diff --git a/src/libcore/iter/adapters/flatten.rs b/src/libcore/iter/adapters/flatten.rs
index 40f6865d38bcf..565546da6a80a 100644
--- a/src/libcore/iter/adapters/flatten.rs
+++ b/src/libcore/iter/adapters/flatten.rs
@@ -226,11 +226,19 @@ impl<I, U> Iterator for FlattenCompat<I, U>
     fn size_hint(&self) -> (usize, Option<usize>) {
         let (flo, fhi) = self.frontiter.as_ref().map_or((0, Some(0)), |it| it.size_hint());
         let (blo, bhi) = self.backiter.as_ref().map_or((0, Some(0)), |it| it.size_hint());
-        let lo = flo.saturating_add(blo);
-        match (self.iter.size_hint(), fhi, bhi) {
-            ((0, Some(0)), Some(a), Some(b)) => (lo, a.checked_add(b)),
-            _ => (lo, None)
-        }
+        let (mlo, mhi) = self.iter.size_hint();
+        let max = U::max_size_hint();
+
+        let lo = flo.saturating_add(blo).saturating_add(mlo.saturating_mul(max.unwrap_or(0)));
+        let hi = match (fhi, bhi, mhi, max) {
+            (Some(f), Some(b), Some(0), None) => f.checked_add(b),
+            (Some(f), Some(b), Some(m), Some(max)) => try {
+                f.checked_add(b)?.checked_add(m.checked_mul(max)?)?
+             },
+            _ => None,
+        };
+
+        (lo, hi)
     }
 
     #[inline]
diff --git a/src/libcore/iter/traits/iterator.rs b/src/libcore/iter/traits/iterator.rs
index 9dfa83f473baf..12b14ffb28ec0 100644
--- a/src/libcore/iter/traits/iterator.rs
+++ b/src/libcore/iter/traits/iterator.rs
@@ -199,6 +199,15 @@ pub trait Iterator {
     #[stable(feature = "rust1", since = "1.0.0")]
     fn size_hint(&self) -> (usize, Option<usize>) { (0, None) }
 
+    /// An estimate of the maximum size of iterators of this type.
+    ///
+    /// This is similar to [`size_hint`], with the exception that it applies to all
+    /// iterators of this type. It can be used to optimize adapters like [`flatten`]
+    /// and provide better size hints.
+    #[inline]
+    #[unstable(feature = "max_size_hint", issue = "0")]
+    fn max_size_hint() -> Option<usize> where Self: Sized { None }
+
     /// Consumes the iterator, counting the number of iterations and returning it.
     ///
     /// This method will evaluate the iterator until its [`next`] returns
diff --git a/src/libcore/option.rs b/src/libcore/option.rs
index 0e54397db0247..ff65a8c0aaade 100644
--- a/src/libcore/option.rs
+++ b/src/libcore/option.rs
@@ -1172,6 +1172,8 @@ impl<'a, A> Iterator for Iter<'a, A> {
     fn next(&mut self) -> Option<&'a A> { self.inner.next() }
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
+    #[inline]
+    fn max_size_hint() -> Option<usize> { Some(1) }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1218,6 +1220,8 @@ impl<'a, A> Iterator for IterMut<'a, A> {
     fn next(&mut self) -> Option<&'a mut A> { self.inner.next() }
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
+    #[inline]
+    fn max_size_hint() -> Option<usize> { Some(1) }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1255,6 +1259,8 @@ impl<A> Iterator for IntoIter<A> {
     fn next(&mut self) -> Option<A> { self.inner.next() }
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
+    #[inline]
+    fn max_size_hint() -> Option<usize> { Some(1) }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
diff --git a/src/libcore/result.rs b/src/libcore/result.rs
index 1ebf0714e23e4..545dc16fe9edc 100644
--- a/src/libcore/result.rs
+++ b/src/libcore/result.rs
@@ -1077,6 +1077,8 @@ impl<'a, T> Iterator for Iter<'a, T> {
         let n = if self.inner.is_some() {1} else {0};
         (n, Some(n))
     }
+    #[inline]
+    fn max_size_hint() -> Option<usize> { Some(1) }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1122,6 +1124,8 @@ impl<'a, T> Iterator for IterMut<'a, T> {
         let n = if self.inner.is_some() {1} else {0};
         (n, Some(n))
     }
+    #[inline]
+    fn max_size_hint() -> Option<usize> { Some(1) }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1165,6 +1169,8 @@ impl<T> Iterator for IntoIter<T> {
         let n = if self.inner.is_some() {1} else {0};
         (n, Some(n))
     }
+    #[inline]
+    fn max_size_hint() -> Option<usize> { Some(1) }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]