[laurentlb]

It was a complete mess, hard to maintain the code, with scalability and performance problems. The semantics were very error-prone because it combined two interpreters (Python interpreter, optional, as a preprocessing step) followed by the interpreter in Bazel.

Bazel started a Python process for each BUILD file to preprocess. If two BUILD files were using the same bzl file, that bzl file would be parsed/evaluated twice. In a graph with lots of dependencies, it was causing a lot of redundant work. It's a bit like #include vs modules in C++: Starlark can evaluate a dependency file once, and provide the result for each BUILD file.

Python can be hard to sandbox, and we got multiple security reports about exploits.

Interestingly, Facebook Buck went through the same way. They originally copied the Python preprocessing approach. When we open-sourced Bazel, the Buck team took our Starlark (aka Skylark) interpreter, and started the migration. See https://buck.build/concept/skylark.html