Bazel toolchain for pre-built protoc

Using Protocol Buffers with Bazel has always been painful.

Nearly every Bazel user has waited for protoc to compile from sources many, MANY times.
The versioning of the protobuf module on Bazel Central Registry has fallen behind and contains many patches.
Relying on the protobuf runtime for each language from the @com_google_protobuf repo forces you to use the same version of the runtime for all languages in a monorepo, and matching protoc.

The key observations:

protoc has always been distributed as pre-built binaries on https://github.com/protocolbuffers/protobuf/releases
the protobuf runtimes for each language are distributed to the appropriate package manager

Bazel 7 introduced --incompatible_enable_proto_toolchain_resolution to allow us fetch protoc rather than re-build it! That flag ALSO decouples how each language rule locates the runtime.

This repo simply contains a toolchain that resolves those pre-built binaries.

See examples for several language rules like py_proto_library and java_proto_library. There is NO dependency on @com_google_protobuf anywhere.

Design

Questioning why Bazel is different

Protobuf works fine under many build tools, using the releases and runtime libraries shipped by the protobuf team. Why is Bazel different?

Our answer is: it should not be. The protobuf team shouldn’t have to own Bazel rules or understand bzlmod. As with many other tools such as Swagger and GraphQL, the Bazel community is self-sufficient to create thin layers to establish a toolchain and execute actions that perform codegen steps.

This toolchain shows that there's no need to treat Bazel as a “special” build system vs. all the others that protobuf users rely on. https://protobuf.dev/reference/ is sufficient documentation for everyone.

Why a separate Bazel module?

This belongs in rules_proto, see

Getting reviews from Googlers is hard so let's not wait for them.

Installation

Make sure your .bazelrc contains

# Introduced in Bazel 7.
common --incompatible_enable_proto_toolchain_resolution

Follow instructions from the release you wish to use: https://github.com/alexeagle/toolchains_protoc/releases