DateTimeFormatter numbering_system getter draft

components/datetime/src/neo.rs

@@ -23,6 +23,7 @@ use core::marker::PhantomData;
 use icu_calendar::any_calendar::IntoAnyCalendar;
 use icu_calendar::{AnyCalendar, AnyCalendarKind, AnyCalendarPreferences};
 use icu_decimal::FixedDecimalFormatterPreferences;
+use icu_locale_core::extensions::unicode::Value;
 use icu_locale_core::preferences::extensions::unicode::keywords::{
     CalendarAlgorithm, HourCycle, NumberingSystem,
 };
@@ -795,6 +796,40 @@ impl<FSet: DateTimeMarkers> DateTimeFormatter<FSet> {
     pub fn calendar_kind(&self) -> AnyCalendarKind {
         self.calendar.kind()
     }
+
+    /// Returns the numbering system used in this formatter.
+    ///
+    /// If the fields do not require a number formatter, this could return `None`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use icu::calendar::AnyCalendarKind;
+    /// use icu::calendar::Date;
+    /// use icu::datetime::fieldsets::YMD;
+    /// use icu::datetime::DateTimeFormatter;
+    /// use icu::locale::locale;
+    /// use writeable::assert_writeable_eq;
+    ///
+    /// let formatter = DateTimeFormatter::try_new(
+    ///     locale!("th").into(),
+    ///     YMD::long(),
+    /// )
+    /// .unwrap();
+    /// 
+    /// assert_writeable_eq!(
+    ///     formatter.format_any_calendar(&Date::try_new_iso(2024, 12, 16).unwrap()),
+    ///     "16 ธันวาคม 2567"
+    /// );
+    ///
+    /// assert_eq!(formatter.numbering_system().unwrap(), "latn");
+    /// ```
+    pub fn numbering_system(&self) -> Option<Value> {
+        self.names
+            .fixed_decimal_formatter
+            .as_ref()
+            .map(|f| f.numbering_system())
+    }
 }
 
 /// A formatter optimized for time and time zone formatting.

components/datetime/src/pattern/names.rs

@@ -257,7 +257,7 @@ pub(crate) struct RawDateTimeNames<FSet: DateTimeNamesMarker> {
     mz_periods:
         <FSet::MetazoneLookup as DateTimeNamesHolderTrait<tz::MzPeriodV1Marker>>::Container<()>,
     // TODO(#4340): Make the FixedDecimalFormatter optional
-    fixed_decimal_formatter: Option<FixedDecimalFormatter>,
+    pub(crate) fixed_decimal_formatter: Option<FixedDecimalFormatter>,
     _marker: PhantomData<FSet>,
 }