diff --git a/src/libcore/char.rs b/src/libcore/char.rs
index bbeebf52a73c19ddeda47bfcc44187b4532ed7a0..718c6b893edf229cfecf8ddb9d337521634136fd 100644
--- a/src/libcore/char.rs
+++ b/src/libcore/char.rs
@@ -902,6 +902,16 @@ fn next(&mut self) -> Option<Result<char, InvalidSequence>> {
             }
         })
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (lower, upper) = self.0.size_hint();
+
+        // A code point is at most 4 bytes long.
+        let min_code_points = lower / 4;
+
+        (min_code_points, upper)
+    }
 }
 
 #[unstable(feature = "decode_utf8", issue = "33906")]
diff --git a/src/libcore/iter/traits.rs b/src/libcore/iter/traits.rs
index 0267fcd375453cbcdd24f9f6e330ebdb3a65787f..c3aebc4fb23ce37eb8c4aaf63c65a4297d798889 100644
--- a/src/libcore/iter/traits.rs
+++ b/src/libcore/iter/traits.rs
@@ -901,6 +901,15 @@ fn next(&mut self) -> Option<Self::Item> {
             None => None,
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.error.is_some() {
+            (0, Some(0))
+        } else {
+            let (_, upper) = self.iter.size_hint();
+            (0, upper)
+        }
+    }
 }
 
 #[stable(feature = "iter_arith_traits_result", since="1.16.0")]
diff --git a/src/libcore/option.rs b/src/libcore/option.rs
index 570f745f8330c78739bd095d97d20416b9e362cc..61ef6798b2efb2062d5c874f531c86171a573c22 100644
--- a/src/libcore/option.rs
+++ b/src/libcore/option.rs
@@ -1188,6 +1188,16 @@ fn next(&mut self) -> Option<T> {
                     None => None,
                 }
             }
+
+            #[inline]
+            fn size_hint(&self) -> (usize, Option<usize>) {
+                if self.found_none {
+                    (0, Some(0))
+                } else {
+                    let (_, upper) = self.iter.size_hint();
+                    (0, upper)
+                }
+            }
         }
 
         let mut adapter = Adapter { iter: iter.into_iter(), found_none: false };
diff --git a/src/librustc/hir/pat_util.rs b/src/librustc/hir/pat_util.rs
index 2bec224362ea64710106683d573240ba914afacc..1f00a3ab1f5ded464dc5fb701b7725ad9afa0a9e 100644
--- a/src/librustc/hir/pat_util.rs
+++ b/src/librustc/hir/pat_util.rs
@@ -31,6 +31,10 @@ fn next(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
             (if i < self.gap_pos { i } else { i + self.gap_len }, elem)
         })
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.enumerate.size_hint()
+    }
 }
 
 pub trait EnumerateAndAdjustIterator {
diff --git a/src/librustc/mir/traversal.rs b/src/librustc/mir/traversal.rs
index 74c3408c4c2f61837d9e3e7fb4841b7080ee0518..666ca5eabe81b93485f5ba5cf3718ceaef1b43e1 100644
--- a/src/librustc/mir/traversal.rs
+++ b/src/librustc/mir/traversal.rs
@@ -77,8 +77,18 @@ fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
 
         None
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // All the blocks, minus the number of blocks we've visited.
+        let remaining = self.mir.basic_blocks().len() - self.visited.count();
+
+        // We will visit all remaining blocks exactly once.
+        (remaining, Some(remaining))
+    }
 }
 
+impl<'a, 'tcx> ExactSizeIterator for Preorder<'a, 'tcx> {}
+
 /// Postorder traversal of a graph.
 ///
 /// Postorder traversal is when each node is visited after all of it's
@@ -210,8 +220,18 @@ fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
 
         next.map(|(bb, _)| (bb, &self.mir[bb]))
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // All the blocks, minus the number of blocks we've visited.
+        let remaining = self.mir.basic_blocks().len() - self.visited.count();
+
+        // We will visit all remaining blocks exactly once.
+        (remaining, Some(remaining))
+    }
 }
 
+impl<'a, 'tcx> ExactSizeIterator for Postorder<'a, 'tcx> {}
+
 /// Reverse postorder traversal of a graph
 ///
 /// Reverse postorder is the reverse order of a postorder traversal.
@@ -276,4 +296,10 @@ fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
 
         self.blocks.get(self.idx).map(|&bb| (bb, &self.mir[bb]))
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.idx, Some(self.idx))
+    }
 }
+
+impl<'a, 'tcx> ExactSizeIterator for ReversePostorder<'a, 'tcx> {}
diff --git a/src/librustc/session/search_paths.rs b/src/librustc/session/search_paths.rs
index 5bbc6841693ea3628c75c8ebf35988b046485ac1..d2dca9f60845a42ee91d7c2d1e47b344633a786a 100644
--- a/src/librustc/session/search_paths.rs
+++ b/src/librustc/session/search_paths.rs
@@ -78,4 +78,11 @@ fn next(&mut self) -> Option<(&'a Path, PathKind)> {
             }
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // This iterator will never return more elements than the base iterator;
+        // but it can ignore all the remaining elements.
+        let (_, upper) = self.iter.size_hint();
+        (0, upper)
+    }
 }
diff --git a/src/librustc/traits/util.rs b/src/librustc/traits/util.rs
index 8f7a24057472c1c2d65b48b4a52e13b366a393e6..9e38911f53c9a0ec179cb93f5428725b1e771349 100644
--- a/src/librustc/traits/util.rs
+++ b/src/librustc/traits/util.rs
@@ -347,6 +347,11 @@ fn next(&mut self) -> Option<ty::PolyTraitRef<'tcx>> {
             }
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (_, upper) = self.base_iterator.size_hint();
+        (0, upper)
+    }
 }
 
