Creating wasm-opt Rust bindings with cxx

wasm-opt is a component of the Binaryen toolkit, written in C++, that optimizes WebAssembly modules, I have recently created a wasm-opt bindings crate for Rust (with the extensive help of my partner Aimeedeer). The wasm-opt crate allows wasm-opt to be installed via cargo, and also includes an idiomatic Rust API to access wasm-opt programmatically.

This was a fun bite-sized project that involved several interesting topics: Rust FFI via the cxx crate, converting C++ abstractions to Rust abstractions, designing Rust APIs, creating tests to catch upstream changes in C++ code, creating tests to verify conformance with upstream behavior, and the experience of writing and fulfilling a grant proposal, all of which I will describe herein.

Thanks to Alon Zakai, David Tolnay, Marcin Górny, Michael Müller, and Robin Freyler for their contributions to and support of this work. Thanks to the Web3 Foundation for funding this project.

Table of Contents

Preface: Installing and using the wasm-opt crate

If you are interested in using this tool, to install wasm-opt via cargo:

cargo install wasm-opt --locked

You’ll end up with a wasm-opt binary in $CARGO_HOME/bin, and it should work exactly the same as the wasm-opt you install from any other source:

$ wasm-opt -Os infile.wasm -o outfile.wasm

To use wasm-opt as a library, follow the API docs.

Basic usage looks like

use wasm_opt::OptimizationOptions;

let infile = "hello_world.wasm";
let outfile = "hello_world_optimized.wasm";

    .run(infile, outfile)?;


We decided to build this after a recent experience with cargo-contract, the tool for building Ink! programs, in which we had to go “outside” the Rust ecosystem to find and install wasm-opt (downloading and extracting a tarball from GitHub, setting up PATH). A minor inconvenience, but as a Rust programmer working with a Rust toolset I want to cargo install whenever I can.

Many platforms that use wasm as their VM end up creating their own tools for building and packaging their wasm programs, and many of those delegate to wasm-opt to shrink their output. So making wasm-opt available as a Rust crate seemed like an obviously useful thing to do for both cargo-contract and all Rust-based wasm-targeting tools.

We proposed a W3F grant to build it, which was accepted gladly. This was a perfect subject for a grant: clear benefit, clear scope, low risk. And it worked out almost exactly as expected.

We had the opportunity to use the cxx crate, which creates safe Rust bindings to C++ code, for the first time; and had to solve a bunch of minor problems including: giving the C++ API a shape suitable for binding to Rust, dealing with the differing move semantics of C++ and Rust, working around C++ const-correctness issues, exception handling between FFI layers. We needed to make one upstream Binaryen change.

We were happy with the decision to use cxx. It worked as advertised: we didn’t have to think much about memory-safety across the FFI as cxx guided us to the safe solutions. The cxx docs are pretty great, with good API docs, and a book. The table showing the correspondence between C++ and Rust types is invaluable.

While leveraging Binaryen’s module readers / writers and optimization passes, we ended up duplicating the application-level logic of the wasm-opt program itself in Rust, as wasm-opt is a command-line program not suitable to use as a library. This duplication necessitated writing carefully chosen tests to both help ensure that the crate’s behavior matches the CLI’s, but also that as Binaryen changes in the future we notice those changes and adapt to them.

In the end we had six layers of Rust abstractions, including C++ shims for all the C++ APIs we needed, a Rust-style builder API, and a nearly-complete reimplementation of wasm-opts own CLI argument parsing on top of the builder. This feels like overkill for such a small project, but they all have a clear role in the stack, and several layers are doing simple transformations around the FFI boundary.

Prior to publication of this blog post we asked David Tolnay (dtolnay), the author of cxx to review our usage of his library. He made many insightful suggestions and contributions, some of which I’ll pass on here.

The next sections discuss our objectives at the outset of the project, then the bulk of this post is about our experience attempting to fulfill them.

Links to wasm-opt-rs point to commit bae78101. Links to Binaryen point to commit c74d5eb6, corresponding to version 110.

The plan: Our bin strategy

The intent of this project was to make wasm-opt available to Rust programmers in two ways: as a command-line program via cargo install, and as a Rust library.

