Make GetField sealed; other datetime docs fixes

components/datetime/src/fieldsets.rs

@@ -7,6 +7,27 @@
 //! Each field set is a struct containing options specified to that field set.
 //! The fields can either be set directly or via helper functions.
 //!
+//! This module contains _static_ field sets, which deliver the smallest binary size.
+//! If the field set is not known until runtime, use a _dynamic_ field set: [`enums`]
+//!
+//! # What is a Field Set?
+//!
+//! A field set determines what datetime fields should be printed in the localized output.
+//!
+//! Examples of field sets include:
+//!
+//! 1. Year, month, and day ([`YMD`])
+//! 2. Weekday and time ([`ET`])
+//!
+//! Field sets fit into four categories:
+//!
+//! 1. Date: fields that specify a particular day in time.
+//! 2. Calendar period: fields that specify a span of time greater than a day.
+//! 3. Time: fields that specify a time within a day.
+//! 4. Zone: fields that specify a time zone or offset from UTC.
+//!
+//! Certain combinations of field sets are allowed, too. See [`Combo`].
+//!
 //! # Examples
 //!
 //! Two ways to configure the same field set:
@@ -1098,7 +1119,8 @@ impl_calendar_period_marker!(
 );
 
 impl_time_marker!(
-    /// Hours can be switched between 12-hour and 24-hour time via the `u-hc` locale keyword.
+    /// Hours can be switched between 12-hour and 24-hour time via the `u-hc` locale keyword
+    /// or [`DateTimeFormatterPreferences`].
     ///
     /// ```
     /// use icu::calendar::Time;
@@ -1182,6 +1204,8 @@ impl_time_marker!(
     ///     "24:00"
     /// );
     /// ```
+    ///
+    /// [`DateTimeFormatterPreferences`]: crate::DateTimeFormatterPreferences
     T,
     description = "time (locale-dependent hour cycle)",
     sample_length = medium,

components/datetime/src/neo.rs

@@ -32,7 +32,28 @@ use icu_provider::prelude::*;
 use writeable::TryWriteable;
 
 define_preferences!(
-    /// The prefs for datetime formatting.
+    /// The user locale preferences for datetime formatting.
+    ///
+    /// # Examples
+    ///
+    /// Two ways to build a preferences bag with a custom hour cycle and calendar system:
+    ///
+    /// ```
+    /// use icu::datetime::DateTimeFormatterPreferences;
+    /// use icu::locale::Locale;
+    /// use icu::locale::preferences::extensions::unicode::keywords::CalendarAlgorithm;
+    /// use icu::locale::preferences::extensions::unicode::keywords::HourCycle;
+    /// use icu::locale::subtags::Language;
+    ///
+    /// let prefs1: DateTimeFormatterPreferences = Locale::try_from_str("fr-u-ca-buddhist-hc-h12").unwrap().into();
+    ///
+    /// let mut prefs2 = DateTimeFormatterPreferences::default();
+    /// prefs2.locale_prefs.language = Language::try_from_str("fr").unwrap();
+    /// prefs2.hour_cycle = Some(HourCycle::H12);
+    /// prefs2.calendar_algorithm = Some(CalendarAlgorithm::Buddhist);
+    ///
+    /// assert_eq!(prefs1, prefs2);
+    /// ```
     [Copy]
     DateTimeFormatterPreferences,
     {

components/datetime/src/scaffold/calendar.rs

@@ -22,7 +22,7 @@ use icu_provider::marker::NeverMarker;
 use icu_provider::prelude::*;
 use icu_timezone::{CustomZonedDateTime, TimeZoneInfo, TimeZoneModel, UtcOffset};
 
-/// A calendar that can be found in CLDR
+/// A calendar that can be found in CLDR.
 ///
 /// New implementors of this trait will likely also wish to modify `get_era_code_map()`
 /// in the CLDR transformer to support any new era maps.

components/datetime/src/scaffold/dynamic_impls.rs

@@ -137,6 +137,8 @@ impl DateTimeMarkers for TimeFieldSet {
     type GluePatternV1Marker = datetime_marker_helper!(@glue,);
 }
 
