# icu [](https://crates.io/crates/icu)
`icu` is the main meta-crate of the `ICU4X` project.
It provides a comprehensive selection of functionality found in
[International Components for Unicode](http://icu.unicode.org/)
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 [`DataProvider`]s.
### Compiled data
Compiled data is exposed through idiomatic Rust constructors like `new` or `try_new`:
```rust
use icu::datetime::DateTimeFormatter;
use icu::locid::locale;
let dtf = DateTimeFormatter::try_new(
&locale!("es-US").into(),
Default::default(),
)
.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 [`DataProvider`]s, which are passed to ICU4X APIs via
special constructors:
```rust
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 =
LocaleFallbackProvider::try_new_with_buffer_provider(provider)
.expect("provider should include fallback data");
let dtf = DateTimeFormatter::try_new_with_buffer_provider(
&provider,
&locale!("es-US").into(),
Default::default(),
)
.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 [`DataProvider`]s 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 [`DataProvider`]s 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.
## Features
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.
- `experimental`: Whether to enable experimental preview features. Modules enabled with
this feature may not be production-ready and could change at any time.
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`.
[CLDR]: http://cldr.unicode.org/
[`DataProvider`]: icu_provider::DataProvider
[`DataPayload`]: icu_provider::DataPayload
[`FsDataProvider`]: https://docs.rs/icu_provider_fs/latest/icu_provider_fs/struct.FsDataProvider.html
[`BlobDataProvider`]: https://docs.rs/icu_provider_blob/latest/icu_provider_blob/struct.BlobDataProvider.html
[`icu_provider_adapters`]: https://docs.rs/icu_provider_adapters/latest/icu_provider_adapters/
[`icu_datagen`]: https://docs.rs/icu_datagen/latest/icu_datagen/
[`icu4x-datagen`]: https://docs.rs/icu_datagen/latest/icu_datagen/
[data management tutorial]: https://github.com/unicode-org/icu4x/blob/main/docs/tutorials/data_provider.md#loading-additional-data-at-runtime
## More Information
For more information on development, authorship, contributing etc. please visit [`ICU4X home page`](https://github.com/unicode-org/icu4x).