When installing the CLI program the resulting binary must behave the same as the “native” Binaryen.

The obvious way to do that is to just use cargo as a frontend to the existing Binaryen build system, building wasm-opt in the cargo build script, and installing wasm-opt during cargo install.

This can’t be done quite so simply though because there are no programmable hooks into cargo install. cargo will install any Rust binaries it builds directly, but it can’t be instructed to install arbitrary additional files.

So to get cargo to install wasm-opt, we would create a Rust crate called wasm-opt whose main function did nothing but call the C++ main function.

There would be some minor wrinkles to this strategy, and it turns out that there is a much simpler way to use a C++ main function than calling it from Rust, but this is the easy part of the project.

The plan: Our cxx lib strategy

The hard part of this project would be wrapping Binaryen APIs in a Rust FFI, layering on top of that an idiomatic Rust API, capturing all the features of the underlying tool, and ensuring that they worked the same.

We proposed to use the cxx crate for the FFI. Though we had never used it, and had no sense of how it worked or would work for our purpose, it is created by dtolnay, who has designed many excellent Rust libraries, so we were enthusiastic to try it.

On top of the cxx API we would layer an idiomatic Rust API, though we did not know at the outside the form it would take.

Both the lib and bin would live in the same wasm-opt crate, a decision with tradeoffs.

Six layers of abstraction

This project ended up defining six clear layers of abstraction, which strikes me as a lot, but on examination I don’t want to get rid of any of them. Some of them are imposed by the nature of FFI, and seem worth enumerating. Most of this post will be walking through the process of building these from bottom to top.

  • The C++ shims. A tiny layer of types and methods that wrap the Binaryen types, but present an interface that is easy to call via cxx bindings.
  • The cxx declarations. The declarations used to auto-generate safe Rust types and C++ glue.
  • The base API. This layer does several bits of API cleanup so other modules don’t have to deal with FFI issues: encapsulates cxx::UniquePtr<SomeBinaryenType> in a Rust struct, uses Rust naming conventions instead of Binaryen’s C++ conventions, handles pinning as required by cxx, handles conversion of Path to platform-specific string types, etc.
  • The OptimizationOptions configuration types and the run method. This is the heart of the Rust API: create OptimizationOptions and call run. The configuration types contained in OptimizationOptions closely mirror the wasm-opt command-line options. Their definitions spill across several modules but they all are reexported at the crate root. The run method duplicates the application-level logic of the wasm-opt binary in Rust.
  • The OptimizationOptions builder methods. Overlaid onto OptimizationOptions. Most methods are obvious one-liners.
  • The Command interpreter. This constructs OptimizationOptions by interpreting the arguments to Rust’s Command type for launching processes. It’s a bit of extravagance: it essentially duplicates wasm-opt’s own command-line parser. Originally intended to make it easier for projects that already invoke the wasm-opt process to integrate the API, It ended up being invaluable for testing: we can run all three of 1) the real wasm-opt, 2) our Rust binary wasm-opt, and 3) our library; all in the same way, ensuring they all behave the identically.

Building Binaryen without cmake

We gave ourselves one extra challenge: do not use any build system external to cargo. We want to impose as few requirements on wasm-opt embedders as possible, and Binaryen is a relatively simple codebase, so we decided not to use Binaryen’s build system (CMake-based) to build Binaryen.

Instead we built Binaryen in a cargo build script using the cc crate. This build script lives in the wasm-opt-sys crate, *-sys crates being special cargo convention for managing access to native libraries.

The cc crate compiles C and C++ source files, packages them into an archive (.a) file, and emits the correct metadata to tell cargo to include the archive in the crate’s library (.rlib) file, and subsequently in the final executable. It is widely used in the Rust ecosystem and contains a great deal of platform-specific, toolchain-specific knowledge about how to drive various parts of the C/C++ toolchain. But it is mostly intended for building small bits of code to supplement Rust crates. It is not a full build system.

Writing our custom build script for Binaryen was the first step of the project. The build script ended up being more complex than I would prefer, and will present more debugging challenges when upgrading Binaryen in the future, but it was all implementable without great difficulty.

