7 +23,7 @@ use convert::AsRef;
 use default::Default;
 use fmt;
 use iter::ExactSizeIterator;
-use iter::{Map, Cloned, Iterator, DoubleEndedIterator};
+use iter::{Map, Cloned, Iterator, DoubleEndedIterator, FusedIterator};
 use marker::Sized;
 use mem;
 use ops::{Fn, FnMut, FnOnce};
@@ -454,6 +454,9 @@ impl<'a> DoubleEndedIterator for Chars<'a> {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a> FusedIterator for Chars<'a> {}
+
 impl<'a> Chars<'a> {
     /// View the underlying data as a subslice of the original data.
     ///
@@ -525,6 +528,9 @@ impl<'a> DoubleEndedIterator for CharIndices<'a> {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a> FusedIterator for CharIndices<'a> {}
+
 impl<'a> CharIndices<'a> {
     /// View the underlying data as a subslice of the original data.
     ///
@@ -593,6 +599,9 @@ impl<'a> ExactSizeIterator for Bytes<'a> {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a> FusedIterator for Bytes<'a> {}
+
 /// This macro generates a Clone impl for string pattern API
 /// wrapper types of the form X<'a, P>
 macro_rules! derive_pattern_clone {
@@ -739,6 +748,13 @@ macro_rules! generate_pattern_iterators {
             }
         }
 
+        #[unstable(feature = "fused", issue = "35602")]
+        impl<'a, P: Pattern<'a>> FusedIterator for $forward_iterator<'a, P> {}
+
+        #[unstable(feature = "fused", issue = "35602")]
+        impl<'a, P: Pattern<'a>> FusedIterator for $reverse_iterator<'a, P>
+            where P::Searcher: ReverseSearcher<'a> {}
+
         generate_pattern_iterators!($($t)* with $(#[$common_stability_attribute])*,
                                                 $forward_iterator,
                                                 $reverse_iterator, $iterty);
@@ -1088,6 +1104,9 @@ impl<'a> DoubleEndedIterator for Lines<'a> {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<'a> FusedIterator for Lines<'a> {}
+
 /// Created with the method [`lines_any()`].
 ///
 /// [`lines_any()`]: ../../std/primitive.str.html#method.lines_any
@@ -1151,6 +1170,10 @@ impl<'a> DoubleEndedIterator for LinesAny<'a> {
     }
 }
 
+#[unstable(feature = "fused", issue = "35602")]
+#[allow(deprecated)]
+impl<'a> FusedIterator for LinesAny<'a> {}
+
 /*
 Section: Comparing strings
 */