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wasm-opt for Rust

  • Team Name: Common Orbit LLC
  • Payment Address: (Ethereum DAI) 0x2de31E52E24Df0588C64B27657D4F75e5462adEf
  • Level: 2

Project Overview 📄


This project makes the wasm-opt tool more accessible to Rust programmers. wasm-opt is a WebAssembly optimizer and is required by most software toolchains that produce WebAssembly binaries.

This project is to package wasm-opt as a cargo crate, so that it can be installed by typing

cargo install wasm-opt

Additionally, it will provide a Rust API so that toolchain developers may, if they desire, call wasm-opt programmatically, so that their users are not required to manually install the tool.

wasm-opt is a part of the binaryen toolset, written in C++. It is almost universally required by all toolchains that compile to WebAssembly. It is required by Parity's cargo-contract tool for ink! development.

This tool can not be aquired in the way Rust programmers expect — with cargo install. Installation of the tool is system-dependent. Many system package managers have some version of it, in some cases an old version; or it can be downloaded in binary form for most platforms, from GitHub, in which case it has to be extracted and added to the environment PATH variable in an ad-hoc way.

In our personal experience, aquiring this tool was a minor, but needless, barrier, to programming with ink!.

In the course of this grant cargo-contract will receive pull requests to optionally enable both the following:

  1. recommend installation via cargo install wasm-opt
  2. use wasm-opt via API and not require installation.

Project Details

Note that although wasm-opt comes from the binaryen suite of tools, this project is only focused on wasm-opt. All other binaryen tools are out of scope. If this project is successful and there is demand, future work can extend the technique to the rest of binaryen.

This project is technically straightforward. The main complication is that within cargo it is not possible to install binaries that were not produced directly by the Rust compiler. This means that it is not possible to simply build wasm-opt in a build script, then have cargo install it.

This is the basic approach we will take, for the executable:

  • a wasm-opt-sys crate builds the C++ code for wasm-opt
  • the C++ source is built with only the C++ compiler all Rust users will have installed, no CMake or other build-system dependency
  • the C++ wasm-opt source is minimally-patched to export its main function with C ABI to be called from Rust
  • a wasm-opt crate contains a tiny Rust module that calls the C wasm-opt main function

We have prototyped the project sufficiently to believe the described approach will succeed.

For the library bindings:

  • wasm-opt-sys will export low-level Rust bindings with the help of one of the common C++-integration crates, likely cxx
  • wasm-opt will provide an idiomatic Rust wrapper that exposes the necessary wasm-opt options programmatically

We will also deliver the following:

  • Full README and API documentation
  • Basic regression tests for the binary and library
  • CI for the platforms
    • aarch64-apple-darwin
    • aarch64-unknown-linux-gnu
    • i686-pc-windows-msvc
    • i686-unknown-linux-gnu
    • x86_64-apple-darwin
    • x86_64-pc-windows-msvc
    • x86_64-unknown-linux-gnu
  • Pull requests adding optional support for the wasm-opt crate to
    • cargo-contract, the ink! build tool
  • One blog post about the tool and its development, at

We will not include the following wasm-opt capabilities in the library bindings:

  • Fuzzing. wasm-opt has multiple options related to fuzz testing the output module. We are aware of no potential clients for this feature. Including these fuzzing features requires code duplication in Rust, for additional maintenance burden and questionable benefit.

Ecosystem Fit

This project is immediately useful to all Rust developers that build for wasm.

It is more specifically useful to developers of Rust-based toolchains that target wasm, and most specifically the cargo-contract tool used to compile ink! programs. It is probably relevant to authors of Substrate runtimes as well, though we do not have that experience yet.

Prior Work and Alternatives

There are existing Rust bindings for binaryen. As-is they don't provide a route to installing wasm-opt via cargo. It is unclear if they provide the APIs needed to expose wasm-opt programmatically, though they probably do. These bindings appear to rely on CMake to build. We do not expect to use them directly, but may reference them during development.

