Fix(serde): properly serialize unit structs.

Previously it was serialized as unit (nil), now there is just empty
array ([]).

Author: 3Hren

rmp-serde/src/encode.rs

@@ -305,6 +305,12 @@ impl<'a, W: VariantWriter> serde::Serializer for Serializer<'a, W> {
         value.serialize(self)
     }
 
+    fn visit_unit_struct(&mut self, _name: &'static str) -> Result<(), Error> {
+        try!(write_array_len(&mut self.wr, 0));
+
+        Ok(())
+    }
+
     fn visit_struct<V>(&mut self, _name: &str, mut visitor: V) -> Result<(), Error>
         where V: serde::ser::MapVisitor,
     {

rmp-serde/tests/se_macro.rs

@@ -24,6 +24,20 @@ fn pass_struct() {
     assert_eq!([0x92, 0x2a, 0xce, 0x0, 0x1, 0x88, 0x94], buf);
 }
 
+#[test]
+fn pass_empty_struct() {
+    // Must be encoded as [].
+    let mut buf = vec![];
+
+    #[derive(Serialize)]
+    struct Struct;
+
+    let val = Struct;
+    val.serialize(&mut Serializer::new(&mut buf)).ok().unwrap();
+
+    assert_eq!(vec![0x90], buf);
+}
+
 #[test]
 fn pass_struct_map() {
     use std::io::Write;