Crate Universe repository rules are backed by a binary called cargo-bazel. This can be built using cargo by simply changing directories to @rules_rust//crate_universe and running the following:
cargo build --bin=cargo-bazel
It's then recommended to export the CARGO_BAZEL_GENERATOR_URL environment variable to be a file:// url to the built binary on disk.
export CARGO_BAZEL_GENERATOR_URL=file://$(pwd)/target/debug/cargo-bazel
From here on, the repository rule can be run
If a project does not get rules_rust from a release artifact from the Github releases page (e.g. using an archive from a commit or branch) then cargo-bazel binaries will have to manually be specified for repository rules that consume it. It's highly recommended to build cargo-bazel binaries yourself and host them somewhere the project can safely access them. Without this, repository rules will attempt to build the binary using cargo_bootstrap_repository as a fallback. This is very time consuming and in no way the recommended workflow for anything other than developing rules_rust directly.