The cargo-wasi tool takes a different strategy to acquiring wasm-opt. It automatically downloads the wasm-opt binary, presumably from the official releases. Other wasm toolchains like cargo-contract could follow a similar strategy, possibly by extracting the existing logic from cargo-wasi.

Team 👥

Team members


  • Registered Address: 16192 Coastal Highway, Lewes, Delaware 19958
  • Registered Legal Entity: Common Orbit LLC

Team's experience

The team lead is one of the original authors of the Rust programming language, with 12 years of Rust experience. They have performed Rust work for Mozilla, Reddit, PingCAP, Solana, MobileCoin, Parity, and Nervos. Both team members are maintainers of the Rust in Blockchain newsletter.

Team Code Repos

Team LinkedIn Profiles (if available)


Development Status 📖

We have created the initial project layout and investigated the feasibility of building the binaryen codebase using only the cc crate, as well as the feasibility of trivially calling the wasm-opt main function from Rust as described.

We have reserved the wasm-opt and wasm-opt-sys crate names on

Development Roadmap 🔩


  • Total Estimated Duration: 3-4 months
  • Full-Time Equivalent (FTE): 0.4
  • Total Costs: 30,000 USD

This will be a part time effort. I have divided this into two logical milestones, though in reality the work will overlap.

Milestone 1 — Proof of Concept

  • Estimated duration: 1-2 month
  • FTE: 0.4
  • Costs: 15,000 USD

During this phase we will prove the concept and produce a wasm-opt Rust binary and API.

0a.LicenseMIT / Apache-2.0
0b.DocumentationBasic README.
0c.Testing GuideManual smoke testing on Linux, Windows, and MacOS, x86_64 and ARM.
0d.DockerDocker is not required for this project.
1.wasm-opt binaryProduce a wasm-opt binary that can be built by cargo but is otherwise identical to stock wasm-opt.
2.APIsWrite an idiomatic, but possibly incomplete, Rust API for loading wasm, optimizing it, and writing it again.

Milestone 2 — Integration

  • Estimated duration: 1-2 month
  • FTE: 0.4
  • Costs: 15,000 USD

During this phase we will prepare the project for production and integrate it with cargo-contract.

0a.LicenseMIT / Apache-2.0
0b.DocumentationFull README and API docs.
0c.Testing GuideBasic integration tests for both binary and library.
0d.DockerDocker is not required for this project.
0e.ArticlePublish a technical blog post about how the project was developed and indicating it is available for use.
1.APIsEnsure the APIs expose all the wasm-opt options, and can be easily integrated into tools like cargo-contract.
2.cargo-contract integrationSubmit a PR to cargo-contract that integrates the wasm-opt crate.
3.CISet up CI for all indicated platforms.

Future Plans

Upon completion of this project we will pursue a maintainence grant to supply hourly funds for maintenance of this project. We expect maintenance to be minimal, primarily updating the code and making new releases to match upstream binaryen releases, and responding to issue reports.

We are interested in pursuing a pure-Rust alternative to wasm-opt with a limited focus of quickly shrinking wasm binaries.

We expect to pursue additional projects related to ink! and Substrate.

Appendix: The wasm-opt installation experience

Upon calling cargo-contract build without wasm-opt installed the build errors with this long explanation:

ERROR: wasm-opt not found! Make sure the binary is in your PATH environment.

We use this tool to optimize the size of your contract's Wasm binary.

wasm-opt is part of the binaryen package. You can find detailed
installation instructions on

There are ready-to-install packages for many platforms:
* Debian/Ubuntu: apt-get install binaryen
* Homebrew: brew install binaryen
* Arch Linux: pacman -S binaryen
* Windows: binary releases at

Despite the effort to explain how to install this tool, following the instructions on our system resulted in an old version of wasm-opt and cargo-contract produced a new error:

ERROR: Your wasm-opt version is 91, but we require a version >= 99.

If you tried installing from your system package manager the best
way forward is to download a recent binary release directly:

Make sure that the `wasm-opt` file from that release is in your `PATH`.

This actually did about the best that could be expected to help us get set up, and we did end up downloading the binary tarball, extracting it, and modifying our path. But the experience could be better.