diff --git a/src/libcollections/lib.rs b/src/libcollections/lib.rs
index 990de541b6783..23d6edd6d794e 100644
--- a/src/libcollections/lib.rs
+++ b/src/libcollections/lib.rs
@@ -50,6 +50,7 @@
 #![feature(specialization)]
 #![feature(staged_api)]
 #![feature(step_by)]
+#![feature(trusted_len)]
 #![feature(unicode)]
 #![feature(unique)]
 #![cfg_attr(test, feature(rand, test))]
diff --git a/src/libcollections/vec.rs b/src/libcollections/vec.rs
index f3d78c20a4d6b..053dcd8272e29 100644
--- a/src/libcollections/vec.rs
+++ b/src/libcollections/vec.rs
@@ -68,7 +68,7 @@ use core::cmp::Ordering;
 use core::fmt;
 use core::hash::{self, Hash};
 use core::intrinsics::{arith_offset, assume};
-use core::iter::{FromIterator, FusedIterator};
+use core::iter::{FromIterator, FusedIterator, TrustedLen};
 use core::mem;
 use core::ops::{Index, IndexMut};
 use core::ops;
@@ -76,7 +76,6 @@ use core::ptr;
 use core::ptr::Shared;
 use core::slice;
 
-use super::SpecExtend;
 use super::range::RangeArgument;
 
 /// A contiguous growable array type, written `Vec<T>` but pronounced 'vector.'
@@ -1212,26 +1211,7 @@ impl<T: Clone> Vec<T> {
     /// ```
     #[stable(feature = "vec_extend_from_slice", since = "1.6.0")]
     pub fn extend_from_slice(&mut self, other: &[T]) {
-        self.reserve(other.len());
-
-        // Unsafe code so this can be optimised to a memcpy (or something
-        // similarly fast) when T is Copy. LLVM is easily confused, so any
-        // extra operations during the loop can prevent this optimisation.
-        unsafe {
-            let len = self.len();
-            let ptr = self.get_unchecked_mut(len) as *mut T;
-            // Use SetLenOnDrop to work around bug where compiler
-            // may not realize the store through `ptr` trough self.set_len()
-            // don't alias.
-            let mut local_len = SetLenOnDrop::new(&mut self.len);
-
-            for i in 0..other.len() {
-                ptr::write(ptr.offset(i as isize), other.get_unchecked(i).clone());
-                local_len.increment_len(1);
-            }
-
-            // len set by scope guard
-        }
+        self.extend(other.iter().cloned())
     }
 }
 
@@ -1573,19 +1553,25 @@ impl<'a, T> IntoIterator for &'a mut Vec<T> {
 impl<T> Extend<T> for Vec<T> {
     #[inline]
     fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
-        <Self as SpecExtend<I>>::spec_extend(self, iter);
+        self.extend_desugared(iter.into_iter())
     }
 }
 
-impl<I: IntoIterator> SpecExtend<I> for Vec<I::Item> {
-    default fn spec_extend(&mut self, iter: I) {
-        self.extend_desugared(iter.into_iter())
-    }
+trait IsTrustedLen : Iterator {
+    fn trusted_len(&self) -> Option<usize> { None }
 }
+impl<I> IsTrustedLen for I where I: Iterator { }
 
-impl<T> SpecExtend<Vec<T>> for Vec<T> {
-    fn spec_extend(&mut self, ref mut other: Vec<T>) {
-        self.append(other);
+impl<I> IsTrustedLen for I where I: TrustedLen
+{
+    fn trusted_len(&self) -> Option<usize> {
+        let (low, high) = self.size_hint();
+        if let Some(high_value) = high {
+            debug_assert_eq!(low, high_value,
+                             "TrustedLen iterator's size hint is not exact: {:?}",
+                             (low, high));
+        }
+        high
     }
 }
 
