Audit sealed traits, take 2 (#6015)

Fixes #4338

This:

 - Adds a doc header to all sealed traits
 - Adds a (different) doc header to all UnstableSealed traits
 - Ensures all sealed traits have hidden methods
- Seals the segmenter type traits since they're implementing a "compile
time enum" pattern
- Audits the rest of the traits and adds comments explaining that they
are explicitly implementable (or de-facto sealed by dint of having a
blanket impl)
 - Makes CldrCalendar Sealed instead of UnstableSealed

components/casemap/src/set.rs

@@ -12,6 +12,9 @@ use icu_collections::codepointinvlist::CodePointInversionListBuilder;
 /// will be some kind of set over codepoints and strings, or something that
 /// can be built into one.
 ///
+/// An implementation is provided for [`CodePointInversionListBuilder`], but users are encouraged
+/// to implement this trait on their own collections as needed.
+///
 /// [`CaseMapCloser::add_string_case_closure_to()`]: crate::CaseMapCloser::add_string_case_closure_to
 /// [`CaseMapCloser::add_case_closure_to()`]: crate::CaseMapCloser::add_case_closure_to
 pub trait ClosureSink {

components/collections/src/codepointtrie/cptrie.rs

@@ -53,6 +53,8 @@ pub enum TrieType {
 
 /// A trait representing the values stored in the data array of a [`CodePointTrie`].
 /// This trait is used as a type parameter in constructing a `CodePointTrie`.
+///
+/// This trait can be implemented on anything that can be represented as a u32s worth of data.
 pub trait TrieValue: Copy + Eq + PartialEq + zerovec::ule::AsULE + 'static {
     /// Last-resort fallback value to return if we cannot read data from the trie.
     ///

components/datetime/src/scaffold/calendar.rs

@@ -20,16 +20,20 @@ use icu_provider::marker::NeverMarker;
 use icu_provider::prelude::*;
 use icu_timezone::{DateTime, Time, TimeZoneInfo, TimeZoneModel, UtcOffset, ZonedDateTime};
 
+mod private {
+    pub trait Sealed {}
+}
+
 /// 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.
 ///
 /// <div class="stab unstable">
-/// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
-/// including in SemVer minor releases. Do not implement this trait in userland.
+/// 🚫 This trait is sealed; it cannot be implemented by user code. If an API requests an item that implements this
+/// trait, please consider using a type from the implementors listed below.
 /// </div>
-pub trait CldrCalendar: UnstableSealed {
+pub trait CldrCalendar: private::Sealed {
     /// The data marker for loading year symbols for this calendar.
     type YearNamesV1Marker: DataMarker<DataStruct = YearNamesV1<'static>>;
 
@@ -40,60 +44,70 @@ pub trait CldrCalendar: UnstableSealed {
     type SkeletaV1Marker: DataMarker<DataStruct = PackedPatternsV1<'static>>;
 }
 
+impl private::Sealed for () {}
 impl CldrCalendar for () {
     type YearNamesV1Marker = NeverMarker<YearNamesV1<'static>>;
     type MonthNamesV1Marker = NeverMarker<MonthNamesV1<'static>>;
     type SkeletaV1Marker = NeverMarker<PackedPatternsV1<'static>>;
 }
 
