Toolchain

A toolchain defines a set of tools, scripts and flags used by certain rules. Their purpose is to enable reuse of rules across projects that source their tools (e.g. compilers and linters) differently.

For example, consider cxx_binary, which is defined in the prelude. Since building C++ code is complex, it is desirable to share the same implementation of cxx_binary across projects. However, not all projects will want to source their C++ compiler, linker, etc. the same way:

• Some projects do not care, and want to pick them from the ambient environment (most likely the tools installed system-wide).
• Some projects want to achieve reproducible builds by running the build within some sort of virtual environment.
• Some projects want to achieve reproducible builds by downloading tools as part of the build itself.
• Some projects want to achieve reproducible builds by accessing tools checked into version control.

Defining those in a toolchain lets us decouple those project-specific concerns from generic build rules.

When running buck2 init, Buck2 sets up some demo toolchains via the system_demo_toolchains macro. Those expect to find the relevant tools in the user's PATH.

For more information about defining toolchains, see the relevant page in the Rule Authors section.