@@ -1596,16 +1582,30 @@ impl<T> Vec<T> {
         //      for item in iterator {
         //          self.push(item);
         //      }
-        while let Some(element) = iterator.next() {
-            let len = self.len();
-            if len == self.capacity() {
-                let (lower, _) = iterator.size_hint();
-                self.reserve(lower.saturating_add(1));
-            }
+        if let Some(additional) = iterator.trusted_len() {
+            self.reserve(additional);
             unsafe {
-                ptr::write(self.get_unchecked_mut(len), element);
-                // NB can't overflow since we would have had to alloc the address space
-                self.set_len(len + 1);
+                let mut ptr = self.as_mut_ptr().offset(self.len() as isize);
+                let mut local_len = SetLenOnDrop::new(&mut self.len);
+                for element in iterator {
+                    ptr::write(ptr, element);
+                    ptr = ptr.offset(1);
+                    // NB can't overflow since we would have had to alloc the address space
+                    local_len.increment_len(1);
+                }
+            }
+        } else {
+            while let Some(element) = iterator.next() {
+                let len = self.len();
+                if len == self.capacity() {
+                    let (lower, _) = iterator.size_hint();
+                    self.reserve(lower.saturating_add(1));
+                }
+                unsafe {
+                    ptr::write(self.get_unchecked_mut(len), element);
+                    // NB can't overflow since we would have had to alloc the address space
+                    self.set_len(len + 1);
+                }
             }
         }
     }
@@ -1614,24 +1614,7 @@ impl<T> Vec<T> {
 #[stable(feature = "extend_ref", since = "1.2.0")]
 impl<'a, T: 'a + Copy> Extend<&'a T> for Vec<T> {
     fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
-        <I as SpecExtendVec<T>>::extend_vec(iter, self);
-    }
-}
-
-// helper trait for specialization of Vec's Extend impl
-trait SpecExtendVec<T> {
-    fn extend_vec(self, vec: &mut Vec<T>);
-}
-
-impl <'a, T: 'a + Copy, I: IntoIterator<Item=&'a T>> SpecExtendVec<T> for I {
-    default fn extend_vec(self, vec: &mut Vec<T>) {
-        vec.extend(self.into_iter().cloned());
-    }
-}
-
-impl<'a, T: Copy> SpecExtendVec<T> for &'a [T] {
-    fn extend_vec(self, vec: &mut Vec<T>) {
-        vec.extend_from_slice(self);
+        self.extend(iter.into_iter().map(|&x| x))
     }
 }
 
@@ -1948,6 +1931,9 @@ impl<T> ExactSizeIterator for IntoIter<T> {}
 #[unstable(feature = "fused", issue = "35602")]
 impl<T> FusedIterator for IntoIter<T> {}
 
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<T> TrustedLen for IntoIter<T> {}
+
 #[stable(feature = "vec_into_iter_clone", since = "1.8.0")]
 impl<T: Clone> Clone for IntoIter<T> {
     fn clone(&self) -> IntoIter<T> {
diff --git a/src/libcore/iter/mod.rs b/src/libcore/iter/mod.rs
index 9eeb2608071c2..58d04218e072c 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 = "37572")]
+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 = "37572")]
+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 = "37572")]
+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 = "37572")]
+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 = "37572")]
+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 = "37572")]
+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 = "37572")]
+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 eaa3d50c88ade..7c96be2facb48 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.
 ///
@@ -480,6 +480,22 @@ macro_rules! range_incl_exact_iter_impl {
     )*)
 }
 
+macro_rules! range_trusted_len_impl {
+    ($($t:ty)*) => ($(
+        #[unstable(feature = "trusted_len", issue = "37572")]
+        unsafe impl TrustedLen for ops::Range<$t> { }
+    )*)
+}
+
+macro_rules! range_incl_trusted_len_impl {
+    ($($t:ty)*) => ($(
+        #[unstable(feature = "inclusive_range",
+                   reason = "recently added, follows RFC",
+                   issue = "28237")]
+        unsafe impl TrustedLen for ops::RangeInclusive<$t> { }
+    )*)
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A: Step> Iterator for ops::Range<A> where
     for<'a> &'a A: Add<&'a A, Output = A>
@@ -513,6 +529,13 @@ impl<A: Step> Iterator for ops::Range<A> where
 range_exact_iter_impl!(usize u8 u16 u32 isize i8 i16 i32);
 range_incl_exact_iter_impl!(u8 u16 i8 i16);
 
+// These macros generate `TrustedLen` impls.
+//
+// They need to guarantee that .size_hint() is either exact, or that
+// the upper bound is None when it does not fit the type limits.
+range_trusted_len_impl!(usize isize u8 i8 u16 i16 u32 i32 i64 u64);
+range_incl_trusted_len_impl!(usize isize u8 i8 u16 i16 u32 i32 i64 u64);
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A: Step + Clone> DoubleEndedIterator for ops::Range<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 b55d6f96af9bf..2b0dd63a5714a 100644
--- a/src/libcore/iter/traits.rs
+++ b/src/libcore/iter/traits.rs
@@ -665,3 +665,22 @@ pub trait FusedIterator: Iterator {}
 
