Fall back from specific non-location to specific location (#5792)

https://unicode-org.atlassian.net/browse/CLDR-18074

components/datetime/src/fieldset.rs

@@ -953,7 +953,8 @@ impl_datetime_marker!(
 );
 
 impl_zone_marker!(
-    /// When a display name is unavailable, falls back to the offset format:
+    /// When a display name is unavailable, falls back to the localized offset format for short lengths, and
+    /// to the location format for long lengths:
     ///
     /// ```
     /// use icu::calendar::{Date, Time};
@@ -965,21 +966,32 @@ impl_zone_marker!(
     /// use tinystr::tinystr;
     /// use writeable::assert_try_writeable_eq;
     ///
+    /// // Time zone info for Europe/Istanbul in the winter
+    /// let zone = TimeZoneBcp47Id(tinystr!(8, "trist"))
+    ///     .with_offset("+02".parse().ok())
+    ///     .at_time((Date::try_new_iso(2022, 1, 29).unwrap(), Time::midnight()))
+    ///     .with_zone_variant(ZoneVariant::Standard);
+    ///
     /// let fmt = FixedCalendarDateTimeFormatter::<Gregorian, _>::try_new(
     ///     &locale!("en").into(),
     ///     Z::short(),
     /// )
     /// .unwrap();
     ///
-    /// // Time zone info for America/Sao_Paulo in the winter
-    /// let zone = TimeZoneBcp47Id(tinystr!(8, "brsao"))
-    ///     .with_offset("-03".parse().ok())
-    ///     .at_time((Date::try_new_iso(2022, 1, 29).unwrap(), Time::midnight()))
-    ///     .with_zone_variant(ZoneVariant::Standard);
+    /// assert_try_writeable_eq!(
+    ///     fmt.format(&zone),
+    ///     "GMT+2"
+    /// );
+    ///
+    /// let fmt = FixedCalendarDateTimeFormatter::<Gregorian, _>::try_new(
+    ///     &locale!("en").into(),
+    ///     Z::long(),
+    /// )
+    /// .unwrap();
     ///
     /// assert_try_writeable_eq!(
     ///     fmt.format(&zone),
-    ///     "GMT-3"
+    ///     "Türkiye Standard Time"
     /// );
     /// ```
     ///
@@ -1015,6 +1027,7 @@ impl_zone_marker!(
     sample_length = long,
     sample = "Central Daylight Time",
     zone_essentials = yes,
+    zone_locations = yes,
     zone_specific_long = yes,
     zone_specific_short = yes,
     metazone_periods = yes,
@@ -1184,21 +1197,20 @@ impl_zone_marker!(
     /// use tinystr::tinystr;
     /// use writeable::assert_try_writeable_eq;
     ///
+    /// // Time zone info for Europe/Istanbul
+    /// let zone = TimeZoneBcp47Id(tinystr!(8, "trist"))
+    ///     .without_offset()
+    ///     .at_time((Date::try_new_iso(2022, 1, 29).unwrap(), Time::midnight()));
+    ///
     /// let fmt = FixedCalendarDateTimeFormatter::<Gregorian, _>::try_new(
     ///     &locale!("en").into(),
     ///     V::short(),
     /// )
     /// .unwrap();
     ///
-    /// // Time zone info for America/Sao_Paulo in the winter
-    /// let zone = TimeZoneBcp47Id(tinystr!(8, "brsao"))
-    ///     .with_offset("-03".parse().ok())
-    ///     .at_time((Date::try_new_iso(2022, 1, 29).unwrap(), Time::midnight()))
-    ///     .with_zone_variant(ZoneVariant::Standard);
-    ///
     /// assert_try_writeable_eq!(
     ///     fmt.format(&zone),
-    ///     "Sao Paulo Time"
+    ///     "Türkiye Time"
     /// );
     /// ```
     ///
@@ -1214,7 +1226,7 @@ impl_zone_marker!(
     /// use icu::locale::locale;
     /// use writeable::assert_try_writeable_eq;
     ///
-    /// let time_zone_basic = TimeZoneBcp47Id(tinystr!(8, "uschi")).with_offset("-06".parse().ok());
+    /// let time_zone_basic = TimeZoneBcp47Id(tinystr!(8, "uschi")).without_offset();
     ///
     /// let formatter = FixedCalendarDateTimeFormatter::try_new(
     ///     &locale!("en-US").into(),

components/datetime/src/format/datetime.rs

@@ -334,6 +334,20 @@ where
                 Ok(s) => Ok(w.write_str(s)?),
             }
         }
+        (FieldSymbol::TimeZone(fields::TimeZone::SpecificNonLocation), FieldLength::Four) => {
+            perform_timezone_fallback(
+                w,
+                input,
+                datetime_names,
+                fdf,
+                field,
+                &[
+                    TimeZoneFormatterUnit::SpecificNonLocation(FieldLength::Four),
+                    TimeZoneFormatterUnit::SpecificLocation,
+                    TimeZoneFormatterUnit::LocalizedOffset(FieldLength::Four),
+                ],
+            )?
+        }
         (FieldSymbol::TimeZone(fields::TimeZone::SpecificNonLocation), l) => {
             perform_timezone_fallback(
                 w,

components/datetime/src/format/neo.rs

@@ -2110,6 +2110,7 @@ impl<R: DateTimeNamesMarker> RawDateTimeNames<R> {
                         mz_period_provider,
                         locale,
                     )?;
+                    self.load_time_zone_location_names(locations_provider, locale)?;
                 }
 
                 // v

components/datetime/src/format/time_zone.rs

@@ -44,7 +44,6 @@ pub(super) enum TimeZoneFormatterUnit {
     GenericNonLocation(FieldLength),
     SpecificNonLocation(FieldLength),
     GenericLocation,
-    #[allow(dead_code)]
     SpecificLocation,
     #[allow(dead_code)]
     GenericPartialLocation(FieldLength),

components/timezone/src/time_zone.rs

@@ -121,6 +121,16 @@ impl TimeZoneBcp47Id {
             zone_variant: (),
         }
     }
+
+    /// Converts this [`TimeZoneBcp47Id`] into a [`TimeZoneInfo`] without an offset.
+    pub const fn without_offset(self) -> TimeZoneInfo<models::Base> {
+        TimeZoneInfo {
+            offset: None,
+            time_zone_id: self,
+            local_time: (),
+            zone_variant: (),
+        }
+    }
 }
 
 impl TimeZoneInfo<models::Base> {

ffi/capi/src/timezone.rs

@@ -114,6 +114,7 @@ pub mod ffi {
         /// Clears the `offset` field.
         #[diplomat::rust_link(icu::timezone::UtcOffset::offset_seconds, FnInStruct)]
         #[diplomat::rust_link(icu::timezone::UtcOffset, Struct, compact)]
+        #[diplomat::rust_link(icu::timezone::TimeZoneBcp47Id::without_offset, FnInStruct, compact)]
         pub fn clear_offset(&mut self) {
             self.offset.take();
         }