diff --git a/serde_macros/src/lib.rs b/serde_macros/src/lib.rs
index 4c6a09a2..23765672 100644
--- a/serde_macros/src/lib.rs
+++ b/serde_macros/src/lib.rs
@@ -265,9 +265,8 @@ fn serialize_tuple_struct(
             }
 
             #[inline]
-            fn size_hint(&self) -> (usize, Option<usize>) {
-                let size = $len - (self.state as usize);
-                (size, Some(size))
+            fn len(&self) -> Option<usize> {
+                Some($len)
             }
         }
 
@@ -349,9 +348,8 @@ fn serialize_struct(
             }
 
             #[inline]
-            fn size_hint(&self) -> (usize, Option<usize>) {
-                let size = $len - (self.state as usize);
-                (size, Some(size))
+            fn len(&self) -> Option<usize> {
+                Some($len)
             }
         }
 
@@ -550,8 +548,8 @@ fn serialize_variant(
 
         ast::MethodImplItem(
             quote_method!(cx,
-                fn size_hint(&self) -> (usize, Option<usize>) {
-                    ($len - self.state as usize, Some($len - self.state as usize))
+                fn len(&self) -> Option<usize> {
+                    Some($len)
                 }
             )
         ),
diff --git a/src/json/value.rs b/src/json/value.rs
index aae566d7..f444c754 100644
--- a/src/json/value.rs
+++ b/src/json/value.rs
@@ -233,11 +233,7 @@ impl ser::Serializer for Serializer {
     fn visit_seq<V>(&mut self, mut visitor: V) -> Result<(), ()>
         where V: ser::SeqVisitor,
     {
-        let len = match visitor.size_hint() {
-            (_, Some(len)) => len,
-            (len, None) => len,
-        };
-
+        let len = visitor.len().unwrap_or(0);
         let values = Vec::with_capacity(len);
 
         self.state.push(State::Array(values));
diff --git a/src/ser.rs b/src/ser.rs
index 6eebba68..c6574649 100644
--- a/src/ser.rs
+++ b/src/ser.rs
@@ -163,9 +163,10 @@ pub trait SeqVisitor {
     fn visit<S>(&mut self, serializer: &mut S) -> Result<Option<()>, S::Error>
         where S: Serializer;
 
+    /// Return the length of the sequence if known.
     #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        (0, None)
+    fn len(&self) -> Option<usize> {
+        None
     }
 }
 
@@ -173,9 +174,10 @@ pub trait MapVisitor {
     fn visit<S>(&mut self, serializer: &mut S) -> Result<Option<()>, S::Error>
         where S: Serializer;
 
+    /// Return the length of the map if known.
     #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        (0, None)
+    fn len(&self) -> Option<usize> {
+        None
     }
 }
 
@@ -247,6 +249,7 @@ impl<T> Serialize for Option<T> where T: Serialize {
 
 pub struct SeqIteratorVisitor<Iter> {
     iter: Iter,
+    len: Option<usize>,
     first: bool,
 }
 
@@ -254,9 +257,10 @@ impl<T, Iter> SeqIteratorVisitor<Iter>
     where Iter: Iterator<Item=T>
 {
     #[inline]
-    pub fn new(iter: Iter) -> SeqIteratorVisitor<Iter> {
+    pub fn new(iter: Iter, len: Option<usize>) -> SeqIteratorVisitor<Iter> {
         SeqIteratorVisitor {
             iter: iter,
+            len: len,
             first: true,
         }
     }
@@ -283,8 +287,8 @@ impl<T, Iter> SeqVisitor for SeqIteratorVisitor<Iter>
     }
 
     #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        self.iter.size_hint()
+    fn len(&self) -> Option<usize> {
+        self.len
     }
 }
 
@@ -297,7 +301,7 @@ impl<'a, T> Serialize for &'a [T]
     fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
         where S: Serializer,
     {
-        serializer.visit_seq(SeqIteratorVisitor::new(self.iter()))
+        serializer.visit_seq(SeqIteratorVisitor::new(self.iter(), Some(self.len())))
     }
 }
 
@@ -310,12 +314,14 @@ impl<T> Serialize for Vec<T> where T: Serialize {
     }
 }
 
-impl<T> Serialize for BTreeSet<T> where T: Serialize {
+impl<T> Serialize for BTreeSet<T>
+    where T: Serialize + Ord,
+{
     #[inline]
     fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
         where S: Serializer,
     {
-        serializer.visit_seq(SeqIteratorVisitor::new(self.iter()))
+        serializer.visit_seq(SeqIteratorVisitor::new(self.iter(), Some(self.len())))
     }
 }
 
@@ -327,7 +333,7 @@ impl<T, H> Serialize for HashSet<T, H>
     fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
         where S: Serializer,
     {
-        serializer.visit_seq(SeqIteratorVisitor::new(self.iter()))
+        serializer.visit_seq(SeqIteratorVisitor::new(self.iter(), Some(self.len())))
     }
 }
 
