Bazel Rules for WebAssembly Component Model

Production-ready Bazel rules for building WebAssembly components across multiple languages with native WASI Preview 2 support.

Why Use This?

• Multi-language: Build components from Rust, Go (TinyGo), C++, JavaScript/TypeScript
• Production Ready: OCI publishing, cryptographic signing, WAC composition, AOT compilation
• Bazel Native: Hermetic builds, aggressive caching, cross-platform (Windows/macOS/Linux)
• Zero Shell Scripts: Pure Bazel implementation for maximum portability

Installation

Add to your MODULE.bazel:

bazel_dep(name = "rules_wasm_component", version = "1.0.0")

Quick Examples

Rust Component

load("@rules_wasm_component//rust:defs.bzl", "rust_wasm_component")

rust_wasm_component(
    name = "my_service",
    srcs = ["src/lib.rs"],
    wit = ":service_wit",
    profiles = ["debug", "release"],
)

Go Component (TinyGo 0.39.0+)

load("@rules_wasm_component//go:defs.bzl", "go_wasm_component")

go_wasm_component(
    name = "calculator",
    srcs = ["main.go"],
    wit = ":calculator_wit",
    world = "calculator",
)

C++ Component (WASI SDK 27+)

load("@rules_wasm_component//cpp:defs.bzl", "cpp_component")

cpp_component(
    name = "calculator",
    srcs = ["calculator.cpp"],
    wit = ":calculator_wit",
    cxx_std = "c++20",
)

WAC Composition

load("@rules_wasm_component//wac:defs.bzl", "wac_compose")

wac_compose(
    name = "full_system",
    components = {
        ":frontend": "app:frontend",
        ":backend": "app:backend",
    },
    composition = '''
        let frontend = new app:frontend { ... };
        let backend = new app:backend { ... };
        connect frontend.request -> backend.handler;
        export frontend as main;
    ''',
)

Features

Language Support

• Rust (1.90.0+): Multi-profile builds, Clippy integration, Wizer pre-initialization
• Go (TinyGo 0.39.0): Native WASI Preview 2, hermetic Go module resolution
• C++ (WASI SDK 27): C++17/20/23, cross-package headers, LTO optimization
• JavaScript/TypeScript (jco 1.4.0, Node.js 20.18.0): NPM dependencies, componentize-js

Production Features

• WAC Composition: Official WebAssembly Composition standard for multi-component systems
• OCI Publishing: Push components to Docker/OCI registries
• Cryptographic Signing: wasmsign2 integration for supply chain security
• AOT Compilation: Wasmtime precompilation for 87% size reduction and faster startup
• Wizer Pre-initialization: 1.35-6x startup improvement for Rust components
• Multi-Profile Builds: Debug, release, and custom optimization profiles

Developer Experience

• Hermetic Toolchains: All tools downloaded automatically, no system dependencies
• Cross-Platform: Native Windows/macOS/Linux support without WSL
• Bazel-First: Zero shell scripts, pure Bazel actions for reproducible builds
• Comprehensive Examples: 20+ working examples from basic to advanced patterns

Toolchain Versions

See MODULE.bazel for current versions - the single source of truth.

All toolchains are hermetically downloaded and version-pinned for reproducible builds. Key toolchains include:

• wasm-tools - Component manipulation (new, validate, compose)
• TinyGo - Go → WASM compilation with WASI Preview 2
• WASI SDK - C/C++ → WASM compilation
• Wasmtime - WASM runtime and AOT compilation
• Wizer - Pre-initialization for faster startup
• wkg - Package management
• jco - JavaScript component compiler

Documentation

📚 Complete Rule Reference - All rules, attributes, and providers

Guides:

• Toolchain Configuration
• Multi-Profile Builds
• Development Guidelines - Bazel-first principles

Examples:

• Basic Component - Getting started
• Multi-Language Composition - WAC composition
• Wizer Pre-initialization - Performance optimization
• OCI Publishing - Production deployment
• See all examples →

Known Limitations

Go WIT Components (Temporary)

• Issue: WIT-enabled Go components currently fail due to upstream TinyGo limitations
• Tracking: GitHub Issue #82
• Status: Excluded from CI until TinyGo PR #4934 lands
• Workaround: Basic Go components (without WIT) work perfectly

C++ Exception Handling

• Design: WASI disables C++ exceptions by default for size/performance
• Solution: Components use error codes instead of exceptions (Issue #83)
• Override: Use enable_exceptions = True for components that require exceptions

Contributing

Contributions welcome! See CONTRIBUTING.md for details.

License

Apache 2.0 - See LICENSE for details.