Add support for optional enum field

src/de.rs

@@ -164,7 +164,18 @@ pub fn deserialize_internal(
                         })*
                         _ => (),
                     }
-                    self.__map.take().ok_or(miniserde::Error)?.finish()?;
+                    let mut map = match self.__map.take() {
+                        Some(map) => map,
+                        None => match tag.as_str() {
+                            #(
+                                #struct_variant_names => <#struct_names as miniserde::Deserialize>::begin(
+                                    unsafe {&mut *(&mut self.#struct_names as *mut #ex::Option<#struct_names>)}
+                                ).map()?,
+                            )*
+                            _ => return #ex::Err(miniserde::Error),
+                        },
+                    };
+                    map.finish()?;
                     match tag.as_str() {
                         #(#struct_variant_names => {
                             self.__out.replace(self.#struct_names.take().ok_or(miniserde::Error)?.as_enum());

tests/deserialize.rs

@@ -51,10 +51,13 @@ fn test_internal() {
             x: i32,
         },
         D,
+        E {
+            value: Option<i32>,
+        },
     }
     use Internal::*;
-    let example = r#"[{"type":"renamedB"},{"type":"C","x":2},{"type":"D"}]"#;
+    let example = r#"[{"type":"renamedB"},{"type":"C","x":2},{"type":"D"},{"type":"E"},{"type":"E","value":5}]"#;
     let actual: Vec<Internal> = json::from_str(example).unwrap();
-    let expected = [B, C { x: 2 }, D];
+    let expected = [B, C { x: 2 }, D, E { value: None }, E { value: Some(5) }];
     assert_eq!(actual, expected);
 }