About

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. Languages developed in this repository are
Rust, C, C++, and C#. For other languages see the documentation
below.

Executing a component in a host is not
managed in this repository, and some options of how to do so are described
below. If you encounter any problems feel free to open an
issue or chat with
us on Zulip.

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 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
my-plugin-api 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 online documentation for
WIT as well as its upstream
reference.

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-wasip1 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 Wasmtime runtime publishes adapter
modules with each release that are suitable to use with
--adapt to implement wasi_snapshot_preview1 in terms of WASI 0.2. On
Wasmtime's releases page you'll see three modules to choose from:

• wasi_snapshot_preview1.command.wasm - use this for CLI applications.
• wasi_snapshot_preview1.reactor.wasm - use this for applications that don't
  have a main function for example: for example a process that responds to an
  event.
• wasi_snapshot_preview1.proxy.wasm - use this for applications fed into
  wasmtime serve for example.

Only one adapter is necessary and be sure to look for the latest
versions as well.

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 since version 1.82 supports a native wasm32-wasip2 target and can be added to
any rustup-based toolchain with:

rustup target add wasm32-wasip2

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

[lib]
crate-type = ["cdylib"]

Projects can then depend on wit-bindgen by executing:

cargo add 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`
wit_bindgen::generate!({
    // the name of the world in the `*.wit` input file
    world: "host",
});

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

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

// export! defines that the `MyHost` struct defined below is going to define
// the exports of the `world`, namely the `run` function.
export!(MyHost);

By using cargo expand or cargo doc you can also explore the generated code. If there's a bug in wit-bindgen
and the generated bindings do not compile or if there's an error in the
generated code (which is probably also a bug in wit-bindgen), you can use
WIT_BINDGEN_DEBUG=1 as an environment variable to help debug this.

This project can then be built with:

cargo build --target wasm32-wasip2

This creates a ./target/wasm32-wasip2/debug/my-project.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 ./target/wasm32-wasip2/debug/my-project.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++

See the wit-bindgen C and C++ Bindings Generator documentation for details.

C and C++ code can be compiled for the wasm32-wasip1 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!");

    host_print(&my_string);
}

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 C#

To generate the bindings:

wit-bindgen csharp -w command -r native-aot --generate-stub wit/

Now you create a c# project file:

dotnet new console -o MyApp
cd MyApp
dotnet new nugetconfig

In the nuget.config after <clear />make sure you have:

<add key="dotnet-experimental" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-experimental/nuget/v3/index.json" />
<add key="nuget" value="https://api.nuget.org/v3/index.json" />

In the MyApp.csproj add the following to the property group:

<RuntimeIdentifier>wasi-wasm</RuntimeIdentifier>
<UseAppHost>false</UseAppHost>
<PublishTrimmed>true</PublishTrimmed>
<InvariantGlobalization>true</InvariantGlobalization>
<SelfContained>true</SelfContained>
<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
<WASI_SDK_PATH>path/to/wasi-sdk</WASI_SDK_PATH>

Add the native-aot compiler (substitute win-x64 for linux-x64 on Linux):

dotnet add package Microsoft.DotNet.ILCompiler.LLVM --prerelease
dotnet add package runtime.win-x64.Microsoft.DotNet.ILCompiler.LLVM --prerelease

Now you can build with:

dotnet publish

Checkout out componentize-dotnet for a simplified experience.

Guest: Java

This project historically had some support for
TeaVM-WASI, but it was unmaintained for
a long time and never was at feature parity with other generators, so it was
removed. The last commit with support for TeaVM-WASI was
bytecodealliance@86e8ae2.

Guest: TinyGo

The new TinyGo WIT bindings generator is currently in development at the
go.bytecodealliance.org repository.

To install the wit-bindgen-go CLI, run:

go install go.bytecodealliance.org/cmd/wit-bindgen-go@latest

Note: it requires wasm-tools to be installed.

Then, you can generate the bindings for your project:

wit-bindgen-go generate <path-to-wit-pkg>

Guest: C++-17+

The cpp crate contains code to generate C++ code which uses the std types
optional, string, string_view, vector, expected to represent generic
WIT types.

This relies on wasi-SDK for guest compilation.

Guest: MoonBit

MoonBit can be compiled to WebAssembly using its toolchain:

moon build --target wasm # --debug to keep symbols

The generated core wasm will be found under target/wasm/release/build/gen/gen.wasm by default. Then you can use wasm-tools to componentize the module:

wasm-tools component embed wit target/wasm/release/build/gen/gen.wasm -o target/gen.wasm
wasm-tools component new target/gen.wasm -o target/gen.component.wasm

You may use --gen-dir to specify which package should be responsible for the exportation. The default is gen as mentioned above.
This can be useful having one project that exports multiple worlds.

When using wit-bindgen moonbit, you may use --derive-show or --derive-eq to derive Show or Eq traits for all types.
You may also use --derive-error, which will make types containing Error as error types in MoonBit.

You will find the files to be modified with the name **/stub.mbt.
To avoid touching the files during regeneration (including moon.pkg.json or moon.mod.json) you may use --ignore-stub.

/!\ MoonBit is still evolving, so please check out the Weekly Updates for any breaking changes or deprecations.

Guest: Other Languages

A (non-exhaustive) list of other languages known to support components are:

• JavaScript through componentize-js
• Python through componentize-py
• Go through wit-bindgen-go.

See also
The WebAssembly Component Model developer's guide
for examples of how to build components using various languages.

Other languages such as Ruby, 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 interested 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 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 jco 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.

• Ruby: the wasmtime-rb
  project has initial support for components since
  v27.

• 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.

Building and Testing

To build the cli:

cargo build

Learn more how to run the tests in the testing document.

Versioning and Releases

This repository's crates and CLI are all currently versioned at 0.X.Y where
Y is frequently 0 and X increases most of the time with publishes. This
means that changes are published as possibly-API-breaking changes as development
continues here.

Also, this repository does not currently have a strict release cadence. Releases
are done on an as-needed basis. If you'd like a release done please feel free to
reach out on Zulip, file an issue, leave a comment on a PR, or otherwise
contact a maintainer.

For maintainers, the release process looks like:

• Go to this link
• Click on "Run workflow" in the UI.
• Use the default bump argument and hit "Run workflow"
• Wait for a PR to be created by CI. You can watch the "Actions" tab for if
  things go wrong.
• When the PR opens, close it then reopen it. Don't ask questions.
• Review the PR, approve it, then queue it for merge.

That should be it, but be sure to keep an eye on CI in case anything goes wrong.

License

This project is triple licenced under the Apache 2/ Apache 2 with LLVM exceptions/ MIT licences. The reasoning for this is:

• Apache 2/ MIT is common in the rust ecosystem.
• Apache 2/ MIT is used in the rust standard library, and some of this code may be migrated there.
• Some of this code may be used in compiler output, and the Apache 2 with LLVM exceptions licence is useful for this.

For more details see

• Apache 2 Licence
• Apache 2 Licence with LLVM exceptions
• MIT Licence

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in this project by you, as defined in the Apache 2/ Apache 2 with LLVM exceptions/ MIT licenses,
shall be licensed as above, without any additional terms or conditions.