Fix parsing json into optional types

Does not yet handle parsing missing values as `None` yet though.
Closes #25.

serde2/src/json/de.rs

@@ -392,6 +392,24 @@ impl<Iter: Iterator<Item=u8>> de::Deserializer for Deserializer<Iter> {
     {
         self.parse_value(visitor)
     }
+
+    #[inline]
+    fn visit_option<V>(&mut self, mut visitor: V) -> Result<V::Value, Error>
+        where V: de::Visitor,
+    {
+        self.parse_whitespace();
+
+        if self.eof() {
+            return Err(self.error(ErrorCode::EOFWhileParsingValue));
+        }
+
+        if self.ch_is(b'n') {
+            try!(self.parse_ident(b"ull"));
+            visitor.visit_none()
+        } else {
+            visitor.visit_some(self)
+        }
+    }
 }
 
 struct SeqVisitor<'a, Iter: 'a> {

serde2/src/json/value.rs

@@ -78,6 +78,18 @@ impl de::Deserialize for Value {
                 Ok(Value::String(value))
             }
 
+            #[inline]
+            fn visit_none<E>(&mut self) -> Result<Value, E> {
+                Ok(Value::Null)
+            }
+
+            #[inline]
+            fn visit_some<D>(&mut self, deserializer: &mut D) -> Result<Value, D::Error>
+                where D: de::Deserializer,
+            {
+                de::Deserialize::deserialize(deserializer)
+            }
+
             #[inline]
             fn visit_seq<V>(&mut self, visitor: V) -> Result<Value, V::Error>
                 where V: de::SeqVisitor,

serde2/tests/test_json.rs

@@ -691,11 +691,10 @@ fn test_parse_struct() {
     ]);
 }
 
-/*
 #[test]
 fn test_parse_option() {
     test_parse_ok(&[
-        ("null", None),
+        ("null", None::<String>),
         ("\"jodhpurs\"", Some("jodhpurs".to_string())),
     ]);
 
@@ -706,21 +705,18 @@ fn test_parse_option() {
         x: Option<isize>,
     }
 
+    /*
     let value: Foo = from_str("{}").unwrap();
     assert_eq!(value, Foo { x: None });
+    */
 
-    let value: Foo = from_str("{ \"x\": 5 }").unwrap();
-    assert_eq!(value, Foo { x: Some(5) });
-}
-
-#[test]
-fn test_json_deserialize_option() {
-    test_json_deserialize_ok(&[
-        None,
-        Some("jodhpurs".to_string()),
+    test_parse_ok(&[
+        ("{\"x\": null}", Foo { x: None }),
+        ("{\"x\": 5}", Foo { x: Some(5) }),
     ]);
 }
 
+/*
 #[test]
 fn test_parse_enum() {
     test_parse_ok(&[