rust-native-wasm-loader

This is a webpack loader that loads Rust code as a WebAssembly module. It uses the native Rust support for compiling to wasm32 and does not require Emscripten.

• rust-native-wasm-loader
• Usage
  • Short version
  • Available loader options
  • Long version
• wasm-bindgen experimental support
• cargo-web experimental support

Usage

If you already know how to use Rust and Webpack, read the "Short version" of this section. If you want a full example, read the "Long version."

Short version

Add both this loader and wasm-loader to your Webpack loaders in webpack.config.js:

module.exports = {
  // ...
  module: {
    rules: [
      {
        test: /\.rs$/,
        use: [{
          loader: 'wasm-loader'
        }, {
          loader: 'rust-native-wasm-loader',
          options: {
            release: true
          }
        }]
      }
    ]
  }
}

Then, specify that your Rust library should be a cdylib in Cargo.toml:

[lib]
crate-type = ["cdylib"]

Now you can import any functions marked with #[no_mangle] as WebAssembly functions:

import loadWasm from './path/to/rustlib/src/lib.rs'
 
loadWasm().then(result => {
  const add = result.instance.exports['add'];
  console.log('return value was', add(2, 3));
});

Available loader options

• release: boolean; whether to compile the WebAssembly module in debug or release mode; defaults to false.
• gc: boolean; whether to run wasm-gc on the WebAssembly output. Reduces binary size but requires installing wasm-gc. Defaults to false and is a no-op in wasmBindgen or cargoWeb mode.
• target: string; the Rust target to use; this defaults to wasm32-unknown-unknown
• wasmBindgen: boolean or object; use wasm-bindgen to post-process the wasm file. This probably means that you need to chain this loader with babel-loader as well since wasm-bindgen outputs ES6 (or typescript).
  • wasmBindgen.debug: boolean; run wasm-bindgen in debug mode.
  • wasmBindgen.wasm2es6js: boolean; use wasm2es6js to inline the wasm file into generated Javascript. Useful if webpack is not configured to load wasm files via some other loader.
  • wasmBindgen.typescript: boolean; emit a typescript declaration file as part of the build. This file will automatically be referenced, and in a way that ts-loader will pick it up but it's still possible to treat the output from this loader like a normal Javascript module compatible with babel-loader.
• cargoWeb: boolean or object; use cargo-web to compile the project instead of only building a wasm module. Defaults to false.
  • cargoWeb.name: string; the file name to use for emitting the wasm file for cargo-web, e.g. 'static/wasm/[name].[hash:8].wasm'.
  • cargoWeb.regExp: string; a regexp to extract additional variables for use in name.

Long version

First, you need Rust installed. The easiest way is to follow the instructions at rustup.rs.

Then, you need to add support for WebAssembly cross-compilation. At the time of writing, this requires using the nightly compiler:

rustup toolchain install nightly
rustup target add wasm32-unknown-unknown --toolchain nightly

The next step is to integrate a cargo/node project. Let's assume we don't already have one, so we create one:

cargo init add
cd add

We can add the Rust code that should be available in the WebAssembly module to src/lib.rs. All functions that should be reachable from WebAssembly should be marked with #[no_mangle]:

#[no_mangle]
pub fn add(a: i32, b: i32) -> i32 {
    eprintln!("add({:?}, {:?}) was called", a, b);
    a + b
}

Then, specify that your Rust library should be a cdylib in Cargo.toml:

[lib]
crate-type = ["cdylib"]

Now you can actually start to use this loader. This loader itself does not create Javascript code for loading a WebAssembly module, so you need to compose it with another loader, like wasm-loader:

yarn init
yarn add --dev webpack
yarn add --dev rust-native-wasm-loader wasm-loader

The loaders can be registered the usual way by adding them to your webpack.config.js:

const path = require('path');
 
module.exports = {
  entry: './src/index.js',
  output: {
    filename: 'index.js',
    path: path.resolve(__dirname, 'dist')
  },
  module: {
    rules: [
      {
        test: /\.rs$/,
        use: [{
          loader: 'wasm-loader'
        }, {
          loader: 'rust-native-wasm-loader',
          options: {
            release: true
          }
        }]
      }
    ]
  }
};

You can now import the WebAssembly module by using the main .rs file from your Cargo project (lib.rs or main.rs); e.g. from your index.js:

import loadAdd from './lib.rs'
 
loadAdd().then(result => {
  const add = result.instance.exports['add'];
  console.log('return value was', add(2, 3));
});

You can now run webpack and the resulting code from node.js or a browser:

$ yarn run webpack
$ node dist/index.js
return value was 5

wasm-bindgen experimental support

You can use experimental wasm-bindgen support with the following options:

{
  test: /\.rs$/,
  use: [
    {
      loader: 'babel-loader',
      options: {
        compact: true,
      }
    },
    {
      loader: 'rust-native-wasm-loader',
      options: {
        release: true,
        wasmBindgen: {
          wasm2es6js: true,
        },
      }
    }
  ]
}

The loader now uses wasm-bindgen to build the project. If you are using webpack 4, it has experimental native support for importing WASM files, so you probably don't need the wasm2es6js flag. If you are using webpack 3 (or bundling for Chrome and it's 4K limit on main thread WASM), it is needed in order to inline the loading of the wasm file correctly. By using wasm2es6js, the loader returns a normal Javascript module that can be loaded like so:

import { add, wasmBooted } from './path/to/rustlib/src/lib.rs'
 
wasmBooted.then(() => {
  console.log('return value was', add(2, 3));
});

cargo-web experimental support

You can use experimental cargo-web support with the following options:

{
  loader: 'rust-native-wasm-loader',
  options: {
    cargoWeb: true,
    name: 'static/wasm/[name].[hash:8].wasm'
  }
}

The loader now uses cargo-web to build the project, and as a result needs to emit the wasm file separately. The loader now returns a normal Javascript module that can be loaded like so:

import loadWasm from './path/to/rustlib/src/lib.rs'
 
loadWasm.then(module => {
  console.log('return value was', module.add(2, 3));
});