Add datetime formatting impls on Weekday with test (#7719)

See #7710

Author: sffc

components/datetime/src/fieldsets.rs

@@ -1022,6 +1022,26 @@ impl_date_marker!(
 );
 
 impl_date_marker!(
+    /// Format a weekday using [`Weekday`]:
+    ///
+    /// ```
+    /// use icu::calendar::Gregorian;
+    /// use icu::calendar::types::Weekday;
+    /// use icu::datetime::fieldsets;
+    /// use icu::datetime::FixedCalendarDateTimeFormatter;
+    /// use icu::locale::locale;
+    /// use writeable::assert_writeable_eq;
+    ///
+    /// let formatter = FixedCalendarDateTimeFormatter::<Gregorian, _>::try_new(
+    ///     locale!("fr").into(),
+    ///     fieldsets::E::long(),
+    /// ).unwrap();
+    ///
+    /// assert_writeable_eq!(
+    ///     formatter.format(&Weekday::Monday),
+    ///     "lundi"
+    /// );
+    /// ```
     E,
     ET,
     description = "weekday (standalone)",

components/datetime/src/scaffold/calendar.rs

@@ -10,6 +10,7 @@ use crate::MismatchedCalendarError;
 use core::marker::PhantomData;
 use icu_calendar::cal;
 use icu_calendar::preferences::{CalendarAlgorithm, CalendarPreferences, HijriCalendarAlgorithm};
+use icu_calendar::types::Weekday;
 use icu_calendar::{AnyCalendar, AnyCalendarKind, AsCalendar, Date, IntoAnyCalendar, Ref};
 use icu_provider::marker::NeverMarker;
 use icu_provider::prelude::*;
@@ -600,6 +601,11 @@ impl CalMarkers<ErasedPackedPatterns> for FullDataCalMarkers {
 }
 
 /// A type that can be converted into a specific calendar system.
+///
+/// You often want to set `Converted` to an ICU4X built-in type.
+///
+/// If the type is not calendar-specific, such as a time or time zone, set `Converted`
+/// to the same type and return it in the implementation.
 // This trait is implementable
 pub trait ConvertCalendar {
     /// The converted type. This can be the same as the receiver type.
@@ -649,6 +655,14 @@ impl<C: IntoAnyCalendar, A: AsCalendar<Calendar = C>, Z: Copy> ConvertCalendar
     }
 }
 
+impl ConvertCalendar for UtcOffset {
+    type Converted<'a> = UtcOffset;
+    #[inline]
+    fn to_calendar<'a>(&self, _: &'a AnyCalendar) -> Self::Converted<'a> {
+        *self
+    }
+}
+
 impl<O: TimeZoneModel> ConvertCalendar for TimeZoneInfo<O> {
     type Converted<'a> = TimeZoneInfo<O>;
     #[inline]
@@ -657,6 +671,14 @@ impl<O: TimeZoneModel> ConvertCalendar for TimeZoneInfo<O> {
     }
 }
 
+impl ConvertCalendar for Weekday {
+    type Converted<'a> = Weekday;
+    #[inline]
+    fn to_calendar<'a>(&self, _: &'a AnyCalendar) -> Self::Converted<'a> {
+        *self
+    }
+}
+
 /// An input that may be associated with a specific runtime calendar.
 // This trait is implementable
 pub trait InSameCalendar {
@@ -727,7 +749,16 @@ impl<O: TimeZoneModel> InSameCalendar for TimeZoneInfo<O> {
     }
 }
 
+impl InSameCalendar for Weekday {
+    #[inline]
+    fn check_any_calendar_kind(&self, _: AnyCalendarKind) -> Result<(), MismatchedCalendarError> {
+        Ok(())
+    }
+}
+
 /// An input associated with a fixed, static calendar.
+///
+/// Inputs that are not calendar-specific should blanket-impl this for all `C`.
 // This trait is implementable
 pub trait InFixedCalendar<C> {}
 
@@ -745,3 +776,5 @@ impl<C, Z> InFixedCalendar<C> for ZonedTime<Z> {}
 impl<C> InFixedCalendar<C> for UtcOffset {}
 
 impl<C, O: TimeZoneModel> InFixedCalendar<C> for TimeZoneInfo<O> {}
+
+impl<C> InFixedCalendar<C> for Weekday {}

components/datetime/src/scaffold/get_field.rs

@@ -390,6 +390,8 @@ where
     }
 }
 
+impl UnstableSealed for Weekday {}
+
 impl<C: Calendar, A: AsCalendar<Calendar = C>> GetField<()> for Date<A> {
     #[inline]
     fn get_field(&self) {}
@@ -425,3 +427,8 @@ impl<O: TimeZoneModel> GetField<()> for TimeZoneInfo<O> {
     #[inline]
     fn get_field(&self) {}
 }
+
+impl GetField<()> for Weekday {
+    #[inline]
+    fn get_field(&self) {}
+}

components/datetime/src/scaffold/mod.rs

@@ -6,6 +6,21 @@
 //!
 //! Items in this module are mostly for trait bounds. Most users should not need to reference
 //! these items in userland code.
+//!
+//! # Implementing a custom input type
+//!
+//! One reason you might find yourself in this module is because you have a custom datetime
+//! type, and you want to plug it into ICU4X for formatting.
+//!
+//! To properly equip your type for ICU4X, implement the following traits:
+//!
+//! - [`UnstableSealed`] must always be implemented to acknowledge that these traits are
+//!   subject to change, even across minor releases.
+//! - [`GetField<()>`] must almost always be implemented. It is used as a placeholder for input fields that a particular fieldset does not require.
+//! - Additional [`GetField`] impls based on what fields your type contains. For a list of all possible fields, see [`AllInputMarkers`].
+//! - [`ConvertCalendar`], for [`DateTimeFormatter::format`](crate::DateTimeFormatter::format)
+//! - [`InSameCalendar`], for [`DateTimeFormatter::format_same_calendar`](crate::DateTimeFormatter::format_same_calendar)
+//! - [`InFixedCalendar`], for [`FixedCalendarDateTimeFormatter::format`](crate::FixedCalendarDateTimeFormatter::format)
 
 mod calendar;
 mod dynamic_impls;