Limitations
What dxn_intl does not format, options that are ignored or limited in v0.1.0, behavior that surprises, and what to do instead.
What the crate leaves out, where it behaves differently from what you might expect, and what to do in each case. The version is v0.1.0; check the releases for changes.
#Not implemented
ICU4X has most of these; they are added when a consumer needs them.
| Missing | Instead |
|---|---|
| Time zones: names (“CET”), offsets, conversion, daylight saving | Resolve the offset with a zone database and pass it to DateTime::from_unix_millis; see Dates and times |
| Relative times (“in 3 days”, “yesterday”) | Compare day numbers and use your own words; see Cookbook |
| Lists (“A, B and C”) | Join with your translations, or use messages in dXn.I18n |
| Units (“5 km”, “3 hours”) | Format the number, add the unit from your translations |
| Compact numbers (“1.2K”), scientific notation | Format the number and divide it yourself |
| Collation (locale-aware sorting) | Not available |
| Parsing localized input (“1.234,5”, “27.10.2026”) | Number reads ASCII only; normalize the input first |
| Date patterns and skeletons of your own, format to parts | Pick DateFields, TimePrecision and Length |
| Date ranges (“Oct 27 – 29”) | Format both ends and join |
| Currency display styles (code, name, narrow symbol), accounting negatives | The locale’s symbol is always used |
| Plural currency names (“5 US dollars”) | Use plural categories with your own words |
| Rounding modes other than half away from zero; significant digits | Round the Number yourself before formatting |
| Choosing which locales’ data ships | The data of every locale is compiled in; see Platforms |
Messages with plural and gender selection, placeholders and a bundle per language belong one level up: dXn.I18n builds on this crate for them.
#Behavior with caveats
#Formatters are not Send or Sync
PluralRules, NumberFormatter and DateTimeFormatter cannot cross threads: they contain Rc values from ICU4X’s data handling. They cannot sit in a static, an Arc or state that a multi-threaded runtime moves between threads. Keep them on the thread that uses them or build one per thread; see Cookbook. Number, DateTime, Locale and IntlError are Send + Sync.
#Floats lose trailing zeros
Number::try_from(1.0_f64) is 1, so the plural rules read it as 1, not 1.0. Use text or the formatter’s resolve. See Numbers.
#Options that do not apply to currencies
A currency formatter uses the currency’s own fraction digits and ignores minimum_fraction_digits, maximum_fraction_digits and with_fraction_digits. In v0.1.0 it also ignores grouping: Grouping::Never still shows $12,345.00. NumberFormatter::resolve does not round or pad a currency amount, so it is not the value shown. See Numbers.
#Grouping::Always is Auto
ICU4X’s decimal formatter treats Always like Auto, so it does not group four-digit numbers in locales that leave them ungrouped (Spanish, Polish, Portuguese in Portugal). The crate’s own documentation says otherwise.
#Currency symbols fall back to the code
When the locale’s data has no symbol for a currency, the ISO code is printed (BHD 1,234.500). ICU4X writes a Missing data for identifier line to standard error when this happens. A syntactically valid but unknown code, such as ZZZ, is not an error.
#POSIX modifiers are dropped
sr_RS@latin reads as sr-RS (Cyrillic), not sr-Latn-RS, and C and POSIX are errors. See Locales.
#Negotiation drops the region
negotiate returns one of the locales you passed in available, with the user’s Unicode extensions added. A user’s de-CH becomes your de, and a formatter built from that answer uses German rather than Swiss conventions. Format with the user’s own locale when it exists. See Locales.
#Calendars follow the locale
The input is always the ISO calendar, and the output follows the locale: th prints Buddhist years (2569) and fa Persian ones (1405). Add -u-ca-gregory to the locale when you need Gregorian dates. See Dates and times.
#Dates have a year range and no zone
DateTime accepts the years -9999 to 9999. from_unix_millis can build a value outside that range and the failure comes at format. The documented range for second is 0 to 59, but 60 is accepted.
#Exact strings change with the data
The text comes from CLDR through ICU4X, which revises separators, abbreviations and joiners between releases (the narrow no-break space in en times is one such change). Dependencies are icu 2.4 or newer within 2.x and icu_experimental pinned to exactly 0.7.0, which provides percent and currency formatting and changes its API in any release. Pin the crate’s tag, and rerun your tests when you bump it.
#It is a 0.x library
Under the 0.x policy a breaking change is released as a minor version (0.1.x to 0.2.0) with a Migration: note. A fix, a performance change or a feature is a patch. See Releases.
#Not covered by tests
The test suite in crates/dxn_intl/tests/ checks plurals, numbers, dates and locales for a handful of locales each, not every language. It runs on Linux, macOS and Windows in CI, and CI checks that the crate builds for wasm32-unknown-unknown but does not run the tests there. The plural rules, calendars and patterns come from ICU4X and CLDR, and their own test suites cover them; this crate’s tests cover how it maps its options onto them.
#Next
Platforms covers targets, threads and data size. Cookbook has recipes for several of the gaps above.