+impl UnstableSealed for DateAndTimeFieldSet {}
+
 impl UnstableSealed for ZoneFieldSet {}
 
 impl DateTimeNamesMarker for ZoneFieldSet {

components/datetime/src/scaffold/fieldset_traits.rs

@@ -26,6 +26,8 @@ use icu_timezone::{TimeZoneBcp47Id, UtcOffset, ZoneVariant};
 
 /// A trait associating types for date formatting in any calendar
 /// (input types only).
+///
+/// This is a sealed trait implemented on field set markers.
 pub trait DateInputMarkers: UnstableSealed {
     /// Marker for resolving the year input field.
     type YearInput: IntoOption<YearInfo>;
@@ -41,6 +43,8 @@ pub trait DateInputMarkers: UnstableSealed {
 
 /// A trait associating types for date formatting in a specific calendar
 /// (data markers only).
+///
+/// This is a sealed trait implemented on field set markers.
 pub trait TypedDateDataMarkers<C>: UnstableSealed {
     /// Marker for loading date skeleton patterns.
     type DateSkeletonPatternsV1Marker: DataMarker<DataStruct = PackedPatternsV1<'static>>;
@@ -54,6 +58,8 @@ pub trait TypedDateDataMarkers<C>: UnstableSealed {
 
 /// A trait associating types for date formatting in any calendar
 /// (data markers only).
+///
+/// This is a sealed trait implemented on field set markers.
 pub trait DateDataMarkers: UnstableSealed {
     /// Cross-calendar data markers for date skeleta.
     type Skel: CalMarkers<ErasedPackedPatterns>;
@@ -67,6 +73,8 @@ pub trait DateDataMarkers: UnstableSealed {
 
 /// A trait associating types for time formatting
 /// (input types and data markers).
+///
+/// This is a sealed trait implemented on field set markers.
 pub trait TimeMarkers: UnstableSealed {
     /// Marker for resolving the day-of-month input field.
     type HourInput: IntoOption<IsoHour>;
@@ -84,6 +92,8 @@ pub trait TimeMarkers: UnstableSealed {
 
 /// A trait associating types for time zone formatting
 /// (input types and data markers).
+///
+/// This is a sealed trait implemented on field set markers.
 pub trait ZoneMarkers: UnstableSealed {
     /// Marker for resolving the time zone id input field.
     type TimeZoneIdInput: IntoOption<TimeZoneBcp47Id>;
@@ -111,6 +121,8 @@ pub trait ZoneMarkers: UnstableSealed {
 
 /// A trait associating constants and types implementing various other traits
 /// required for datetime formatting.
+///
+/// This is a sealed trait implemented on field set markers.
 pub trait DateTimeMarkers: UnstableSealed + DateTimeNamesMarker {
     /// Associated types for date formatting.
     ///
@@ -128,7 +140,22 @@ pub trait DateTimeMarkers: UnstableSealed + DateTimeNamesMarker {
     type GluePatternV1Marker: DataMarker<DataStruct = GluePatternV1<'static>>;
 }
 
-/// Trait to consolidate input markers.
+/// Trait implemented on formattable datetime types.
+///
+/// This trait allows for only those types compatible with a particular field set to be passed
+/// as arguments to the formatting function for that field set. For example, this trait prevents
+/// [`Time`] from being passed to a formatter parameterized with [`fieldsets::YMD`].
+///
+/// The following types implement this trait:
+///
+/// - [`Date`](icu_calendar::Date)
+/// - [`Time`](icu_calendar::Time)
+/// - [`DateTime`](icu_calendar::DateTime)
+/// - [`CustomZonedDateTime`](icu_timezone::CustomZonedDateTime)
+/// - [`UtcOffset`](icu_timezone::UtcOffset)
+/// - [`TimeZoneInfo`](icu_timezone::TimeZoneInfo)
+///
+/// [`fieldsets::YMD`]: crate::fieldsets::YMD
 pub trait AllInputMarkers<R: DateTimeMarkers>:
     GetField<<R::D as DateInputMarkers>::YearInput>
     + GetField<<R::D as DateInputMarkers>::MonthInput>
@@ -174,6 +201,9 @@ where
 
