How to convert a DateTime<Iso> instance into a date-time suitable to pass to DateTimeFormatter::format()?

[rizzen-yazston]

I am definitely missing something important.

I managed to create the date-time formater, but when I pass date_time_iso (&DateTime<Iso>) to format(), I get this error, and I'm struggling to figure out what is missing.

let formatted = DateTimeFormatter::try_new(*preference, *composite)?.format(date_time_iso);

the trait bound `icu_time::DateTime<icu_calendar::Ref<'_, AnyCalendar>>: AllInputMarkers<CompositeFieldSet>` is not satisfied
the following other types implement trait `GetField<T>`:
  `icu_time::DateTime<A>` implements `GetField<()>`
  `icu_time::DateTime<A>` implements `GetField<MonthInfo>`
  `icu_time::DateTime<A>` implements `GetField<Nanosecond>`
  `icu_time::DateTime<A>` implements `GetField<YearInfo>`
  `icu_time::DateTime<A>` implements `GetField<icu_calendar::types::DayOfMonth>`
  `icu_time::DateTime<A>` implements `GetField<icu_calendar::types::DayOfYear>`
  `icu_time::DateTime<A>` implements `GetField<icu_calendar::types::RataDie>`
  `icu_time::DateTime<A>` implements `GetField<icu_calendar::types::Weekday>`
and 3 others
required for `icu_time::DateTime<icu_calendar::Ref<'_, AnyCalendar>>` to implement `AllInputMarkers<CompositeFieldSet>`

[sffc]

If you are using CompositeFieldSet, you must use ZonedDateTime, which is the only built-in input type that supports all fields.

If you want to format datetimes (not zoned datetimes), you can use CompositeDateTimeFieldSet.

It would be possible to write an input type that: is backed by DateTime, can be passed to a CompositeFieldSet formatter, and raises a runtime error if the composite field set requires time zone fields. We just do not have such a type yet.

[rizzen-yazston]

Thank you for the information.

My best approach is to start with the placeholder value enum to convert Date, Time, or DateTime to ZonedDateTime, using the application's preferences for any missing fields. Also, add a new ZonedDateTime variant to the placeholder value enum.

To obtain the fieldset for creating the date-time formatter DateTimeFormatter::try_new(), I used the builder FieldSetBuilder::new(), populated the fields as stipulated by the date-time pattern options, and finally called FieldSetBuilder::build_composite() to get the fieldset.

However, my test zone-date-time ZonedDateTime<Iso, UtcOffset> is still not being accepted by .format(), with the new error:

the trait bound `UtcOffset: GetField<ZoneNameTimestamp>` is not satisfied
required for `ZonedDateTime<icu_calendar::Ref<'_, AnyCalendar>, UtcOffset>` to implement `GetField<ZoneNameTimestamp>`
required for `ZonedDateTime<icu_calendar::Ref<'_, AnyCalendar>, UtcOffset>` to implement `AllInputMarkers<CompositeFieldSet>`

[sffc]

More specifically, you need ZonedDateTime<Iso, TimeZoneInfo<AtTime>>

[rizzen-yazston]

Yay! That solved the issue. I would not have figured out that TimeZoneInfo<AtTime> is required, and not UtcOffset.

Maybe include a note, perhaps in the description of the FieldSetBuilder::build_composite(), that the value passed to the DateTimeFormatter::format(zoned_date_time) should be ZonedDateTime<A, TimeZoneInfo<AtTime>> where A is a calendar, for example Iso as ZonedDateTime<Iso, TimeZoneInfo<AtTime>>.