@@ -394,8 +400,8 @@ macro_rules! tuple_impls {
                     }
                 }
 
-                fn size_hint(&self) -> (usize, Option<usize>) {
-                    ($len, Some($len))
+                fn len(&self) -> Option<usize> {
+                    Some($len)
                 }
             }
 
@@ -520,6 +526,7 @@ tuple_impls! {
 
 pub struct MapIteratorVisitor<Iter> {
     iter: Iter,
+    len: Option<usize>,
     first: bool,
 }
 
@@ -527,9 +534,10 @@ impl<K, V, Iter> MapIteratorVisitor<Iter>
     where Iter: Iterator<Item=(K, V)>
 {
     #[inline]
-    pub fn new(iter: Iter) -> MapIteratorVisitor<Iter> {
+    pub fn new(iter: Iter, len: Option<usize>) -> MapIteratorVisitor<Iter> {
         MapIteratorVisitor {
             iter: iter,
+            len: len,
             first: true,
         }
     }
@@ -557,8 +565,8 @@ impl<K, V, I> MapVisitor for MapIteratorVisitor<I>
     }
 
     #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        self.iter.size_hint()
+    fn len(&self) -> Option<usize> {
+        self.len
     }
 }
 
@@ -572,7 +580,7 @@ impl<K, V> Serialize for BTreeMap<K, V>
     fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
         where S: Serializer,
     {
-        serializer.visit_map(MapIteratorVisitor::new(self.iter()))
+        serializer.visit_map(MapIteratorVisitor::new(self.iter(), Some(self.len())))
     }
 }
 
@@ -585,7 +593,7 @@ impl<K, V, H> Serialize for HashMap<K, V, H>
     fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
         where S: Serializer,
     {
-        serializer.visit_map(MapIteratorVisitor::new(self.iter()))
+        serializer.visit_map(MapIteratorVisitor::new(self.iter(), Some(self.len())))
     }
 }
 
diff --git a/tests/test_ser.rs b/tests/test_ser.rs
index b7d86af9..9e5ea9ba 100644
--- a/tests/test_ser.rs
+++ b/tests/test_ser.rs
@@ -33,15 +33,15 @@ pub enum Token<'a> {
     NamedUnit(&'a str),
     EnumUnit(&'a str, &'a str),
 
-    SeqStart(usize),
-    NamedSeqStart(&'a str, usize),
-    EnumSeqStart(&'a str, &'a str, usize),
+    SeqStart(Option<usize>),
+    NamedSeqStart(&'a str, Option<usize>),
+    EnumSeqStart(&'a str, &'a str, Option<usize>),
     SeqSep(bool),
     SeqEnd,
 
-    MapStart(usize),
-    NamedMapStart(&'a str, usize),
-    EnumMapStart(&'a str, &'a str, usize),
+    MapStart(Option<usize>),
+    NamedMapStart(&'a str, Option<usize>),
+    EnumMapStart(&'a str, &'a str, Option<usize>),
     MapSep(bool),
     MapEnd,
 }
@@ -187,7 +187,7 @@ impl<'a> Serializer for AssertSerializer<'a> {
     fn visit_seq<V>(&mut self, visitor: V) -> Result<(), ()>
         where V: SeqVisitor
     {
-        let (len, _) = visitor.size_hint();
+        let len = visitor.len();
 
         assert_eq!(self.iter.next(), Some(Token::SeqStart(len)));
 
@@ -197,7 +197,7 @@ impl<'a> Serializer for AssertSerializer<'a> {
     fn visit_named_seq<V>(&mut self, name: &str, visitor: V) -> Result<(), ()>
         where V: SeqVisitor
     {
-        let (len, _) = visitor.size_hint();
+        let len = visitor.len();
 
         assert_eq!(self.iter.next().unwrap(), Token::NamedSeqStart(name, len));
 
@@ -210,7 +210,7 @@ impl<'a> Serializer for AssertSerializer<'a> {
                          visitor: V) -> Result<(), ()>
         where V: SeqVisitor
     {
-        let (len, _) = visitor.size_hint();
+        let len = visitor.len();
 
         assert_eq!(self.iter.next().unwrap(), Token::EnumSeqStart(name, variant, len));
 
@@ -227,7 +227,7 @@ impl<'a> Serializer for AssertSerializer<'a> {
     fn visit_map<V>(&mut self, visitor: V) -> Result<(), ()>
         where V: MapVisitor
     {
-        let (len, _) = visitor.size_hint();
+        let len = visitor.len();
 
         assert_eq!(self.iter.next(), Some(Token::MapStart(len)));
 
@@ -237,7 +237,7 @@ impl<'a> Serializer for AssertSerializer<'a> {
     fn visit_named_map<V>(&mut self, name: &str, visitor: V) -> Result<(), ()>
         where V: MapVisitor
     {
-        let (len, _) = visitor.size_hint();
+        let len = visitor.len();
 
         assert_eq!(self.iter.next().unwrap(), Token::NamedMapStart(name, len));
 