 /// Trait to consolidate data provider markers defined by this crate
 /// for datetime formatting with a fixed calendar.
+///
+/// This trait is implemented on all providers that support datetime formatting,
+/// including [`crate::provider::Baked`].
 pub trait AllFixedCalendarFormattingDataMarkers<C: CldrCalendar, FSet: DateTimeMarkers>:
     DataProvider<<FSet::D as TypedDateDataMarkers<C>>::YearNamesV1Marker>
     + DataProvider<<FSet::D as TypedDateDataMarkers<C>>::MonthNamesV1Marker>
@@ -223,6 +253,9 @@ where
 
 /// Trait to consolidate data provider markers defined by this crate
 /// for datetime formatting with any calendar.
+///
+/// This trait is implemented on all providers that support datetime formatting,
+/// including [`crate::provider::Baked`].
 pub trait AllAnyCalendarFormattingDataMarkers<FSet: DateTimeMarkers>:
     DataProvider<<<FSet::D as DateDataMarkers>::Year as CalMarkers<YearNamesV1Marker>>::Buddhist>
     + DataProvider<<<FSet::D as DateDataMarkers>::Year as CalMarkers<YearNamesV1Marker>>::Chinese>

components/datetime/src/scaffold/get_field.rs

@@ -13,22 +13,28 @@ use icu_timezone::{
     CustomZonedDateTime, TimeZoneBcp47Id, TimeZoneInfo, TimeZoneModel, UtcOffset, ZoneVariant,
 };
 
+use super::UnstableSealed;
+
 /// A type that can return a certain field `T`.
-pub trait GetField<T> {
+///
+/// This is used as a bound on various datetime functions.
+pub trait GetField<T>: UnstableSealed {
     /// Returns the value of this trait's field `T`.
     fn get_field(&self) -> T;
 }
 
 impl<T> GetField<T> for T
 where
-    T: Copy,
+    T: Copy + UnstableSealed,
 {
     #[inline]
     fn get_field(&self) -> T {
         *self
     }
 }
 
+impl<C: Calendar, A: AsCalendar<Calendar = C>> UnstableSealed for Date<A> {}
+
 impl<C: Calendar, A: AsCalendar<Calendar = C>> GetField<YearInfo> for Date<A> {
     #[inline]
     fn get_field(&self) -> YearInfo {
@@ -64,6 +70,8 @@ impl<C: Calendar, A: AsCalendar<Calendar = C>> GetField<DayOfYearInfo> for Date<
     }
 }
 
+impl UnstableSealed for Time {}
+
 impl GetField<IsoHour> for Time {
     #[inline]
     fn get_field(&self) -> IsoHour {
@@ -92,6 +100,8 @@ impl GetField<NanoSecond> for Time {
     }
 }
 
+impl<C: Calendar, A: AsCalendar<Calendar = C>> UnstableSealed for DateTime<A> {}
+
 impl<C: Calendar, A: AsCalendar<Calendar = C>> GetField<YearInfo> for DateTime<A> {
     #[inline]
     fn get_field(&self) -> YearInfo {
@@ -155,6 +165,8 @@ impl<C: Calendar, A: AsCalendar<Calendar = C>> GetField<NanoSecond> for DateTime
     }
 }
 
+impl<C: Calendar, A: AsCalendar<Calendar = C>, Z> UnstableSealed for CustomZonedDateTime<A, Z> {}
+
 impl<C: Calendar, A: AsCalendar<Calendar = C>, Z> GetField<YearInfo> for CustomZonedDateTime<A, Z> {
     #[inline]
     fn get_field(&self) -> YearInfo {
@@ -276,13 +288,17 @@ where
     }
 }
 
+impl UnstableSealed for UtcOffset {}
+
 impl GetField<Option<UtcOffset>> for UtcOffset {
     #[inline]
     fn get_field(&self) -> Option<UtcOffset> {
         Some(*self)
     }
 }
 
+impl<O: TimeZoneModel> UnstableSealed for TimeZoneInfo<O> {}
+
 impl<O> GetField<TimeZoneBcp47Id> for TimeZoneInfo<O>
 where
     O: TimeZoneModel,