Mark Calendar UnstableSealed (#6483)

Note: I used a different header than in datetime for the trait, since I
think unlike datetime this is a non-scaffolding trait that people will
reference often.

Personally my rule of thumb for UnstableSealed is that we expect people
to want to implement it (check) and we have some idea that this is the
direction in which we want to go for custom impls. I hadn't put full
thought into that in the past for this crate but with some of the
formatting stuff split out I'm more confident. So I'm fine with this
being unstable sealed.


I'm not sure if UnstableSealed should live in the cal module or at the
top level.



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Author: Manishearth

components/calendar/src/any_calendar.rs

@@ -218,6 +218,7 @@ macro_rules! match_cal {
     };
 }
 
+impl crate::cal::scaffold::UnstableSealed for AnyCalendar {}
 impl Calendar for AnyCalendar {
     type DateInner = AnyDateInner;
     type Year = YearInfo;

components/calendar/src/cal/buddhist.rs

@@ -49,6 +49,7 @@ const BUDDHIST_ERA_OFFSET: i32 = 543;
 #[allow(clippy::exhaustive_structs)] // this type is stable
 pub struct Buddhist;
 
+impl crate::cal::scaffold::UnstableSealed for Buddhist {}
 impl Calendar for Buddhist {
     type DateInner = IsoDateInner;
     type Year = types::EraYear;

components/calendar/src/cal/chinese.rs

@@ -151,6 +151,7 @@ impl Chinese {
     pub(crate) const DEBUG_NAME: &'static str = "Chinese";
 }
 
+impl crate::cal::scaffold::UnstableSealed for Chinese {}
 impl Calendar for Chinese {
     type DateInner = ChineseDateInner;
     type Year = types::CyclicYear;

components/calendar/src/cal/coptic.rs

@@ -92,6 +92,7 @@ impl CalendarArithmetic for Coptic {
     }
 }
 
+impl crate::cal::scaffold::UnstableSealed for Coptic {}
 impl Calendar for Coptic {
     type DateInner = CopticDateInner;
     type Year = types::EraYear;

components/calendar/src/cal/dangi.rs

@@ -149,6 +149,7 @@ impl Dangi {
     pub(crate) const DEBUG_NAME: &'static str = "Dangi";
 }
 
+impl crate::cal::scaffold::UnstableSealed for Dangi {}
 impl Calendar for Dangi {
     type DateInner = DangiDateInner;
     type Year = CyclicYear;

components/calendar/src/cal/ethiopian.rs

@@ -110,6 +110,7 @@ impl CalendarArithmetic for Ethiopian {
     }
 }
 
+impl crate::cal::scaffold::UnstableSealed for Ethiopian {}
 impl Calendar for Ethiopian {
     type DateInner = EthiopianDateInner;
     type Year = types::EraYear;

components/calendar/src/cal/gregorian.rs

@@ -44,6 +44,7 @@ pub struct Gregorian;
 /// The inner date type used for representing [`Date`]s of [`Gregorian`]. See [`Date`] and [`Gregorian`] for more details.
 pub struct GregorianDateInner(pub(crate) IsoDateInner);
 
+impl crate::cal::scaffold::UnstableSealed for Gregorian {}
 impl Calendar for Gregorian {
     type DateInner = GregorianDateInner;
     type Year = types::EraYear;

components/calendar/src/cal/hebrew.rs

@@ -130,6 +130,7 @@ impl PrecomputedDataSource<HebrewYearInfo> for () {
     }
 }
 
+impl crate::cal::scaffold::UnstableSealed for Hebrew {}
 impl Calendar for Hebrew {
     type DateInner = HebrewDateInner;
     type Year = types::EraYear;

components/calendar/src/cal/hijri.rs

@@ -361,6 +361,7 @@ impl CalendarArithmetic for HijriSimulated {
     }
 }
 
+impl crate::cal::scaffold::UnstableSealed for HijriSimulated {}
 impl Calendar for HijriSimulated {
     type DateInner = HijriSimulatedDateInner;
     type Year = types::EraYear;
@@ -642,6 +643,7 @@ impl CalendarArithmetic for HijriUmmAlQura {
     }
 }
 
+impl crate::cal::scaffold::UnstableSealed for HijriUmmAlQura {}
 impl Calendar for HijriUmmAlQura {
     type DateInner = HijriUmmAlQuraDateInner;
     type Year = types::EraYear;
@@ -1171,6 +1173,7 @@ impl CalendarArithmetic for HijriTabular {
     }
 }
 
+impl crate::cal::scaffold::UnstableSealed for HijriTabular {}
 impl Calendar for HijriTabular {
     type DateInner = HijriTabularDateInner;
     type Year = types::EraYear;

components/calendar/src/cal/indian.rs

@@ -90,6 +90,7 @@ const DAY_OFFSET: u16 = 80;
 /// The Saka calendar is 78 years behind Gregorian. This number should be added to Gregorian dates
 const YEAR_OFFSET: i32 = 78;
 
+impl crate::cal::scaffold::UnstableSealed for Indian {}
 impl Calendar for Indian {
     type DateInner = IndianDateInner;
     type Year = types::EraYear;