Rename method to set_date_fields_unchecked (#6488)

This cleans up the format_unchecked invariants a little.

Author: sffc

components/datetime/src/format/input.rs

@@ -61,7 +61,13 @@ pub struct DateTimeInputUnchecked {
 
 impl DateTimeInputUnchecked {
     /// Sets all fields from a [`Date`] input.
-    pub fn set_date_fields<C: Calendar, A: AsCalendar<Calendar = C>>(&mut self, input: Date<A>) {
+    ///
+    /// This method does not check the calendar of the date! The caller is
+    /// responsible for making sure the calendar matches the formatter.
+    pub fn set_date_fields_unchecked<C: Calendar, A: AsCalendar<Calendar = C>>(
+        &mut self,
+        input: Date<A>,
+    ) {
         self.year = Some(input.year());
         self.month = Some(input.month());
         self.day_of_month = Some(input.day_of_month());

components/datetime/src/neo.rs

@@ -404,11 +404,15 @@ impl<C: CldrCalendar, FSet: DateTimeNamesMarker> FixedCalendarDateTimeFormatter<
     /// .cast_into_fset::<CompositeFieldSet>();
     ///
     /// // Create a date and convert it to the correct calendar:
-    /// let date = Date::try_new_iso(2025, 3, 7).unwrap().to_calendar(Buddhist);
-    ///
-    /// // Extract the fields and use it with format_unchecked:
     /// let mut input = DateTimeInputUnchecked::default();
-    /// input.set_date_fields(date);
+    /// let date = Date::try_new_iso(2025, 3, 7)
+    ///     .unwrap()
+    ///     .to_calendar(Buddhist);
+    ///
+    /// // Safe because the calendar matches the formatter:
+    /// input.set_date_fields_unchecked(date);
+    ///
+    /// // Safe because YMD needs only date fields, which are in the input:
     /// let result = formatter.format_unchecked(input);
     ///
     /// assert_try_writeable_eq!(result, "7 มีนาคม 2568");
@@ -810,11 +814,15 @@ impl<FSet: DateTimeNamesMarker> DateTimeFormatter<FSet> {
     /// .cast_into_fset::<CompositeFieldSet>();
     ///
     /// // Create a date and convert it to the correct calendar:
-    /// let date = Date::try_new_iso(2025, 3, 7).unwrap().to_calendar(formatter.calendar());
-    ///
-    /// // Extract the fields and use it with format_unchecked:
     /// let mut input = DateTimeInputUnchecked::default();
-    /// input.set_date_fields(date);
+    /// let date = Date::try_new_iso(2025, 3, 7)
+    ///     .unwrap()
+    ///     .to_calendar(formatter.calendar());
+    ///
+    /// // Safe because the calendar matches the formatter:
+    /// input.set_date_fields_unchecked(date);
+    ///
+    /// // Safe because YMD needs only date fields, which are in the input:
     /// let result = formatter.format_unchecked(input);
     ///
     /// assert_try_writeable_eq!(result, "7 มีนาคม 2568");

ffi/capi/src/zoned_date_formatter.rs

@@ -518,7 +518,7 @@ pub mod ffi {
         ) -> Result<(), DateTimeWriteError> {
             let mut input = icu_datetime::DateTimeInputUnchecked::default();
             let date = date.0.to_calendar(self.0.calendar());
-            input.set_date_fields(date);
+            input.set_date_fields_unchecked(date); // calendar conversion on previous line
             input.set_time_zone_id(zone.id);
             if let Some(offset) = zone.offset {
                 input.set_time_zone_utc_offset(offset);
@@ -1010,7 +1010,7 @@ pub mod ffi {
         ) -> Result<(), DateTimeWriteError> {
             let mut input = icu_datetime::DateTimeInputUnchecked::default();
             let date = date.0.to_calendar(Gregorian);
-            input.set_date_fields(date);
+            input.set_date_fields_unchecked(date); // calendar conversion on previous line
             input.set_time_zone_id(zone.id);
             if let Some(offset) = zone.offset {
                 input.set_time_zone_utc_offset(offset);

ffi/capi/src/zoned_date_time_formatter.rs

@@ -519,7 +519,7 @@ pub mod ffi {
         ) -> Result<(), DateTimeWriteError> {
             let mut input = icu_datetime::DateTimeInputUnchecked::default();
             let date = date.0.to_calendar(self.0.calendar());
-            input.set_date_fields(date);
+            input.set_date_fields_unchecked(date); // calendar conversion on previous line
             input.set_time_fields(time.0);
             input.set_time_zone_id(zone.id);
             if let Some(offset) = zone.offset {
@@ -1013,7 +1013,7 @@ pub mod ffi {
         ) -> Result<(), DateTimeWriteError> {
             let mut input = icu_datetime::DateTimeInputUnchecked::default();
             let date = date.0.to_calendar(Gregorian);
-            input.set_date_fields(date);
+            input.set_date_fields_unchecked(date); // calendar conversion on previous line
             input.set_time_fields(time.0);
             input.set_time_zone_id(zone.id);
             if let Some(offset) = zone.offset {

tools/make/codegen/templates/zoned_formatter.rs.jinja

@@ -194,7 +194,7 @@ pub mod ffi {
             {%- else %}
             let date = date.0.to_calendar(self.0.calendar());
             {%- endif %}
-            input.set_date_fields(date);
+            input.set_date_fields_unchecked(date); // calendar conversion on previous line
             {%- endif %}
             {%- if flavor.has_time() %}
             input.set_time_fields(time.0);