#wit #interface-types #module-linking #wasi #wasmtime #bindgen #webassembly


Rust bindings generator and runtime support for WIT and the component model. Used when compiling Rust programs to the component model

10 breaking releases

0.12.0 Sep 18, 2023
0.10.0 Aug 18, 2023
0.9.0 Jul 15, 2023
0.4.0 Mar 7, 2023
0.0.0 Nov 16, 2021

#804 in WebAssembly

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Guest language bindings generator for WIT and the Component Model

A Bytecode Alliance project

build status supported rustc stable


Note: this project is still relatively young and active development with large changes is still happening. If you're considering depending on this at this time it's recommended to reach out to the authors on zulip and get more information first.

This project is a suite of bindings generators for languages that are compiled to WebAssembly and use the component model. Bindings are described with *.wit files which specify imports, exports, and facilitate reuse between bindings definitions.

The wit-bindgen repository is currently focused on guest programs which are those compiled to WebAssembly. Executing a component in a host is not managed in this repository, and some options of how to do so are described below.

WIT as an IDL

The wit-bindgen project extensively uses WIT definitions to describe imports and exports. The items supported by WIT directly map to the component model which allows core WebAssembly binaries produced by native compilers to be transformed into a component. All imports into a WebAssembly binary and all exports must be described with WIT. An example file looks like:

package example:host

world host {
  import print: func(msg: string)

  export run: func()

This describes a "world" which describes both imports and exports that the WebAssembly component will have available. In this case the host will provide a print function and the component itself will provide a run function.

Functionality in WIT can also be organized into interfaces:

package example:my-game

interface my-plugin-api {
  record coord {
    x: u32,
    y: u32,

  get-position: func() -> coord
  set-position: func(pos: coord)

  record monster {
    name: string,
    hp: u32,
    pos: coord,

  monsters: func() -> list<monster>

world my-game {
  import print: func(msg: string)
  import plugin: self.my-plugin-api

  export run: func()

Here the my-plugin-api interface encapsulates a group of functions, types, etc. This can then be imported wholesale into the my-game world via the plugin namespace. The structure of a WIT document and world will affect the generated bindings per-language.

For more information about WIT and its syntax see the upstream description of WIT.

Creating a Component

The end-goal of wit-bindgen is to facilitate creation of a component. Once a component is created it can then be handed off to any one of a number of host runtimes for execution. Creating a component is not supported natively by any language today, however, so wit-bindgen is only one of the pieces in the process of creating a component. The general outline for the build process of a component for a compiled language is:

  1. Using wit-bindgen source code for the language is generated representing bindings to the specified APIs. This source code is then compiled by the native compiler and used by user-written code as well.
  2. The native language toolchain is used to emit a core WebAssembly module. This core wasm module is the "meat" of a component and contains all user-defined code compiled to WebAssembly. The most common native target to use for compilation today is the wasm32-wasi target.
  3. The output core wasm module is transformed into a component using the wasm-tools project, notably the wasm-tools component new subcommand. This will ingest the native core wasm output and wrap the output into the component model binary format.

The precise tooling and commands at each of these steps differs language by language, but this is the general idea. With a component in-hand the binary can then be handed off to a host runtimes for execution.

Creating components: WASI

An important consideration when creating a component today is WASI. All current native toolchains for languages which have WASI support are using the wasi_snapshot_preview1 version of WASI. This definition of WASI was made with historical *.witx files and is not compatible with the component model. There is, however, a means by which to still create components from modules that are using wasi_snapshot_preview1 APIs.

The wasm-tools component new subcommand takes an --adapt argument which acts as a way to polyfill non-component-model APIs, like wasi_snapshot_preview1, with component model APIs. The preview2-prototyping project is the current go-to location to acquire a polyfill from wasi_snapshot_preview1 to an in-development version of "wasi preview2" which is specified with WIT and the component model.

Notably you'll want to download one of the adapter modules and name it wasi_snapshot_preview1.wasm locally to pass as an --adapt argument to wasm-tools component new. Note that there are two modules provided on the downloads page, one is for non-commands which don't have a _start entrypoint in the generated core wasm module (e.g. the cdylib crate type in Rust) and one that is for command modules which has a _start entrypoint (e.g. a src/main.rs in Rust).

Supported Guest Languages

The wit-bindgen project is primarily focused on guest languages which are those compiled to WebAssembly. Each language here already has native support for execution in WebAssembly at the core wasm layer (e.g. targets the current core wasm specification). Brief instructions are listed here for each language of how to use it as well.

Each project below will assume the following *.wit file in the root of your project.

// wit/host.wit
package example:host

world host {
  import print: func(msg: string)

  export run: func()

Guest: Rust

The Rust compiler supports a native wasm32-wasi target and can be added to any rustup-based toolchain with:

rustup target add wasm32-wasi

In order to compile a wasi dynamic library, the following must be added to the Cargo.toml file:

crate-type = ["cdylib"]

Projects can then depend on wit-bindgen by executing:

cargo add --git https://github.com/bytecodealliance/wit-bindgen wit-bindgen

WIT files are currently added to a wit/ folder adjacent to your Cargo.toml file. Example code using this then looks like:

// src/lib.rs

// Use a procedural macro to generate bindings for the world we specified in
// `host.wit`

// Define a custom type and implement the generated `Host` trait for it which
// represents implementing all the necesssary exported interfaces for this
// component.
struct MyHost;

impl Host for MyHost {
    fn run() {
        print("Hello, world!");


By using cargo expand or cargo doc you can also explore the generated code.

