Overview

cargo component is a cargo subcommand for creating WebAssembly components using Rust as the component's implementation language.

Notice

cargo component is considered to be experimental and is not currently stable in terms of the code it supports building.

Until the component model stabilizes, upgrading to a newer cargo component may cause build errors for existing component projects.

Requirements

1. The cargo component subcommand is written in Rust, so you'll want the latest stable Rust installed.

Installation

To install the cargo component subcommand from source, run the following command:

cargo install cargo-component

If you have the cargo-binstall utility installed, cargo component can also be installed via a prebuilt release artifact, saving time on the installation:

cargo binstall cargo-component

Motivation

Today, developers that target WebAssembly typically compile a monolithic program written in a single source language to a WebAssembly module. The WebAssembly module can then be used in all sorts of places: from web browsers to cloud compute platforms. WebAssembly was intentionally designed to provide the portability and security properties required for such environments.

However, WebAssembly modules are not easily composed with other modules into a single program or service. WebAssembly only has a few primitive value types (integer and floating point types) and those are inadequate to describe the complex types that developers would desire to exchange between modules.

To make things even more challenging, WebAssembly modules typically define their own local linear memories, meaning one module can't access the (conceptual) address space of another. Something must sit between the two modules to facilitate communication when pointers are passed around.

While it is possible to solve these challenges with the existing WebAssembly standard, doing so is burdensome, error-prone, and requires foreknowledge of how the WebAssembly modules are implemented.

WebAssembly Component Model

The WebAssembly component model proposal provides a way to simplify the process of building WebAssembly applications and services out of reusable pieces of functionality using a variety of source languages, all while still maintaining the portability and security properties of WebAssembly.

At its most fundamental level, WebAssembly components may be used to wrap a WebAssembly module in a way that describes how its interface, a set of functions using complex value types (e.g. strings, variants, records, lists, etc.), is translated to and from the lower-level representation required of the WebAssembly module.

This enables WebAssembly runtimes to know specifically how they must facilitate the exchange of data between the discrete linear memories of components, eliminating the need for developers to do so by hand.

Additionally, components can describe their dependencies in a way that modules simply cannot today; they can even control how their dependencies are instantiated, enabling a component to virtualize functionality needed by a dependency. And because different components might have a shared dependency, hosts may even share the same implementation of that dependency to save on host memory usage.

Cargo Component

A primary goal of cargo component is to try to imagine what first-class support for WebAssembly components might look like for Rust.

That means being able to reference WebAssembly components via Cargo.toml and have WebAssembly component dependencies used in the same way as Rust crate dependencies:

• add a dependency on a WebAssembly component to Cargo.toml
• reference it like you would an external crate (via bindings::<name>::...) in your source code
• build using cargo component build and out pops your component!

To be able to use a WebAssembly component from any particular programming language, bindings must be created by translating a WebAssembly component's interface to a representation that a specific programming language can understand.

Tools like wit-bindgen exist to generate those bindings for different languages, including Rust.

wit-bindgen even provides procedural macros to generate the bindings "inline" with the component's source code.

Unlike wit-bindgen, cargo component generates bindings directly into your project at src/bindings.rs so that bindings are generated based on the resolved dependencies from Cargo.toml rather than parsing a local definition of the component's interface.

The hope is that one day (in the not too distant future...) that WebAssembly components might become an important part of the Rust ecosystem such that cargo itself might support them.

Until that time, there's cargo component!

WASI Support

Currently cargo component targets wasm32-wasi by default.

As this target is for a preview1 release of WASI, the WebAssembly module produced by the Rust compiler must be adapted to the preview2 version of WASI supported by the component model.

The adaptation is automatically performed when wasm32-wasi is targeted using a built-in WASI adapter snapshotted out of the Wasmtime repository.

You may override the built-in adapter cargo component uses by setting the adapter setting in the [package.metadata.component] table in Cargo.toml to the path to the adapter module to use.

