diff --git a/src/libcore/iter/mod.rs b/src/libcore/iter/mod.rs
index 9eeb2608071c..9fa950cd94d9 100644
--- a/src/libcore/iter/mod.rs
+++ b/src/libcore/iter/mod.rs
@@ -328,6 +328,8 @@ pub use self::traits::{FromIterator, IntoIterator, DoubleEndedIterator, Extend};
 pub use self::traits::{ExactSizeIterator, Sum, Product};
 #[unstable(feature = "fused", issue = "35602")]
 pub use self::traits::FusedIterator;
+#[unstable(feature = "trusted_len", issue = "0")]
+pub use self::traits::TrustedLen;
 
 mod iterator;
 mod range;
@@ -372,6 +374,10 @@ impl<I> ExactSizeIterator for Rev<I>
 impl<I> FusedIterator for Rev<I>
     where I: FusedIterator + DoubleEndedIterator {}
 
+#[unstable(feature = "trusted_len", issue = "0")]
+unsafe impl<I> TrustedLen for Rev<I>
+    where I: TrustedLen + DoubleEndedIterator {}
+
 /// An iterator that clones the elements of an underlying iterator.
 ///
 /// This `struct` is created by the [`cloned()`] method on [`Iterator`]. See its
@@ -432,6 +438,12 @@ unsafe impl<'a, I, T: 'a> TrustedRandomAccess for Cloned<I>
     fn may_have_side_effect() -> bool { true }
 }
 
+#[unstable(feature = "trusted_len", issue = "0")]
+unsafe impl<'a, I, T: 'a> TrustedLen for Cloned<I>
+    where I: TrustedLen<Item=&'a T>,
+          T: Clone
+{}
+
 /// An iterator that repeats endlessly.
 ///
 /// This `struct` is created by the [`cycle()`] method on [`Iterator`]. See its
@@ -642,6 +654,11 @@ impl<A, B> FusedIterator for Chain<A, B>
           B: FusedIterator<Item=A::Item>,
 {}
 
+#[unstable(feature = "trusted_len", issue = "0")]
+unsafe impl<A, B> TrustedLen for Chain<A, B>
+    where A: TrustedLen, B: TrustedLen<Item=A::Item>,
+{}
+
 /// An iterator that iterates two other iterators simultaneously.
 ///
 /// This `struct` is created by the [`zip()`] method on [`Iterator`]. See its
@@ -859,6 +876,11 @@ unsafe impl<A, B> TrustedRandomAccess for Zip<A, B>
 impl<A, B> FusedIterator for Zip<A, B>
     where A: FusedIterator, B: FusedIterator, {}
 
+#[unstable(feature = "trusted_len", issue = "0")]
+unsafe impl<A, B> TrustedLen for Zip<A, B>
+    where A: TrustedLen, B: TrustedLen,
+{}
+
 /// An iterator that maps the values of `iter` with `f`.
 ///
 /// This `struct` is created by the [`map()`] method on [`Iterator`]. See its
@@ -959,6 +981,11 @@ impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F>
 impl<B, I: FusedIterator, F> FusedIterator for Map<I, F>
     where F: FnMut(I::Item) -> B {}
 
+#[unstable(feature = "trusted_len", issue = "0")]
+unsafe impl<B, I, F> TrustedLen for Map<I, F>
+    where I: TrustedLen,
+          F: FnMut(I::Item) -> B {}
+
 #[doc(hidden)]
 unsafe impl<B, I, F> TrustedRandomAccess for Map<I, F>
     where I: TrustedRandomAccess,
@@ -1195,6 +1222,12 @@ unsafe impl<I> TrustedRandomAccess for Enumerate<I>
 #[unstable(feature = "fused", issue = "35602")]
 impl<I> FusedIterator for Enumerate<I> where I: FusedIterator {}
 
+#[unstable(feature = "trusted_len", issue = "0")]
+unsafe impl<I> TrustedLen for Enumerate<I>
+    where I: TrustedLen,
+{}
+
+
 /// An iterator with a `peek()` that returns an optional reference to the next
 /// element.
 ///
diff --git a/src/libcore/iter/range.rs b/src/libcore/iter/range.rs
index eaa3d50c88ad..39da578d5491 100644
--- a/src/libcore/iter/range.rs
+++ b/src/libcore/iter/range.rs
@@ -12,7 +12,7 @@ use mem;
 use ops::{self, Add, Sub};
 use usize;
 
-use super::FusedIterator;
+use super::{FusedIterator, TrustedLen};
 
 /// Objects that can be stepped over in both directions.
 ///
@@ -533,6 +533,9 @@ impl<A: Step + Clone> DoubleEndedIterator for ops::Range<A> where
 impl<A> FusedIterator for ops::Range<A>
     where A: Step, for<'a> &'a A: Add<&'a A, Output = A> {}
 
+#[unstable(feature = "trusted_len", issue = "0")]
+unsafe impl TrustedLen for ops::Range<usize> { }
+
 #[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>
diff --git a/src/libcore/iter/traits.rs b/src/libcore/iter/traits.rs
index b55d6f96af9b..da150f1d57a6 100644
--- a/src/libcore/iter/traits.rs
+++ b/src/libcore/iter/traits.rs
@@ -665,3 +665,10 @@ pub trait FusedIterator: Iterator {}
 
 #[unstable(feature = "fused", issue = "35602")]
 impl<'a, I: FusedIterator + ?Sized> FusedIterator for &'a mut I {}
+
+/// An iterator that has correct length
+#[unstable(feature = "trusted_len", issue = "0")]
+pub unsafe trait TrustedLen : Iterator {}
+
+#[unstable(feature = "trusted_len", issue = "0")]
+unsafe impl<'a, I: TrustedLen + ?Sized> TrustedLen for &'a mut I {}
diff --git a/src/libcore/slice.rs b/src/libcore/slice.rs
index 31be404ba905..64d38c54c476 100644
--- a/src/libcore/slice.rs
+++ b/src/libcore/slice.rs
@@ -988,6 +988,9 @@ impl<'a, T> ExactSizeIterator for Iter<'a, T> {}
 #[unstable(feature = "fused", issue = "35602")]
 impl<'a, T> FusedIterator for Iter<'a, T> {}
 
+#[unstable(feature = "trusted_len", issue = "0")]
+unsafe impl<'a, T> TrustedLen 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 } }
@@ -1109,6 +1112,9 @@ impl<'a, T> ExactSizeIterator for IterMut<'a, T> {}
 #[unstable(feature = "fused", issue = "35602")]
 impl<'a, T> FusedIterator for IterMut<'a, T> {}
 
+#[unstable(feature = "trusted_len", issue = "0")]
+unsafe impl<'a, T> TrustedLen for IterMut<'a, T> {}
+
 /// An internal abstraction over the splitting iterators, so that
 /// splitn, splitn_mut etc can be implemented once.
 #[doc(hidden)]