Setting up Protocol Buffer compilation with Bazel and Rust

#bazel #rust #protobuf #build-systems #grpc #tonic

In this post, I’ll explain how we set up Protocol Buffer compilation in our Rust project using Bazel, and why we had to build a custom solution.

The Problem with Official Documentation

The official rules_rust documentation for rust_prost (https://bazelbuild.github.io/rules_rust/rust_prost.html) unfortunately no longer works with recent versions of tonic. The toolchain setup and proto compilation rules are incompatible with the current tonic API and dependencies, leading to compilation errors and version conflicts.

This forced us to create our own solution that works reliably with modern versions of both prost and tonic.

Our Custom Solution

We built a three-part system in the proto/ folder:

1. Custom Compiler Binary

Instead of relying on rules_rust’s prost toolchain, we use the official tonic-build ecosystem:

// proto/src/main.rs
fn main() -> Result<(), Box<dyn std::error::Error>> {
    let out_dir = std::env::var("OUT_DIR")?;
    std::fs::create_dir_all(&out_dir)?;

    tonic_prost_build::configure()
        .out_dir(&out_dir)
        .compile_protos(&["./protos/hello.proto"], &["./protos"])?;

    Ok(())
}

This compiler:

  • Reads the OUT_DIR environment variable to determine where to generate code
  • Uses tonic_prost_build directly to compile proto files
  • Generates both message structs and gRPC service traits

2. Bazel Build Configuration

The BUILD.bazel file in the proto folder orchestrates the compilation:

rust_binary(
    name = "compiler",
    srcs = ["src/main.rs"],
    deps = ["@crates//:tonic-prost-build"],
)

genrule(
    name = "protos",
    srcs = glob(["protos/*.proto"]),
    outs = ["generated/hello.rs"],
    cmd = 'export COMPILER="$$PWD/$(location :compiler)"; '
          'export OUT_DIR=$$PWD/$$(dirname $(location :generated/hello.rs)); '
          'cd proto; $$COMPILER',
    tools = [":compiler"],
)

rust_library(
    name = "proto",
    srcs = [":generated/hello.rs"],
    crate_name = "hello_proto",
    deps = [
        "@crates//:prost",
        "@crates//:tonic",
    ],
)

Usage in the Project

Once compiled, the generated proto code integrates seamlessly:

use hello_proto::hello_service_server::HelloService;
use hello_proto::{HelloRequest, HelloResponse};

#[tonic::async_trait]
impl HelloService for MyService {
    async fn say_hello(
        &self,
        request: Request<HelloRequest>,
    ) -> Result<Response<HelloResponse>, Status> {
        // Implementation
    }
}

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