Buck2 vs Pants

Buck2 is a free build system from Meta for large-scale repositories that combine multiple programming languages. It uses Starlark rules, a dynamic dependency graph, and incremental actions to support extensible builds. Its language table lists prelude rules for C/C++, Erlang, Go, Haskell, Java, Kotlin, OCaml, Python, and Rust, though usability varies by language. Build actions can run remotely through services exposing Bazel's remote execution API; the project lists BuildBarn, BuildBuddy, EngFlow, and NativeLink as compatible with its distributed compilation support. Buck2 supports filesystem watching and can integrate with Meta's optional Eden virtual filesystem. Rules are hermetic by default and missing dependencies are errors, but local-only build steps are not sandboxed. The project describes Buck2 as early-stage and says it has no stable release tag; its latest binary tag changes with repository pushes. It warns that outside users may encounter rough edges, missing features, and missing toolchains. Source and binaries are available under MIT and Apache-2.0 licenses. The project directs users to GitHub for issue reports and offers a community Discord server.

Pants is a free build system for codebases of different sizes, focused on Python, Go, Java, Scala, Kotlin, Shell, and Docker. It installs and coordinates development tools such as compilers, code generators, dependency resolvers, test runners, linters, formatters, packagers, and REPLs. By examining import statements, it can infer file dependencies and reduce manual declarations. The engine is written in Rust, and build rules use typed Python 3. Execution includes concurrency, fine-grained invalidation, hermetic runs, caching, and remote execution. Custom rules use the Pants plugin API and share the core rules’ execution semantics. Built-in integrations include Docker, Pex, Protodoc, Thrift, Protobuf, Helm, linting and formatting tools, packaging, and coverage. Lockfiles pin transitive dependency versions and expected SHA256 hashes, which Pants checks against downloaded artifacts. It runs on Linux, macOS, and Windows with WSL 2, subject to stated system requirements. The project is free under the Apache License 2.0 and offers community support through Slack, mailing lists, GitHub, and other channels.