 #[unstable(feature = "fused", issue = "35602")]
 impl<'a, I: FusedIterator + ?Sized> FusedIterator for &'a mut I {}
+
+/// An iterator that reports an accurate length using size_hint.
+///
+/// The iterator reports a size hint where it is either exact
+/// (lower bound is equal to upper bound), or the upper bound is `None`.
+/// The upper bound must only be `None` if the actual iterator length is
+/// larger than `usize::MAX`.
+///
+/// The iterator must produce exactly the number of elements it reported.
+///
+/// # Safety
+///
+/// This trait must only be implemented when the contract is upheld.
+/// Consumers of this trait must inspect `.size_hint()`’s upper bound.
+#[unstable(feature = "trusted_len", issue = "37572")]
+pub unsafe trait TrustedLen : Iterator {}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<'a, I: TrustedLen + ?Sized> TrustedLen for &'a mut I {}
diff --git a/src/libcore/option.rs b/src/libcore/option.rs
index cb18feff73422..063adc2ac9f8a 100644
--- a/src/libcore/option.rs
+++ b/src/libcore/option.rs
@@ -145,7 +145,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use iter::{FromIterator, FusedIterator};
+use iter::{FromIterator, FusedIterator, TrustedLen};
 use mem;
 
 // Note that this is not a lang item per se, but it has a hidden dependency on
@@ -803,6 +803,7 @@ impl<A> DoubleEndedIterator for Item<A> {
 
 impl<A> ExactSizeIterator for Item<A> {}
 impl<A> FusedIterator for Item<A> {}
+unsafe impl<A> TrustedLen for Item<A> {}
 
 /// An iterator over a reference of the contained item in an [`Option`].
 ///
@@ -833,6 +834,9 @@ impl<'a, A> ExactSizeIterator for Iter<'a, A> {}
 #[unstable(feature = "fused", issue = "35602")]
 impl<'a, A> FusedIterator for Iter<'a, A> {}
 
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<'a, A> TrustedLen 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> {
@@ -868,6 +872,8 @@ impl<'a, A> ExactSizeIterator for IterMut<'a, A> {}
 
 #[unstable(feature = "fused", issue = "35602")]
 impl<'a, A> FusedIterator for IterMut<'a, A> {}
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<'a, A> TrustedLen for IterMut<'a, A> {}
 
 /// An iterator over the item contained inside an [`Option`].
 ///
@@ -898,6 +904,9 @@ impl<A> ExactSizeIterator for IntoIter<A> {}
 #[unstable(feature = "fused", issue = "35602")]
 impl<A> FusedIterator for IntoIter<A> {}
 
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<A> TrustedLen for IntoIter<A> {}
+
 /////////////////////////////////////////////////////////////////////////////
 // FromIterator
 /////////////////////////////////////////////////////////////////////////////
diff --git a/src/libcore/result.rs b/src/libcore/result.rs
index 96845259299be..bae1eda7c4a9b 100644
--- a/src/libcore/result.rs
+++ b/src/libcore/result.rs
@@ -249,7 +249,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use fmt;
-use iter::{FromIterator, FusedIterator};
+use iter::{FromIterator, FusedIterator, TrustedLen};
 
 /// `Result` is a type that represents either success (`Ok`) or failure (`Err`).
 ///
@@ -886,6 +886,9 @@ 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 = "37572")]
+unsafe impl<'a, A> TrustedLen for Iter<'a, A> {}
+
 #[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 } }
@@ -924,6 +927,9 @@ 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 = "37572")]
+unsafe impl<'a, A> TrustedLen for IterMut<'a, A> {}
+
 /// An iterator over the value in a [`Ok`] variant of a [`Result`]. This struct is
 /// created by the [`into_iter`] method on [`Result`][`Result`] (provided by
 /// the [`IntoIterator`] trait).
@@ -961,6 +967,9 @@ impl<T> ExactSizeIterator for IntoIter<T> {}
 #[unstable(feature = "fused", issue = "35602")]
 impl<T> FusedIterator for IntoIter<T> {}
 
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<A> TrustedLen for IntoIter<A> {}
+
 /////////////////////////////////////////////////////////////////////////////
 // FromIterator
 /////////////////////////////////////////////////////////////////////////////
diff --git a/src/libcore/slice.rs b/src/libcore/slice.rs
index 31be404ba905a..871b63145ca6d 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 = "37572")]
+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 = "37572")]
+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)]