@@ -250,7 +250,7 @@ impl<'a> Serializer for AssertSerializer<'a> {
                          visitor: V) -> Result<(), ()>
         where V: MapVisitor
     {
-        let (len, _) = visitor.size_hint();
+        let len = visitor.len();
 
         assert_eq!(self.iter.next().unwrap(), Token::EnumMapStart(name, variant, len));
 
@@ -370,11 +370,11 @@ declare_tests! {
     }
     test_slice {
         &[0][..0] => vec![
-            Token::SeqStart(0),
+            Token::SeqStart(Some(0)),
             Token::SeqEnd,
         ],
         &[1, 2, 3][..] => vec![
-            Token::SeqStart(3),
+            Token::SeqStart(Some(3)),
                 Token::SeqSep(true),
                 Token::I32(1),
 
@@ -388,23 +388,23 @@ declare_tests! {
     }
     test_vec {
         Vec::<isize>::new() => vec![
-            Token::SeqStart(0),
+            Token::SeqStart(Some(0)),
             Token::SeqEnd,
         ],
         vec![vec![], vec![1], vec![2, 3]] => vec![
-            Token::SeqStart(3),
+            Token::SeqStart(Some(3)),
                 Token::SeqSep(true),
-                Token::SeqStart(0),
+                Token::SeqStart(Some(0)),
                 Token::SeqEnd,
 
                 Token::SeqSep(false),
-                Token::SeqStart(1),
+                Token::SeqStart(Some(1)),
                     Token::SeqSep(true),
                     Token::I32(1),
                 Token::SeqEnd,
 
                 Token::SeqSep(false),
-                Token::SeqStart(2),
+                Token::SeqStart(Some(2)),
                     Token::SeqSep(true),
                     Token::I32(2),
 
@@ -416,13 +416,13 @@ declare_tests! {
     }
     test_tuple {
         (1,) => vec![
-            Token::SeqStart(1),
+            Token::SeqStart(Some(1)),
                 Token::SeqSep(true),
                 Token::I32(1),
             Token::SeqEnd,
         ],
         (1, 2, 3) => vec![
-            Token::SeqStart(3),
+            Token::SeqStart(Some(3)),
                 Token::SeqSep(true),
                 Token::I32(1),
 
@@ -436,14 +436,14 @@ declare_tests! {
     }
     test_btreemap {
         btreemap![1 => 2] => vec![
-            Token::MapStart(1),
+            Token::MapStart(Some(1)),
                 Token::MapSep(true),
                 Token::I32(1),
                 Token::I32(2),
             Token::MapEnd,
         ],
         btreemap![1 => 2, 3 => 4] => vec![
-            Token::MapStart(2),
+            Token::MapStart(Some(2)),
                 Token::MapSep(true),
                 Token::I32(1),
                 Token::I32(2),
@@ -454,15 +454,15 @@ declare_tests! {
             Token::MapEnd,
         ],
         btreemap![1 => btreemap![], 2 => btreemap![3 => 4, 5 => 6]] => vec![
-            Token::MapStart(2),
+            Token::MapStart(Some(2)),
                 Token::MapSep(true),
                 Token::I32(1),
-                Token::MapStart(0),
+                Token::MapStart(Some(0)),
                 Token::MapEnd,
 
                 Token::MapSep(false),
                 Token::I32(2),
-                Token::MapStart(2),
+                Token::MapStart(Some(2)),
                     Token::MapSep(true),
                     Token::I32(3),
                     Token::I32(4),
@@ -479,7 +479,7 @@ declare_tests! {
     }
     test_named_seq {
         NamedSeq(1, 2, 3) => vec![
-            Token::NamedSeqStart("NamedSeq", 3),
+            Token::NamedSeqStart("NamedSeq", Some(3)),
                 Token::SeqSep(true),
                 Token::I32(1),
 
@@ -493,7 +493,7 @@ declare_tests! {
     }
     test_named_map {
         NamedMap { a: 1, b: 2, c: 3 } => vec![
-            Token::NamedMapStart("NamedMap", 3),
+            Token::NamedMapStart("NamedMap", Some(3)),
                 Token::MapSep(true),
                 Token::Str("a"),
                 Token::I32(1),
@@ -511,7 +511,7 @@ declare_tests! {
     test_enum {
         Enum::Unit => vec![Token::EnumUnit("Enum", "Unit")],
         Enum::Seq(1, 2) => vec![
-            Token::EnumSeqStart("Enum", "Seq", 2),
+            Token::EnumSeqStart("Enum", "Seq", Some(2)),
                 Token::SeqSep(true),
                 Token::I32(1),
 
@@ -520,7 +520,7 @@ declare_tests! {
             Token::SeqEnd,
         ],
         Enum::Map { a: 1, b: 2 } => vec![
-            Token::EnumMapStart("Enum", "Map", 2),
+            Token::EnumMapStart("Enum", "Map", Some(2)),
                 Token::MapSep(true),
                 Token::Str("a"),
                 Token::I32(1),