Add numbering_system getter to FixedDecimalFormatter (unicode-org#5904)

Part of unicode-org#5900

components/decimal/src/lib.rs

@@ -104,6 +104,7 @@ pub use format::FormattedFixedDecimal;
 
 use alloc::string::String;
 use fixed_decimal::SignedFixedDecimal;
+use icu_locale_core::extensions::unicode::Value;
 use icu_locale_core::locale;
 use icu_locale_core::preferences::{
     define_preferences, extensions::unicode::keywords::NumberingSystem,
@@ -214,4 +215,47 @@ impl FixedDecimalFormatter {
     pub fn format_to_string(&self, value: &SignedFixedDecimal) -> String {
         self.format(value).write_to_string().into_owned()
     }
+
+    /// Gets the resolved numbering system identifier of this formatter.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use icu::decimal::FixedDecimalFormatter;
+    /// use icu::locale::locale;
+    ///
+    /// let fmt_en = FixedDecimalFormatter::try_new(
+    ///     locale!("en").into(),
+    ///     Default::default()
+    /// )
+    /// .unwrap();
+    ///
+    /// let fmt_bn = FixedDecimalFormatter::try_new(
+    ///     locale!("bn").into(),
+    ///     Default::default()
+    /// )
+    /// .unwrap();
+    ///
+    /// let fmt_zh_nu = FixedDecimalFormatter::try_new(
+    ///     locale!("zh-u-nu-hanidec").into(),
+    ///     Default::default()
+    /// )
+    /// .unwrap();
+    ///
+    /// assert_eq!(fmt_en.numbering_system(), "latn");
+    /// assert_eq!(fmt_bn.numbering_system(), "beng");
+    /// assert_eq!(fmt_zh_nu.numbering_system(), "hanidec");
+    /// ```
+    pub fn numbering_system(&self) -> Value {
+        match Value::try_from_str(self.symbols.get().numsys()) {
+            Ok(v) => v,
+            Err(e) => {
+                debug_assert!(
+                    false,
+                    "Problem converting numbering system ID to Value: {e}"
+                );
+                Value::new_empty()
+            }
+        }
+    }
 }

components/locale_core/src/extensions/unicode/value.rs

@@ -142,6 +142,19 @@ impl Value {
         self.0.len()
     }
 
+    /// Creates an empty [`Value`], which corresponds to a "true" value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use icu::locale::extensions::unicode::{value, Value};
+    ///
+    /// assert_eq!(value!("true"), Value::new_empty());
+    /// ```
+    pub const fn new_empty() -> Self {
+        Self(ShortBoxSlice::new())
+    }
+
     /// Returns `true` if the Value has no subtags.
     ///
     /// # Examples

ffi/capi/src/decimal.rs

@@ -176,5 +176,10 @@ pub mod ffi {
         ) {
             let _infallible = self.0.format(&value.0).write_to(write);
         }
+
+        #[diplomat::rust_link(icu::decimal::FixedDecimalFormatter::numbering_system, FnInStruct)]
+        pub fn numbering_system(&self, write: &mut diplomat_runtime::DiplomatWrite) {
+            let _infallible = self.0.numbering_system().write_to(write);
+        }
     }
 }