Haven't looked at the Bazel codebase, but it would strongly surprise me if the language support rules were implemented in Starlark. More likely, I'd suppose the

Haven't looked at the Bazel codebase, but it would strongly surprise me if the language support rules were implemented in Starlark. More likely, I'd suppose them to be almost exclusively written in Java (and highly based on quite Bazel-specific classes), with Starlark only coming into play in the form of bindings for being used by the users for their BUILD/bzl definitions. Edit/Add: cf the part about language rules in this comment from a person who says they're a former bazel developer: https://news.ycombinator.com/item?id=35477309 https://news.ycombinator.com/item?id=35477309