The initial task was to simply discover all the .cpp files that needed to be compiled into wasm-opt — Binaryen encompasses multiple tools and not all of them require all of the source files. We did this the brute-force way: write a build script that compiled only wasm-opt.cpp, run the build until the link step, at which point the linker would emit many errors about missing symbols; grep the source for the likely source of a single symbol; add the new source, build again and repeat until there were no more linker errors.

That strategy nearly worked completely, except for a few obstacles:

The first was an unexpected surprise: The cc crate, generally intended for building a handful of files, not entire applications, puts all of its output object files in the same directory (cargo’s OUT_DIR for that crate), regardless of the original source structure. This bit us because Binaryen has two source files called intrinsics.cpp, causing cc to want to create two object files in the same location called intrinsics.o.

We created a hacky workaround: for one of these two files we call a function, disambiguate_file, that copies a source file to a new location while giving it an unambiguous name:

    let file_intrinsics = disambiguate_file(&ir_dir.join("intrinsics.cpp"), "intrinsics-ir.cpp")?;

Like the object files, the disambiguated source file gets put in OUT_DIR:

fn disambiguate_file(input_file: &Path, new_file_name: &str) -> anyhow::Result<PathBuf> {
    let output_dir = std::env::var("OUT_DIR")?;
    let output_dir = Path::new(&output_dir);
    let output_file = output_dir.join(new_file_name);

    fs::copy(input_file, &output_file)?;


The second and third obstacle were due to preprocessing done by Binaryen’s build system. When configuring its build Binaryen creates a config.h file that contains Binaryen’s version number. This number can either come from the CMakeLists.txt configuration file, or from parsing the output of git. Since we already knew one of our prospective clients was parsing the wasm-opt version, we decided to reproduce this behavior exactly, and our build script has two functions that pull the version number from each place and put them into config.h as appropriate.

Binaryen’s build configuration also hex-encodes and embeds a binary called wasm-intrinsics.wat into its source code. So we again had to reproduce that logic.

Adapting our custom build process to Binaryen as it changes in the future will likely be the most difficult ongoing maintenance task on this project. We have already performed one upgrade, from version 109 to 110, and it involved resolving link errors and adding source files until everything linked again.

Dividing the FFI between crates

From our experience getting the wasm-opt-sys crate to build we knew that rebuilding that crate took a long time, multiple minutes on my underpowered laptop.

This is because the cc crate doesn’t support any kind of incremental recompilation: any time the wasm-opt-sys crate needs to rebuild, it compiles every C++ file in the project. The lack of incremental recompilation within cc is intentional — cc is not a full build system.

We could just use an external CMake, and we also considered adding a basic, if imperfect, caching layer on top of cc that would make development faster. But we still did not want to introduce an external build tool, and while creating that caching build system sounded fun, it seemed out of scope for our grant.

Instead we decided to create a division of responsibilities between the crates in our project that would minimize the amount of rebuilding we needed to do during development: we put essentially no Rust code in the wasm-opt-sys crate, no Rust bindings at all.

That way, once we were done figuring out how to successfully build Binaryen, we wouldn’t continually invalidate the Binaryen build while hacking on Rust code.

This is not how -sys crates typically work. They usually include the lowest level of Rust bindings. But we just used wasm-opt-sys to build and link our native code.

Instead we settled on this division of responsibilities between crates:

  • wasm-opt-sys - Just build the native library.
  • wasm-opt-cxx-sys - Create the Rust bindings to the native library via cxx. cxx emits both Rust and C++ at build time, so this crate is also performing a native build. In typical bindings this crate would be part of wasm-opt-sys.
  • wasm-opt - The library and binary, with both a and, calling Binaryen code via wasm-opt-cxx-sys.

Editing either wasm-opt-cxx-sys or wasm-opt does not invalidate wasm-opt-sys, so during development we don’t need to sit through repeated complete builds of Binaryen.

One awkward result of the division between wasm-opt-sys and wasm-opt-cxx-sys is that they both need access to the Binaryen source code, as cxx generates C++ code that accesses Binaryen headers. This complicates our deploy process slightly, and also implies that we must be careful about managing the version numbers of these two crates such that compatible versions always use compatible Binaryen source code.