+impl private::Sealed for Buddhist {}
 impl CldrCalendar for Buddhist {
     type YearNamesV1Marker = BuddhistYearNamesV1Marker;
     type MonthNamesV1Marker = BuddhistMonthNamesV1Marker;
     type SkeletaV1Marker = BuddhistDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for Chinese {}
 impl CldrCalendar for Chinese {
     type YearNamesV1Marker = ChineseYearNamesV1Marker;
     type MonthNamesV1Marker = ChineseMonthNamesV1Marker;
     type SkeletaV1Marker = ChineseDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for Coptic {}
 impl CldrCalendar for Coptic {
     type YearNamesV1Marker = CopticYearNamesV1Marker;
     type MonthNamesV1Marker = CopticMonthNamesV1Marker;
     type SkeletaV1Marker = CopticDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for Dangi {}
 impl CldrCalendar for Dangi {
     type YearNamesV1Marker = DangiYearNamesV1Marker;
     type MonthNamesV1Marker = DangiMonthNamesV1Marker;
     type SkeletaV1Marker = DangiDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for Ethiopian {}
 impl CldrCalendar for Ethiopian {
     type YearNamesV1Marker = EthiopianYearNamesV1Marker;
     type MonthNamesV1Marker = EthiopianMonthNamesV1Marker;
     type SkeletaV1Marker = EthiopianDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for Gregorian {}
 impl CldrCalendar for Gregorian {
     type YearNamesV1Marker = GregorianYearNamesV1Marker;
     type MonthNamesV1Marker = GregorianMonthNamesV1Marker;
     type SkeletaV1Marker = GregorianDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for Hebrew {}
 impl CldrCalendar for Hebrew {
     type YearNamesV1Marker = HebrewYearNamesV1Marker;
     type MonthNamesV1Marker = HebrewMonthNamesV1Marker;
     type SkeletaV1Marker = HebrewDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for Indian {}
 impl CldrCalendar for Indian {
     type YearNamesV1Marker = IndianYearNamesV1Marker;
     type MonthNamesV1Marker = IndianMonthNamesV1Marker;
     type SkeletaV1Marker = IndianDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for IslamicCivil {}
 impl CldrCalendar for IslamicCivil {
     // this value is not actually a valid identifier for this calendar,
     // however since we are overriding is_identifier_allowed_for_calendar we are using
@@ -103,42 +117,49 @@ impl CldrCalendar for IslamicCivil {
     type SkeletaV1Marker = IslamicDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for IslamicObservational {}
 impl CldrCalendar for IslamicObservational {
     type YearNamesV1Marker = IslamicYearNamesV1Marker;
     type MonthNamesV1Marker = IslamicMonthNamesV1Marker;
     type SkeletaV1Marker = IslamicDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for IslamicTabular {}
 impl CldrCalendar for IslamicTabular {
     type YearNamesV1Marker = IslamicYearNamesV1Marker;
     type MonthNamesV1Marker = IslamicMonthNamesV1Marker;
     type SkeletaV1Marker = IslamicDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for IslamicUmmAlQura {}
 impl CldrCalendar for IslamicUmmAlQura {
     type YearNamesV1Marker = IslamicYearNamesV1Marker;
     type MonthNamesV1Marker = IslamicMonthNamesV1Marker;
     type SkeletaV1Marker = IslamicDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for Japanese {}
 impl CldrCalendar for Japanese {
     type YearNamesV1Marker = JapaneseYearNamesV1Marker;
     type MonthNamesV1Marker = JapaneseMonthNamesV1Marker;
     type SkeletaV1Marker = JapaneseDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for JapaneseExtended {}
 impl CldrCalendar for JapaneseExtended {
     type YearNamesV1Marker = JapaneseExtendedYearNamesV1Marker;
     type MonthNamesV1Marker = JapaneseExtendedMonthNamesV1Marker;
     type SkeletaV1Marker = JapaneseExtendedDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for Persian {}
 impl CldrCalendar for Persian {
     type YearNamesV1Marker = PersianYearNamesV1Marker;
     type MonthNamesV1Marker = PersianMonthNamesV1Marker;
     type SkeletaV1Marker = PersianDateNeoSkeletonPatternsV1Marker;
 }
 
+impl private::Sealed for Roc {}
 impl CldrCalendar for Roc {
     type YearNamesV1Marker = RocYearNamesV1Marker;
     type MonthNamesV1Marker = RocMonthNamesV1Marker;
@@ -169,6 +190,11 @@ impl UnstableSealed for Roc {}
 /// [`DynamicDataMarker`]. For example, this trait can be implemented for [`YearNamesV1Marker`].
 ///
 /// This trait serves as a building block for a cross-calendar [`BoundDataProvider`].
+///
+/// <div class="stab unstable">
+/// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
+/// including in SemVer minor releases. Do not implement this trait in userland unless you are prepared for things to occasionally break.
+/// </div>
 pub trait CalMarkers<M>: UnstableSealed
 where
     M: DynamicDataMarker,
@@ -382,6 +408,7 @@ impl_load_any_calendar!([
 ]);
 
 /// A type that can be converted into a specific calendar system.
+// This trait is implementable
 pub trait ConvertCalendar {
     /// The converted type. This can be the same as the receiver type.
     type Converted<'a>: Sized;
@@ -439,6 +466,7 @@ impl<O: TimeZoneModel> ConvertCalendar for TimeZoneInfo<O> {
 }
 
 /// An input that may be associated with a specific runtime calendar.
+// This trait is implementable
 pub trait InSameCalendar {
     /// Checks whether this type is compatible with the given calendar.
     ///
@@ -512,6 +540,7 @@ impl<O: TimeZoneModel> InSameCalendar for TimeZoneInfo<O> {
 }
 
