rules_boost is a Bazel repository providing build rules for the Boost C++ libraries. It allows Bazel users to depend on Boost libraries using labels like @boost//:algorithm. The repository fetches Boost source code and provides platform-specific build configurations for hundreds of Boost components, handling separate compilation requirements for libraries like ASIO and Beast. While the README indicates it's legacy compared to Bazel Central Registry, it remains maintained for compatibility. The core reusable component is the boost_library macro in boost/boost.bzl, which encapsulates patterns for building C++ libraries with Bazel.