Crate icu

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icu is the main meta-crate of the ICU4X project.

It provides a comprehensive selection of functionality found in International Components for Unicode in their canonical configurations intended to enable software internationalization capabilities.

This crate exists to collect the most important functionality for users together in one place. It does not bring any unique functionality, but rather, it re-exports the relevant crates as modules. The exported crate corresponding to each module is also available in a stand-alone manner, i.e. icu::list as icu::list.

§Data Management

Most internationalization algorithms are data-driven based on surveys of locale experts. ICU4X offers multiple ways to manage locale data: many clients can start by using the extensive data compiled into the library, while users with additional requirements can provide data explicitly using DataProviders.

§Compiled data

Compiled data is exposed through idiomatic Rust constructors like new or try_new:

use icu::datetime::DateTimeFormatter;
use icu::locid::locale;

let dtf = DateTimeFormatter::try_new(
.expect("compiled data should include 'es-US'");

Clients using compiled data benefit from simple code and optimal zero-cost data loading. Additionally, ICU4X’s APIs are designed such that dead-code elimination can optimize away unused compiled data.

By default, most of the data available in CLDR is included. Users can customize data by using the icu4x-datagen tool (with the -⁠-format mod flag) to, for example, select a smaller set of locales, and then compiling with the ICU4X_DATA_DIR variable.

§Explicit data

Powerful data management is possible with DataProviders, which are passed to ICU4X APIs via special constructors:

use icu::datetime::DateTimeFormatter;
use icu::locid::locale;
use icu_provider_adapters::fallback::LocaleFallbackProvider;
use icu_provider_blob::BlobDataProvider;

let data: Box<[u8]> = todo!();

let provider = BlobDataProvider::try_new_from_blob(data)
    .expect("data should be valid");

let provider =
        .expect("provider should include fallback data");

let dtf = DateTimeFormatter::try_new_with_buffer_provider(
.expect("data should include 'es-US', 'es', or 'und'");

Explicit data management can be used if the compiled-data constructors are too limiting. It allows:

  • Accessing data without fallback
  • Custom DataProviders backed by sources like the operating system
  • Lazily loading or updating data from I/O
  • Composing data providers from different sources
  • Manually including/excluding data
  • … and more. See our data management tutorial

The following DataProviders are available in separate crates:

  • BlobDataProvider: deserializes data from an in-memory blob, which can be updated at runtime.
  • BakedDataProvider: a code-generated provider that contains the data directly in Rust code. This is the same provider that is used internally by compiled data.
  • FsDataProvider: uses a file system tree of Serde files. This is mostly useful for development and not recommended in production for performance reasons.
  • icu_provider_adapters: this crate contains provider adapters to combine providers, provide additional functionality such as locale fallback, and more.

The data that is required by these providers (in BakedDataProvider’s case, the provider itself) can be generated and customized using the icu4x-datagen tool.


ICU4X components share a set of common Cargo features that control whether core pieces of functionality are compiled. These features are:

  • compiled_data (default): Whether to include compiled data. Without this flag, only constructors with explicit provider arguments are available.
  • std: Whether to include std support. Without this Cargo feature, icu is #[no_std]-compatible.
  • sync: makes most ICU4X objects implement Send + Sync. Has a small performance impact when used with non-static data.
  • logging: Enables logging through the log crate.
  • serde: Activates serde implementations for core library types, such as Locale, as well as *_with_buffer_provider constructors for explicit data management.

The following Cargo features are only available on the individual crates, but not on this meta-crate:

  • datagen: Whether to implement functionality that is only required during data generation.
  • bench: Whether to enable exhaustive benchmarks. This can be enabled on individual crates when running cargo bench.

§Experimental modules

Experimental, unstable functionality can be found in the icu::experimental crate. The modules in that crate are on track to be eventually stabilized into this crate.


  • Types for dealing with dates, times, and custom calendars.
  • Case mapping for Unicode characters and strings.
  • Comparing strings according to language-dependent conventions.
  • Efficient collections for Unicode data.
  • Formatting date and time.
  • Formatting basic decimal numbers.
  • 🚧 The experimental development module of the ICU4X project.
  • Formatting lists in a locale-sensitive way.
  • Parsing, manipulating, and serializing Unicode Language and Locale Identifiers.
  • Canonicalization of locale identifiers based on CLDR data.
  • Normalizing text into Unicode Normalization Forms.
  • Determine the plural category appropriate for a given number in a given language.
  • Definitions of Unicode Properties and APIs for retrieving property data in an appropriate data structure.
  • Segment strings by lines, graphemes, words, and sentences.
  • Types for resolving and manipulating time zones.