 /// An input associated with a fixed, static calendar.
+// This trait is implementable
 pub trait InFixedCalendar<C> {}
 
 impl<C: CldrCalendar, A: AsCalendar<Calendar = C>> InFixedCalendar<C> for Date<A> {}

components/datetime/src/scaffold/fieldset_traits.rs

@@ -28,6 +28,11 @@ use icu_timezone::{
 /// (input types only).
 ///
 /// This is a sealed trait implemented on field set markers.
+///
+/// <div class="stab unstable">
+/// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
+/// including in SemVer minor releases. Do not implement this trait in userland unless you are prepared for things to occasionally break.
+/// </div>
 pub trait DateInputMarkers: UnstableSealed {
     /// Marker for resolving the year input field.
     type YearInput: IntoOption<YearInfo>;
@@ -45,6 +50,11 @@ pub trait DateInputMarkers: UnstableSealed {
 /// (data markers only).
 ///
 /// This is a sealed trait implemented on field set markers.
+///
+/// <div class="stab unstable">
+/// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
+/// including in SemVer minor releases. Do not implement this trait in userland unless you are prepared for things to occasionally break.
+/// </div>
 pub trait TypedDateDataMarkers<C>: UnstableSealed {
     /// Marker for loading date skeleton patterns.
     type DateSkeletonPatternsV1Marker: DataMarker<DataStruct = PackedPatternsV1<'static>>;
@@ -60,6 +70,11 @@ pub trait TypedDateDataMarkers<C>: UnstableSealed {
 /// (data markers only).
 ///
 /// This is a sealed trait implemented on field set markers.
+///
+/// <div class="stab unstable">
+/// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
+/// including in SemVer minor releases. Do not implement this trait in userland unless you are prepared for things to occasionally break.
+/// </div>
 pub trait DateDataMarkers: UnstableSealed {
     /// Cross-calendar data markers for date skeleta.
     type Skel: CalMarkers<ErasedPackedPatterns>;
@@ -75,6 +90,11 @@ pub trait DateDataMarkers: UnstableSealed {
 /// (input types and data markers).
 ///
 /// This is a sealed trait implemented on field set markers.
+///
+/// <div class="stab unstable">
+/// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
+/// including in SemVer minor releases. Do not implement this trait in userland unless you are prepared for things to occasionally break.
+/// </div>
 pub trait TimeMarkers: UnstableSealed {
     /// Marker for resolving the day-of-month input field.
     type HourInput: IntoOption<IsoHour>;
@@ -94,6 +114,11 @@ pub trait TimeMarkers: UnstableSealed {
 /// (input types and data markers).
 ///
 /// This is a sealed trait implemented on field set markers.
+///
+/// <div class="stab unstable">
+/// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
+/// including in SemVer minor releases. Do not implement this trait in userland unless you are prepared for things to occasionally break.
+/// </div>
 pub trait ZoneMarkers: UnstableSealed {
     /// Marker for resolving the time zone id input field.
     type TimeZoneIdInput: IntoOption<TimeZoneBcp47Id>;
@@ -123,6 +148,11 @@ pub trait ZoneMarkers: UnstableSealed {
 /// required for datetime formatting.
 ///
 /// This is a sealed trait implemented on field set markers.
+///
+/// <div class="stab unstable">
+/// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
+/// including in SemVer minor releases. Do not implement this trait in userland unless you are prepared for things to occasionally break.
+/// </div>
 pub trait DateTimeMarkers: UnstableSealed + DateTimeNamesMarker {
     /// Associated types for date formatting.
     ///
@@ -156,6 +186,7 @@ pub trait DateTimeMarkers: UnstableSealed + DateTimeNamesMarker {
 /// - [`TimeZoneInfo`](icu_timezone::TimeZoneInfo)
 ///
 /// [`fieldsets::YMD`]: crate::fieldsets::YMD
+// This trait is implicitly sealed due to sealed supertraits
 pub trait AllInputMarkers<R: DateTimeMarkers>:
     GetField<<R::D as DateInputMarkers>::YearInput>
     + GetField<<R::D as DateInputMarkers>::MonthInput>
@@ -204,6 +235,7 @@ where
 ///
 /// This trait is implemented on all providers that support datetime formatting,
 /// including [`crate::provider::Baked`].
+// This trait is implicitly sealed due to sealed supertraits
 pub trait AllFixedCalendarFormattingDataMarkers<C: CldrCalendar, FSet: DateTimeMarkers>:
     DataProvider<<FSet::D as TypedDateDataMarkers<C>>::YearNamesV1Marker>
     + DataProvider<<FSet::D as TypedDateDataMarkers<C>>::MonthNamesV1Marker>
@@ -256,6 +288,7 @@ where
 ///
 /// This trait is implemented on all providers that support datetime formatting,
 /// including [`crate::provider::Baked`].
+// This trait is implicitly sealed due to sealed supertraits
 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>
@@ -400,6 +433,7 @@ where
 
