Introduce api::v2::calendar::ListReqInit

The api::v2::calendar::ListReq type doesn't have a logical default
implementation: the start and end date should always be provided. At the
same time, we would like to declare it as non-exhaustive to have some
better backward compatibility story moving forward.
To somehow unite these two constraints, this change introduces the
ListReqInit type, which implements Default and requires the start and
end date to be turned into the final ListReq.

src/api/v2/calendar.rs

@@ -76,13 +76,42 @@ pub struct ListReq {
   // is not.
   #[serde(rename = "end")]
   pub end: NaiveDate,
+  /// The type is non-exhaustive and open to extension.
+  #[doc(hidden)]
+  #[serde(skip)]
+  pub _non_exhaustive: (),
 }
 
 impl From<Range<NaiveDate>> for ListReq {
   fn from(range: Range<NaiveDate>) -> Self {
     Self {
       start: range.start,
       end: range.end,
+      _non_exhaustive: (),
+    }
+  }
+}
+
+
+/// A helper for initializing [`ListReq`] objects.
+#[derive(Clone, Debug, Default, Eq, PartialEq)]
+#[allow(missing_copy_implementations)]
+pub struct ListReqInit {
+  /// The type is non-exhaustive and open to extension.
+  #[doc(hidden)]
+  pub _non_exhaustive: (),
+}
+
+impl ListReqInit {
+  /// Create a [`ListReq`] from a `ListReqInit`.
+  #[inline]
+  pub fn init(self, start: NaiveDate, end: NaiveDate) -> ListReq {
+    let Self { _non_exhaustive } = self;
+
+    ListReq {
+      start,
+      end,
+      _non_exhaustive: (),
     }
   }
 }
@@ -147,10 +176,9 @@ mod tests {
   /// Check that we can serialize and deserialize a [`CalendarReq`].
   #[test]
   fn serialize_deserialize_calendar_request() {
-    let request = ListReq {
-      start: NaiveDate::from_ymd_opt(2020, 4, 6).unwrap(),
-      end: NaiveDate::from_ymd_opt(2020, 4, 10).unwrap(),
-    };
+    let start = NaiveDate::from_ymd_opt(2020, 4, 6).unwrap();
+    let end = NaiveDate::from_ymd_opt(2020, 4, 10).unwrap();
+    let request = ListReqInit::default().init(start, end);
 
     let json = to_json(&request).unwrap();
     assert_eq!(from_json::<ListReq>(&json).unwrap(), request);