This project can then be built with:

cargo build --target wasm32-wasi
wasm-tools component new ./target/wasm32-wasi/debug/my-project.wasm \
    -o my-component.wasm --adapt ./wasi_snapshot_preview1.wasm

This creates a my-component.wasm file which is suitable to execute in any component runtime. Using wasm-tools you can inspect the binary as well, for example inferring the WIT world that is the component:

wasm-tools component wit my-component.wasm
# world my-component {
#  import print: func(msg: string)
#  export run: func()
# }

which in this case, as expected, is the same as the input world.

Guest: C/C++

C and C++ code can be compiled for the wasm32-wasi target using the WASI SDK project. The releases on that repository have precompiled clang binaries which are pre-configured to compile for WebAssembly.

To start in C and C++ a *.c and *.h header file is generated for your project to use. These files are generated with the wit-bindgen CLI command in this repository.

wit-bindgen c ./wit
# Generating "host.c"
# Generating "host.h"
# Generating "host_component_type.o"

Some example code using this would then look like

// my-component.c

#include "host.h"

void host_run() {
    host_string_t my_string;
    host_string_set(&my_string, "Hello, world!");


This can then be compiled with clang from the WASI SDK and assembled into a component with:

clang host.c host_component_type.o my-component.c -o my-core.wasm -mexec-model=reactor
wasm-tools component new ./my-core.wasm -o my-component.wasm

Like with Rust, you can then inspect the output binary:

wasm-tools component wit ./my-component.wasm

Guest: Java

Java bytecode can be compiled to WebAssembly using TeaVM-WASI. With this generator, wit-bindgen will emit *.java files which may be used with any JVM language, e.g. Java, Kotlin, Clojure, Scala, etc.

Guest: TinyGo

Go code can be compiled for the wasm32-wasi target using the TinyGo compiler. For example, the following command compiles main.go to a wasm modules with WASI support:

tinygo build -target=wasi main.go

Note: the current TinyGo bindgen only supports TinyGo version v0.27.0 or later.

To start in Go a *.go and *.h C header file are generated for your project to use. These files are generated with the wit-bindgen CLI command in this repository.

wit-bindgen tiny-go ./wit
# Generating "host.go"
# Generating "host.c"
# Generating "host.h"
# Generating "host_component_type.o"

If your Go code uses result or option type, a second Go file host_types.go will be generated. This file contains the Go types that correspond to the result and option types in the WIT file.

Some example code using this would then look like

// my-component.go
package main

import (
    gen "host/gen"

func init() {
    a := HostImpl{}

type HostImpl struct {

func (e HostImpl) Run() {
  gen.Print("Hello, world!")

//go:generate wit-bindgen tiny-go ../wit --out-dir=gen
func main() {}

This can then be compiled with tinygo and assembled into a component with:

go generate # generate bindings for Go
tinygo build -target=wasi -o main.wasm my-component.go # compile
wasm-tools component embed --world host ./wit main.wasm -o main.embed.wasm # create a component
wasm-tools component new main.embed.wasm --adapt wasi_snapshot_preview1.wasm -o main.component.wasm
wasm-tools validate main.component.wasm --features component-model

Guest: Other Languages

Other languages such as JS, Ruby, Python, etc, are hoped to be supported one day with wit-bindgen or with components in general. It's recommended to reach out on zulip if you're intersted in contributing a generator for one of these langauges. It's worth noting, however, that turning an interpreted language into a component is significantly different from how compiled languages currently work (e.g. Rust or C/C++). It's expected that the first interpreted language will require a lot of design work, but once that's implemented the others can ideally relatively quickly follow suit and stay within the confines of the first design.

CLI Installation

To install the CLI for this tool (which isn't the only way it can be used), run the following cargo command. This will let you generate the bindings for any supported language.

cargo install --git https://github.com/bytecodealliance/wit-bindgen wit-bindgen-cli

This CLI IS NOT stable and may change, do not expect it to be or rely on it being stable. Please reach out to us on zulip if you'd like to depend on it, so we can figure out a better alternative for your use case.

Host Runtimes for Components

The wit-bindgen project is intended to facilitate in generating a component, but once a component is in your hands the next thing to do is to actually execute that somewhere. This is not under the purview of wit-bindgen itself but these are some resources and runtimes which can help you work with components:

  • Rust: the wasmtime crate is an implementation of a native component runtime that can run any WIT world. It additionally comes with a bindgen! macro which acts similar to the generate! macro in this repository. This macro takes a WIT package as input and generates trait-based bindings for the runtime to implement and use.

  • JS: the js-component-tools project can be used to execute components in JS either on the web or outside the browser in a runtime such as node. This project generates a polyfill for a single concrete component to execute in a JS environment by extracting the core WebAssembly modules that make up a component and generating JS glue to interact between the host and these modules.

  • Python: the wasmtime project on PyPI has a bindgen mode that works similar to the JS integration. Given a concrete component this will generate Python source code to interact with the component using an embedding of Wasmtime for its core WebAssembly support.

  • Tooling: the wasm-tools project can be used to inspect and modify low-level details of components. For example as previously mentioned you can inspect the WIT-based interface of a component with wasm-tools component wit. You can link two components together with wasm-tools compose as well.

Note that the runtimes above are generally intended to work with arbitrary components, not necessarily only those created by wit-bindgen. This is also not necessarily an exhaustive listing of what can execute a component.


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