remove dummy field from ParseWeedayError and ParseMonthError

impl Error trait for ParseMonthError

src/format/mod.rs

@@ -483,7 +483,7 @@ impl FromStr for Weekday {
         if let Ok(("", w)) = scan::short_or_long_weekday(s) {
             Ok(w)
         } else {
-            Err(ParseWeekdayError { _dummy: () })
+            Err(ParseWeekdayError {})
         }
     }
 }
@@ -531,10 +531,10 @@ impl FromStr for Month {
                 9 => Ok(Month::October),
                 10 => Ok(Month::November),
                 11 => Ok(Month::December),
-                _ => Err(ParseMonthError { _dummy: () }),
+                _ => Err(ParseMonthError {}),
             }
         } else {
-            Err(ParseMonthError { _dummy: () })
+            Err(ParseMonthError {})
         }
     }
 }

src/month.rs

@@ -231,8 +231,16 @@ impl Months {
 
 /// An error resulting from reading `<Month>` value with `FromStr`.
 #[derive(Clone, PartialEq, Eq)]
-pub struct ParseMonthError {
-    pub(crate) _dummy: (),
+pub struct ParseMonthError {}
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for ParseMonthError {}
+
+impl fmt::Display for ParseMonthError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "ParseMonthError {{ .. }}")
+    }
 }
 
 impl fmt::Debug for ParseMonthError {

src/weekday.rs

@@ -209,9 +209,7 @@ impl num_traits::FromPrimitive for Weekday {
 
 /// An error resulting from reading `Weekday` value with `FromStr`.
 #[derive(Clone, PartialEq, Eq)]
-pub struct ParseWeekdayError {
-    pub(crate) _dummy: (),
-}
+pub struct ParseWeekdayError {}
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]