This file documents the procedures for building with ICU4X.

Enabling ICU4X

  1. Add the ac_add_options --enable-icu4x mozconfig. (This is the default)

  2. Do a full build.

Updating the bundled ICU4X data

ICU4X data is bundled directly as a rust crate with compiled_data feature.

But each icu_*_data crate has all locale data, so it might include unnecessary data. icu_segmenter uses custom crates in Gecko since it includes unnecessary data such as the word dictionaries for East Asian.

The script intl/ can generate and update this binary data. If you want to add the new data type, you modify this script and then, run it.

When using ICU4X 1.4.0 data with the latest data that is hard-coded, you have to run the following. The baked data of icu_segmenter is generated into intl/icu_segmenter_data/data.

$ cd $(TOPSRCDIR)/intl
$ ./ icu@1.4.0 44.0.0 release-74-1 1.4.0

Updating ICU4X

If you update ICU4X crate into Gecko, you have to check Cargo.toml in Gecko’s root directory. We might have some hacks to replace version with a custom version.


ICU4X provides icu_capi crate for C/C++ FFI. mozilla::intl::GetDataProvider returns capi::ICU4XDataProvider of icu_capi. It can return valid data until shutting down.

Accessing the data provider from Rust

Use compiled_data feature. You don’t consider data provider.

Adding new ICU4X features to Gecko

To reduce build time and binary size, embedded ICU4X in Gecko is minimal configuration. If you have to add new features, you have to update some files.

  1. Adding the feature to icu_capi entry in js/src/rust/shared/Cargo.toml.

  2. Modify [features] section in intl/icu_capi/Cargo.toml to enable compiled_data feature of added crate.

  3. Modify the patch file for intl/icu_capi/Cargo.toml into intl/icu4x-patches.

  4. (Optional) Modify intl/ to add generated ICU4X data if you want to modify ICU4X baked data.