Clean up FormattableAnyCalendar

[robertbastian]

Trying to reduce usage of AnyCalendarKind and FormattableAnyCalendarKind.

[sffc]

Can you give an overview? What problem does this solve?

/gemini review

[gemini-code-assist]

Hi @sffc, thanks for the question! This pull request, titled "Clean up FormattableAnyCalendar", aims to streamline the internal handling of calendar preferences within the datetime component.

Problem Solved:
Previously, the system used an intermediate FormattableAnyCalendarKind enum to represent calendar types. This PR refactors the code to directly leverage icu_calendar::preferences::CalendarPreferences for calendar selection and instantiation. This approach offers several benefits:

1. Direct API Alignment: It aligns the datetime component more closely with the icu_calendar API by directly using CalendarPreferences.
2. Simplified Logic: It removes the need for an intermediate enum, reducing potential for inconsistencies and simplifying the calendar creation logic.
3. Improved Fallback Handling: The calendar construction methods (new, try_new_with_buffer_provider, try_new_unstable) now directly implement fallback logic for unsupported calendar algorithms (e.g., Iso8601, specific Hijri types) by modifying the CalendarPreferences and retrying, ensuring a valid calendar is always produced or a more informative error is returned.
4. Reduced Redundancy: The FormattableAnyCalendar struct no longer stores a separate kind field, as this information can be derived directly from the wrapped AnyCalendar.
5. Better Error Reporting: The try_from_any_calendar method now returns a Result that includes the original AnyCalendar in case of an unsupported type, providing more context for error handling.

In essence, this change makes the calendar handling more robust, maintainable, and consistent with the broader icu_calendar framework.

Regarding your /gemini review command, please note that I am currently responding to your comment. If you would like a full review of the pull request, please submit the /gemini review command in a new, separate comment.

[sffc]

/gemini review

[gemini-code-assist]

Code Review

This pull request is a nice cleanup of FormattableAnyCalendar. It simplifies the implementation by removing the UntaggedFormattableAnyCalendar wrapper and the cached kind field. The logic for selecting a calendar based on preferences is now encapsulated within the FormattableAnyCalendar constructors. The changes are consistent and improve the overall structure. I have one suggestion to reduce code duplication in components/datetime/src/scaffold/calendar.rs.

[sffc]

If we don't need UntaggedFormattableAnyCalendar any more, I'm in favor of that change, and would prefer to see it in a standalone PR. I'm less convinced by the motivation for the other changes.

[sffc]

Observation: I think I had introduced UntaggedFormattableAnyCalendar because it reduced the stack size of DateTimeFormatter. But, the tests that enforce stack size are still passing, so maybe there's a new niche where the extra field can fit now.

[robertbastian]

FormattableAnyCalendar is now just untagged

[sffc]

Observation/Issue: This logic that used to live in 1 place now lives in 3 places