Allow passing `Result<T, E>` to Lua

For this to work, both `T` and `E` need to implement `ToLua`. An `Ok(t)`
passes the contained `T`, while an `Err(e)` passes `nil` followed by the
contained `E`. This matches the common Lua idiom used by functions like
`io.open`.

Closes #3

src/multi.rs

@@ -15,6 +15,22 @@ impl<'lua> FromLuaMulti<'lua> for () {
     }
 }
 
+impl<'lua, T: ToLua<'lua>, E: ToLua<'lua>> ToLuaMulti<'lua> for Result<T, E> {
+    fn to_lua_multi(self, lua: &'lua Lua) -> LuaResult<LuaMultiValue<'lua>> {
+        let mut result = LuaMultiValue::new();
+
+        match self {
+            Ok(v) => result.push_back(v.to_lua(lua)?),
+            Err(e) => {
+                result.push_back(LuaNil);
+                result.push_back(e.to_lua(lua)?);
+            }
+        }
+
+        Ok(result)
+    }
+}
+
 impl<'lua, T: ToLua<'lua>> ToLuaMulti<'lua> for T {
     fn to_lua_multi(self, lua: &'lua Lua) -> LuaResult<LuaMultiValue<'lua>> {
         let mut v = LuaMultiValue::new();

src/tests.rs

@@ -609,3 +609,32 @@ fn test_table_error() {
     assert!(bad_table.pairs::<i64, i64>().is_ok());
     assert!(bad_table.array_values::<i64>().is_ok());
 }
+
+#[test]
+fn test_result_conversions() {
+    let lua = Lua::new();
+    let globals = lua.globals().unwrap();
+
+    let err = lua.create_function(|lua, args| {
+        lua.pack(Result::Err::<String, String>("only through failure can we succeed".to_string()))
+    }).unwrap();
+    let ok = lua.create_function(|lua, args| {
+        lua.pack(Result::Ok::<String, String>("!".to_string()))
+    }).unwrap();
+
+    globals.set("err", err).unwrap();
+    globals.set("ok", ok).unwrap();
+
+    lua.load::<()>(
+        r#"
+            local err, msg = err()
+            assert(err == nil)
+            assert(msg == "only through failure can we succeed")
+
+            local ok, extra = ok()
+            assert(ok == "!")
+            assert(extra == nil)
+        "#,
+        None,
+    ).unwrap();
+}