ICU already has a number of BUILD.bazel files, but they are mostly meant for use during ICU development for building ICU data. This means that they only Bazelfied what they needed for that to work and not everything in the icu4c library has a build target yet. So if something you need is missing, you will either need to patch it in to an existing upstream ICU BUILD.bazel file or add one in the overlay if it doesn't exist upstream.
ICU requires data files to function correctly. It can either use a prebuilt .dat file or link a shared object file that wraps the data generated using icu4c/source/tools/genccode. Since genccode isn‘t Bazelfied, it was easier to use the prebuilt .dat file that comes with each ICU release. The prebuilt .dat files are only available via the release src asset and don’t exist in the repo. But the ICU BUILD.bazel files are only available in the repo, but not the src release asset. Thus we pull from the repo in sources.json to obtain the code with the BUILD.bazel files and in MODULE.bazel we use http_archive to get the prebuilt .dat file that comes with each ICU release.
Normally in order to tell ICU where the prebuilt .dat file is located, you can set the define directive ICU_DATA_DIR. However, since we‘re passing this data file in with Bazel, we need to use the Bazel C++ runfiles library to look up its actual location. The patches we’ve added allows for this.
The ICU tests are not Bazelfied, so we've just added a simple test file to excercise that the data loading works correctly at runtime.