Add encode_empty_tables_as_array serialize option.

This will change the behaviour of encoding empty Lua tables into array instead of map.

src/serde/de.rs

@@ -49,6 +49,11 @@ pub struct Options {
     ///
     /// Default: **false**
     pub sort_keys: bool,
+
+    /// If true, empty Lua tables will be encoded as array, instead of map.
+    ///
+    /// Default: **false**
+    pub encode_empty_tables_as_array: bool,
 }
 
 impl Default for Options {
@@ -64,6 +69,7 @@ impl Options {
             deny_unsupported_types: true,
             deny_recursive_tables: true,
             sort_keys: false,
+            encode_empty_tables_as_array: false,
         }
     }
 
@@ -93,6 +99,15 @@ impl Options {
         self.sort_keys = enabled;
         self
     }
+
+    /// Sets [`encode_empty_tables_as_array`] option.
+    ///
+    /// [`encode_empty_tables_as_array`]: #structfield.encode_empty_tables_as_array
+    #[must_use]
+    pub const fn encode_empty_tables_as_array(mut self, enabled: bool) -> Self {
+        self.encode_empty_tables_as_array = enabled;
+        self
+    }
 }
 
 impl Deserializer {
@@ -141,6 +156,9 @@ impl<'de> serde::Deserializer<'de> for Deserializer {
                 Err(_) => visitor.visit_bytes(&s.as_bytes()),
             },
             Value::Table(ref t) if t.raw_len() > 0 || t.is_array() => self.deserialize_seq(visitor),
+            Value::Table(ref t) if self.options.encode_empty_tables_as_array && t.is_empty() => {
+                self.deserialize_seq(visitor)
+            }
             Value::Table(_) => self.deserialize_map(visitor),
             Value::LightUserData(ud) if ud.0.is_null() => visitor.visit_none(),
             Value::UserData(ud) if ud.is_serializable() => {

src/table.rs

@@ -1020,7 +1020,10 @@ impl Serialize for SerializableTable<'_> {
 
         // Array
         let len = self.table.raw_len();
-        if len > 0 || self.table.is_array() {
+        if len > 0
+            || self.table.is_array()
+            || (self.options.encode_empty_tables_as_array && self.table.is_empty())
+        {
             let mut seq = serializer.serialize_seq(Some(len))?;
             let mut serialize_err = None;
             let res = self.table.for_each_value::<Value>(|value| {

src/value.rs

@@ -700,6 +700,15 @@ impl<'a> SerializableValue<'a> {
         self.options.sort_keys = enabled;
         self
     }
+
+    /// If true, empty Lua tables will be encoded as array, instead of map.
+    ///
+    /// Default: **false**
+    #[must_use]
+    pub const fn encode_empty_tables_as_array(mut self, enabled: bool) -> Self {
+        self.options.encode_empty_tables_as_array = enabled;
+        self
+    }
 }
 
 #[cfg(feature = "serialize")]

tests/serde.rs

@@ -249,6 +249,26 @@ fn test_serialize_same_table_twice() -> LuaResult<()> {
     Ok(())
 }
 
+#[test]
+fn test_serialize_empty_table() -> LuaResult<()> {
+    let lua = Lua::new();
+
+    let table = Value::Table(lua.create_table()?);
+    let json = serde_json::to_string(&table.to_serializable()).unwrap();
+    assert_eq!(json, "{}");
+
+    // Set the option to encode empty tables as array
+    let json = serde_json::to_string(&table.to_serializable().encode_empty_tables_as_array(true)).unwrap();
+    assert_eq!(json, "[]");
+
+    // Check hashmap table with this option
+    table.as_table().unwrap().set("hello", "world")?;
+    let json = serde_json::to_string(&table.to_serializable().encode_empty_tables_as_array(true)).unwrap();
+    assert_eq!(json, r#"{"hello":"world"}"#);
+
+    Ok(())
+}
+
 #[test]
 fn test_to_value_struct() -> LuaResult<()> {
     let lua = Lua::new();
@@ -667,6 +687,37 @@ fn test_from_value_userdata() -> Result<(), Box<dyn StdError>> {
     Ok(())
 }
 
+#[test]
+fn test_from_value_empty_table() -> Result<(), Box<dyn StdError>> {
+    let lua = Lua::new();
+
+    // By default we encode empty tables as objects
+    let t = lua.create_table()?;
+    let got = lua.from_value::<serde_json::Value>(Value::Table(t.clone()))?;
+    assert_eq!(got, serde_json::json!({}));
+
+    // Set the option to encode empty tables as array
+    let got = lua
+        .from_value_with::<serde_json::Value>(
+            Value::Table(t.clone()),
+            DeserializeOptions::new().encode_empty_tables_as_array(true),
+        )
+        .unwrap();
+    assert_eq!(got, serde_json::json!([]));
+
+    // Check hashmap table with this option
+    t.raw_set("hello", "world")?;
+    let got = lua
+        .from_value_with::<serde_json::Value>(
+            Value::Table(t),
+            DeserializeOptions::new().encode_empty_tables_as_array(true),
+        )
+        .unwrap();
+    assert_eq!(got, serde_json::json!({"hello": "world"}));
+
+    Ok(())
+}
+
 #[test]
 fn test_from_value_sorted() -> Result<(), Box<dyn StdError>> {
     let lua = Lua::new();