how_to_use_prebuilt_protoc.md

Normally, protoc compiler itself is compiled from source code when we use proto_library. However, we can speed up the build by using prebuilt protoc compiler.

Declare a repository for prebuilt protoc compiler in WORKSPACE:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
  name = "com_google_protobuf_protoc_linux_x86_64",
  build_file_content = 'exports_files(["bin/protoc"])',
  sha256 = "179a759581bf4b32cc5edae4ffce6b8ee16ba4f4ab99ad3a309c31113f98d472",
  urls = ["https://github.com/protocolbuffers/protobuf/releases/download/v23.2/protoc-23.2-linux-x86_64.zip"],
)

where the zip file link is obtained from GitHub's Protobuf 23.2 release page "Asset" section: https://github.com/protocolbuffers/protobuf/releases/tag/v23.2.

Then you can run normal build command with --proto_compiler=@com_google_protobuf_protoc_linux_x86_64//:bin/protoc:

suztomo@suztomo:~/work_proto_lib$ bazelisk build --proto_compiler=@com_google_protobuf_protoc_linux_x86_64//:bin/protoc //google/type:postal_address_proto
INFO: Analyzed target //google/type:postal_address_proto (5 packages loaded, 8 targets configured).
INFO: Found 1 target...
Target //google/type:postal_address_proto up-to-date:
  bazel-bin/google/type/postal_address_proto-descriptor-set.proto.bin
INFO: Elapsed time: 0.379s, Critical Path: 0.02s

This skips around 50 seconds of protoc compilation time in my Linux 16 vCPU machine.