Allow Vecs, BTreeMaps, and HashMaps to be deserialized from a unit

Closes #36

src/de.rs

@@ -659,6 +659,13 @@ impl<T> VecVisitor<T> {
 impl<T> Visitor for VecVisitor<T> where T: Deserialize {
     type Value = Vec<T>;
 
+    #[inline]
+    fn visit_unit<E>(&mut self) -> Result<Vec<T>, E>
+        where E: Error,
+    {
+        Ok(vec![])
+    }
+
     #[inline]
     fn visit_seq<V>(&mut self, mut visitor: V) -> Result<Vec<T>, V::Error>
         where V: SeqVisitor,
@@ -767,6 +774,13 @@ impl<K, V> Visitor for HashMapVisitor<K, V>
 {
     type Value = HashMap<K, V>;
 
+    #[inline]
+    fn visit_unit<E>(&mut self) -> Result<HashMap<K, V>, E>
+        where E: Error,
+    {
+        Ok(HashMap::new())
+    }
+
     #[inline]
     fn visit_map<V_>(&mut self, mut visitor: V_) -> Result<HashMap<K, V>, V_::Error>
         where V_: MapVisitor,
@@ -816,6 +830,13 @@ impl<K, V> Visitor for BTreeMapVisitor<K, V>
 {
     type Value = BTreeMap<K, V>;
 
+    #[inline]
+    fn visit_unit<E>(&mut self) -> Result<BTreeMap<K, V>, E>
+        where E: Error,
+    {
+        Ok(BTreeMap::new())
+    }
+
     #[inline]
     fn visit_map<Visitor>(&mut self, mut visitor: Visitor) -> Result<BTreeMap<K, V>, Visitor::Error>
         where Visitor: MapVisitor,

tests/test_de.rs

@@ -499,6 +499,9 @@ declare_tests! {
         ],
     }
     test_vec {
+        Vec::<isize>::new() => vec![
+            Token::Unit,
+        ],
         Vec::<isize>::new() => vec![
             Token::SeqStart(0),
             Token::SeqEnd,
@@ -525,6 +528,15 @@ declare_tests! {
                 Token::SeqEnd,
             Token::SeqEnd,
         ],
+        Vec::<isize>::new() => vec![
+            Token::Name("Anything"),
+            Token::Unit,
+        ],
+        Vec::<isize>::new() => vec![
+            Token::Name("Anything"),
+            Token::SeqStart(0),
+            Token::SeqEnd,
+        ],
     }
     test_tuple {
         (1,) => vec![
@@ -547,6 +559,13 @@ declare_tests! {
         ],
     }
     test_btreemap {
+        BTreeMap::<isize, isize>::new() => vec![
+            Token::Unit,
+        ],
+        BTreeMap::<isize, isize>::new() => vec![
+            Token::MapStart(0),
+            Token::MapEnd,
+        ],
         btreemap![1 => 2] => vec![
             Token::MapStart(1),
                 Token::MapSep(true),
@@ -585,6 +604,15 @@ declare_tests! {
                 Token::MapEnd,
             Token::MapEnd,
         ],
+        BTreeMap::<isize, isize>::new() => vec![
+            Token::Name("Anything"),
+            Token::Unit,
+        ],
+        BTreeMap::<isize, isize>::new() => vec![
+            Token::Name("Anything"),
+            Token::MapStart(0),
+            Token::MapEnd,
+        ],
     }
     test_named_map {
         NamedMap { a: 1, b: 2, c: 3 } => vec![

tests/test_json.rs

@@ -861,8 +861,8 @@ fn test_parse_object() {
 #[test]
 fn test_parse_struct() {
     test_parse_err::<Outer>(vec![
-        ("[]", Error::SyntaxError(ErrorCode::ExpectedSomeValue, 0, 0)),
-        ("{}", Error::SyntaxError(ErrorCode::ExpectedSomeValue, 0, 0)),
+        ("5", Error::SyntaxError(ErrorCode::ExpectedSomeValue, 0, 0)),
+        ("\"hello\"", Error::SyntaxError(ErrorCode::ExpectedSomeValue, 0, 0)),
         ("{\"inner\": true}", Error::SyntaxError(ErrorCode::ExpectedSomeValue, 0, 0)),
     ]);
 
@@ -888,6 +888,13 @@ fn test_parse_struct() {
             },
         )
     ]);
+
+    assert_eq!(
+        from_str("{}"),
+        Ok(Outer {
+            inner: vec![],
+        })
+    );
 }
 
 #[test]