To build the adapter module, clone the Wasmtime repository and run the following commands:

# Add the wasm32-unknown-unknown target if you haven't already
rustup target add wasm32-unknown-unknown

git checkout $REV

git submodule update --init

cargo build -p wasi-preview1-component-adapter --target wasm32-unknown-unknown --release

cp target/wasm32-unknown-unknown/release/wasi_snapshot_preview1.wasm $PROJECT

where $REV is the Wasmtime commit hash you want to use and $PROJECT is the path to your component project.

Next, edit Cargo.toml to point at the adapter:

[package.metadata.component]
adapter = "wasi_snapshot_preview1.wasm"

When the Rust compiler supports a preview2 version of the WASI target, support in cargo component for adapting a preview1 module will be removed.

Getting Started

Use cargo component new --lib <name> to create a new library (reactor) component.

A library component doesn't have a run (i.e. main in Rust) function exported and is meant to be used as a library rather than a command that runs and exits. Without the --lib flag, cargo component defaults to creating a command component.

This will create a wit/world.wit file describing the world that the component will target:

package my-org:my-component;

/// An example world for the component to target.
world example {
    export hello-world: func() -> string;
}

The component will export a hello-world function returning a string.

The implementation of the component will be in src/lib.rs:

#[allow(warnings)]
mod bindings;

use bindings::Guest;

struct Component;

impl Guest for Component {
    /// Say hello!
    fn hello_world() -> String {
        "Hello, World!".to_string()
    }
}

bindings::export!(Component with_types_in bindings);

The bindings module contains the the types and traits that correspond to the world targeted by the component; it is automatically generated by cargo component.

Usage

The cargo component subcommand has some analogous commands to cargo itself:

• cargo component new — creates a new WebAssembly component Rust project.
• cargo component add — adds a component interface dependency to a cargo manifest file.
• cargo component update — same as cargo update but also updates the dependencies in the component lock file.
• cargo component publish - publishes a WebAssembly component to a warg component registry.
• cargo component key - manages signing keys for publishing WebAssembly components.

Unrecognized commands are passed through to cargo itself, but only after the bindings information for component packages has been updated.

Some examples of commands that are passed directly to cargo are: build, check, doc, clippy and extension commands such as expand from cargo-expand.

Certain command line options, like --target and --release, are detected by cargo component to determine what output files of a build command should be componentized.

Using rust-analyzer

rust-analyzer is an extremely useful tool for analyzing Rust code and is used in many different editors to provide code completion and other features.

rust-analyzer depends on cargo metadata and cargo check to discover workspace information and to check for errors.

To ensure that rust-analyzer is able to discover the latest bindings information, rust-analyzer must be configured to use cargo component check as the check command.

To configure rust-analyzer to use the cargo component executable, set the rust-analyzer.check.overrideCommand setting to the following:

{
    "rust-analyzer.check.overrideCommand": [
        "cargo",
        "component",
        "check",
        "--workspace",
        "--all-targets",
        "--message-format=json"
    ],
}

By default, cargo component new will configure Visual Studio Code to use cargo component check by creating a .vscode/settings.json file for you. To prevent this, pass --editor none to cargo component new.

Please check the documentation for rust-analyzer regarding how to set settings for other IDEs.

Contributing to cargo component

cargo component is a Bytecode Alliance project, and follows the Bytecode Alliance's Code of Conduct and Organizational Code of Conduct.

Getting the Code

You'll clone the code via git:

git clone https://github.com/bytecodealliance/cargo-component

Testing Changes

We'd like tests ideally to be written for all changes. Test can be run via:

cargo test

You'll be adding tests primarily to the tests/ directory.

Submitting Changes

Changes to cargo component are managed through pull requests (PRs). Everyone is welcome to submit a pull request! We'll try to get to reviewing it or responding to it in at most a few days.

Code Formatting

Code is required to be formatted with the current Rust stable's cargo fmt command. This is checked on CI.

Continuous Integration

The CI for the cargo component repository is relatively significant. It tests changes on Windows, macOS, and Linux.

Publishing

Publication of this crate is entirely automated via CI. A publish happens whenever a tag is pushed to the repository, so to publish a new version you'll want to make a PR that bumps the version numbers (see the ci/publish.rs script), merge the PR, then tag the PR and push the tag. That should trigger all that's necessary to publish all the crates and binaries to crates.io.