 /// Trait to consolidate data provider markers external to this crate
 /// for datetime formatting with a fixed calendar.
+// This trait is implicitly sealed due to sealed supertraits
 pub trait AllFixedCalendarExternalDataMarkers:
     DataProvider<DecimalSymbolsV2Marker> + DataProvider<DecimalDigitsV1Marker>
 {
@@ -412,6 +446,7 @@ impl<T> AllFixedCalendarExternalDataMarkers for T where
 
 /// Trait to consolidate data provider markers external to this crate
 /// for datetime formatting with any calendar.
+// This trait is implicitly sealed due to sealed supertraits
 pub trait AllAnyCalendarExternalDataMarkers:
     DataProvider<ChineseCacheV1Marker>
     + DataProvider<DangiCacheV1Marker>

components/datetime/src/scaffold/get_field.rs

@@ -17,6 +17,11 @@ use super::UnstableSealed;
 /// A type that can return a certain field `T`.
 ///
 /// This is used as a bound on various datetime functions.
+///
+/// <div class="stab unstable">
+/// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
+/// including in SemVer minor releases. Do not implement this trait in userland unless you are prepared for things to occasionally break.
+/// </div>
 pub trait GetField<T>: UnstableSealed {
     /// Returns the value of this trait's field `T`.
     fn get_field(&self) -> T;

components/datetime/src/scaffold/mod.rs

@@ -51,6 +51,6 @@ pub(crate) use names_storage::OptionalNames;
 ///
 /// <div class="stab unstable">
 /// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
-/// including in SemVer minor releases. Do not implement this trait in userland.
+/// including in SemVer minor releases. Do not implement this trait in userland unless you are prepared for things to occasionally break.
 /// </div>
 pub trait UnstableSealed {}

components/datetime/src/scaffold/names_storage.rs

@@ -19,6 +19,11 @@ use super::UnstableSealed;
 /// This trait allows for types that contain data for some but not all types of datetime names,
 /// allowing for reduced stack size. For example, a type could contain year and month names but
 /// not weekday, day period, or time zone names.
+///
+/// <div class="stab unstable">
+/// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
+/// including in SemVer minor releases. Do not implement this trait in userland unless you are prepared for things to occasionally break.
+/// </div>
 #[allow(missing_docs)]
 pub trait DateTimeNamesMarker: UnstableSealed {
     type YearNames: NamesContainer<YearNamesV1Marker, YearNameLength>;
@@ -35,6 +40,11 @@ pub trait DateTimeNamesMarker: UnstableSealed {
 }
 
 /// Trait that associates a container for a payload parameterized by the given variables.
+///
+/// <div class="stab unstable">
+/// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
+/// including in SemVer minor releases. Do not implement this trait in userland unless you are prepared for things to occasionally break.
+/// </div>
 #[allow(missing_docs)]
 pub trait NamesContainer<M: DynamicDataMarker, Variables>: UnstableSealed
 where
@@ -100,6 +110,11 @@ impl MaybePayloadError {
 /// a value depending on the type parameter `Variables`.
 ///
 /// Helper trait for [`DateTimeNamesMarker`].
+///
+/// <div class="stab unstable">
+/// 🚧 This trait is considered unstable; it may change at any time, in breaking or non-breaking ways,
+/// including in SemVer minor releases. Do not implement this trait in userland unless you are prepared for things to occasionally break.
+/// </div>
 #[allow(missing_docs)]
 pub trait MaybePayload<M: DynamicDataMarker, Variables>: UnstableSealed {
     fn new_empty() -> Self;
@@ -334,6 +349,7 @@ where
 /// is_trait_implemented::<TimeFieldSet, CompositeFieldSet>();
 /// ```
 #[allow(missing_docs)]
+// This trait is implicitly sealed due to sealed supertraits
 pub trait DateTimeNamesFrom<M: DateTimeNamesMarker>: DateTimeNamesMarker {
     fn map_year_names(
         other: <M::YearNames as NamesContainer<YearNamesV1Marker, YearNameLength>>::Container,

components/experimental/src/transliterate/transliterator/replaceable.rs

@@ -579,6 +579,11 @@ pub(super) struct Reverse;
 /// The direction a match can be applied. Used in [`Utf8Matcher`].
 ///
 /// See [`Forward`] and [`Reverse`] for implementors.
+///
+/// <div class="stab unstable">
+/// 🚫 This trait is sealed; it cannot be implemented by user code. If an API requests an item that implements this
+/// trait, please consider using a type from the implementors listed below.
+/// </div>
 pub(super) trait MatchDirection: sealed::Sealed {}
 impl MatchDirection for Forward {}
 impl MatchDirection for Reverse {}