close #199: add header information for empty structs

- also add test which fails without this change
- also add a few helpful unit tests to table

src/ser.rs

@@ -930,6 +930,15 @@ impl<'a, 'b> ser::SerializeStruct for SerializeTable<'a, 'b> {
     }
 
     fn end(self) -> Result<(), Error> {
+        match self {
+            SerializeTable::Datetime(_) => {},
+            SerializeTable::Table { mut ser, first, ..  } => {
+                if first.get() {
+                    let state = ser.state.clone();
+                    ser.emit_table_header(&state)?;
+                }
+            }
+        }
         Ok(())
     }
 }

tests/serde.rs

@@ -554,3 +554,25 @@ fn newtypes2() {
         }),
     }
 }
+
+#[derive(Debug, Default, PartialEq, Serialize, Deserialize)]
+struct CanBeEmpty {
+    a: Option<String>,
+    b: Option<String>,
+}
+
+#[test]
+fn table_structs_empty() {
+    let text = "[bar]\n\n[baz]\n\n[bazv]\na = \"foo\"\n\n[foo]\n";
+    let value: BTreeMap<String, CanBeEmpty> = toml::from_str(text).unwrap();
+    let mut expected: BTreeMap<String, CanBeEmpty> = BTreeMap::new();
+    expected.insert("bar".to_string(), CanBeEmpty::default());
+    expected.insert("baz".to_string(), CanBeEmpty::default());
+    expected.insert(
+        "bazv".to_string(), 
+        CanBeEmpty {a: Some("foo".to_string()), b: None},
+    );
+    expected.insert("foo".to_string(), CanBeEmpty::default());
+    assert_eq!(value, expected);
+    assert_eq!(toml::to_string(&value).unwrap(), text);
+}

tests/valid.rs

@@ -162,6 +162,9 @@ test!(table_empty,
 test!(table_sub_empty,
        include_str!("valid/table-sub-empty.toml"),
        include_str!("valid/table-sub-empty.json"));
+test!(table_multi_empty,
+       include_str!("valid/table-multi-empty.toml"),
+       include_str!("valid/table-multi-empty.json"));
 test!(table_whitespace,
        include_str!("valid/table-whitespace.toml"),
        include_str!("valid/table-whitespace.json"));

tests/valid/table-multi-empty.json

@@ -0,0 +1,5 @@
+{
+    "a": { "b": {} },
+    "b": {},
+    "c": { "a": {} }
+}

tests/valid/table-multi-empty.toml

@@ -0,0 +1,5 @@
+[a]
+[a.b]
+[b]
+[c]
+[c.a]