Buck / Buck2
Meta's build system — Buck1 (2013, Java) is now superseded by Buck2 (2021, Rust). Built for Meta's giant monorepo.
Buck2 open-sourced 2023. Differs from Buck1: Starlark instead of Python configs, Rust core, dramatically faster (2× faster build, 5-10× faster incremental). Used internally by Meta for all of Facebook, Instagram, WhatsApp code. External adopters few (high learning curve). Competitors: Bazel (Google), Pants.