 ///////////////////////////////////////////////////////////////////////////
diff --git a/src/librustc_data_structures/bitvec.rs b/src/librustc_data_structures/bitvec.rs
index 54565afa4c6c7be4154cd9c5f15529c4b61c41d1..28e3180063c170d83a497e4d0307e5d1bbd8c72d 100644
--- a/src/librustc_data_structures/bitvec.rs
+++ b/src/librustc_data_structures/bitvec.rs
@@ -132,6 +132,11 @@ fn next(&mut self) -> Option<usize> {
         self.idx += offset + 1;
         return Some(self.idx - 1);
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (_, upper) = self.iter.size_hint();
+        (0, upper)
+    }
 }
 
 impl FromIterator<bool> for BitVector {
diff --git a/src/librustc_data_structures/graph/mod.rs b/src/librustc_data_structures/graph/mod.rs
index 1945b82c03145218b9ac3c3730ff9ac0e4425182..e2b393071ff5c4ec0333aaf2f343ed31db734902 100644
--- a/src/librustc_data_structures/graph/mod.rs
+++ b/src/librustc_data_structures/graph/mod.rs
@@ -334,6 +334,11 @@ fn next(&mut self) -> Option<(EdgeIndex, &'g Edge<E>)> {
         self.next = edge.next_edge[self.direction.repr];
         Some((edge_index, edge))
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // At most, all the edges in the graph.
+        (0, Some(self.graph.len_edges()))
+    }
 }
 
 pub struct DepthFirstTraversal<'g, N, E>
@@ -383,8 +388,16 @@ fn next(&mut self) -> Option<NodeIndex> {
         }
         next
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // We will visit every node in the graph exactly once.
+        let remaining = self.graph.len_nodes() - self.visited.count();
+        (remaining, Some(remaining))
+    }
 }
 
+impl<'g, N: Debug, E: Debug> ExactSizeIterator for DepthFirstTraversal<'g, N, E> {}
+
 impl<E> Edge<E> {
     pub fn source(&self) -> NodeIndex {
         self.source
diff --git a/src/librustc_driver/pretty.rs b/src/librustc_driver/pretty.rs
index 4ae6a93d6983a12bd8535c7b19f1fca136ce4401..c5e7fdb30d36ade6dd716322037674395a9fe0d4 100644
--- a/src/librustc_driver/pretty.rs
+++ b/src/librustc_driver/pretty.rs
@@ -584,6 +584,13 @@ fn next(&mut self) -> Option<ast::NodeId> {
             &mut NodesMatchingSuffix(ref mut iter) => iter.next(),
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        match self {
+            &NodesMatchingDirect(ref iter) => iter.size_hint(),
+            &NodesMatchingSuffix(ref iter) => iter.size_hint(),
+        }
+    }
 }
 
 impl UserIdentifiedItem {
diff --git a/src/librustc_typeck/check/method/suggest.rs b/src/librustc_typeck/check/method/suggest.rs
index c4d98c8a2abb536b7bdc36caf9436eb7035b7286..4bc2d8ce123a7527091376b97b95b207aa5f49da 100644
--- a/src/librustc_typeck/check/method/suggest.rs
+++ b/src/librustc_typeck/check/method/suggest.rs
@@ -730,8 +730,15 @@ fn next(&mut self) -> Option<TraitInfo> {
             TraitInfo::new(*info)
         })
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.borrow.as_ref().unwrap().len() - self.idx;
+        (len, Some(len))
+    }
 }
 
+impl<'a> ExactSizeIterator for AllTraits<'a> {}
+
 
 struct UsePlacementFinder<'a, 'tcx: 'a, 'gcx: 'tcx> {
     target_module: ast::NodeId,
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index 7f3a50cf561c9d4358f2afb3141dcc9fcb054cbf..e3ba482eecfeef90058febf1fe74a6a6ee6aaabc 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -604,6 +604,11 @@ fn next(&mut self) -> Option<Self::Item> {
 
         None
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let lower = self.current_list.len();
+        (lower, None)
+    }
 }
 
 pub trait AttributesExt {
diff --git a/src/libsyntax_ext/format_foreign.rs b/src/libsyntax_ext/format_foreign.rs
index 0476d7d4fcc1c53b45d14f0e6f1e94187efafbd1..e95c6f2e1243fe7f5a2556d64d243eb3c30e12b4 100644
--- a/src/libsyntax_ext/format_foreign.rs
+++ b/src/libsyntax_ext/format_foreign.rs
@@ -272,6 +272,11 @@ fn next(&mut self) -> Option<Self::Item> {
             self.s = tail;
             Some(sub)
         }
+
+        fn size_hint(&self) -> (usize, Option<usize>) {
+            // Substitutions are at least 2 characters long.
+            (0, Some(self.s.len() / 2))
+        }
     }
 
     enum State {
@@ -782,6 +787,10 @@ fn next(&mut self) -> Option<Self::Item> {
                 None => None,
             }
         }
+
+        fn size_hint(&self) -> (usize, Option<usize>) {
+            (0, Some(self.s.len()))
+        }
     }
 
     /// Parse the next substitution from the input string.