We originally packaged the full Binaryen source code with both wasm-opt-sys and wasm-opt-cxx-sys, and this amounted to 72 MB of C++ source code.

After review, cxx author dtolnay clued us in that

1) We don’t need to package the 26 MB Binaryen test suite, and 2) cxx_build has a solution to sharing headers between crates

Linking to crates that contain no Rust code

The decision to put no Rust code into wasm-opt-sys led to one big oddity. And it would be difficult to understand and debug if I wasn’t previously aware of it.

After cc compiles all of its C and C++ (.c / .cpp) files into object (.o) files, it then packages them with the ar tool into an archive (.a) file, and then instructs cargo to package that archive into the .rlib file that represents the compiled Rust library for the crate being built.

Later, rustc will link all of the code inside the .rlib into the final executable.

What isn’t obvious though is that rustc doesn’t actually attempt to link every crate it is told to: it will notice when no Rust code ever calls into a crate, decide that crate is not used, and quietly not link it.

This manifests as linker errors with many missing C++ symbols.

With our decision to put the FFI bindings in a separate crate from wasm-opt-sys, rustc did not actually link to our native library.

The way to solve this is to “fool” rustc into thinking the crate in question is used.

There are multiple ways to do so, but the recommended way is to just mention it in an extern crate declaration. So for us, both the wasm-opt lib and wasm-opt-cxx-sys crates contain

extern crate wasm_opt_sys;

And otherwise do nothing with the crate.

This is also useful when activating the unused_crate_dependencies lint, to tell the compiler about a crate that is only used in some configurations (e.g. Windows-only).

I have also seen crates use unnamed imports for this:

use wasm_opt_sys as _;

It’s not clear to me if there is any meaningful difference between these two patterns.

Creating a Rust bin with a non-Rust main function

With a working build of all the source needed by wasm-opt, we needed to create a file that would delegate to the C++ main function.

Firstly, I’ll describe the easy, and arguably right, way to do it. Then I’ll describe how we did it!

The easy way is to have cc compile the source containing the main function just as you would any other source; then create a that contains

// Use a foreign main function.

// Make sure to link to it.
extern crate wasm_opt_sys;

That’s it. For just running a foreign main you don’t need anything else. The Rust compiler won’t look for a Rust main function, and it won’t emit any startup code. It’ll just assume you have set everything up correctly.

We did not do that. We did not think to do that, but if we had it would have influenced our initial design. Instead we wrote a Rust main function that called the C++ main function, and the final design ended up requiring us to do this unless we split the wasm-opt library and binary into two different crates.

Anyway, what we did:

We had to write our Rust file and call the C++ main function. For this we did not use cxx as the FFI was easy to write by hand (and cxx wouldn’t have helped us much with the argc and argv parameters anyway). The wasm-opt crate’s calls the FFI directly, bypassing the cxx layer and the wasm-opt-cxx-sys crate.

This is mostly straightforward, except that we can’t simply call the existing main function from Rust, for at least one reason, possibly several.

wasm-opt’s main function is declared as:

int main(int argc, const char* argv[]) {

This is a C++ function, and Rust can only call C functions.

To declare it a C function it needs to be:

extern "C" int main(int argc, const char* argv[]) {

At the least this will disable name mangling for the main function, so that the symbol main can be linked. I am not clear on whether it has any impact on the calling convention of the function.

The above about name mangling is true generally, but may not be true for a function named main. In brief experiments, even without declaring main as extern "C", I did see that the resulting object file contained an unmangled function named main, that appeared to be this function. This is on linux with gcc. So maybe gcc doesn’t name-mangle the main function.

I don’t know how MSVC treats main on Windows.

But it’s probably best to declare this function we want to call extern "C".

Regardless, I need this function to be extern "C" because I need to rename it anyway: rustc also wants to create an unmangled function named main, so I have to rename this one to something else. (The main function emitted by rustc is not the main function you write though — it is a synthetic function generated by the compiler that calls the standard library, which later calls your name-mangled main function).

I want this main to be called wasm_opt_main and declared like:

extern "C" int wasm_opt_main(int argc, const char* argv[]) {

So that Rust code can declare it as an extern like

    extern "C" {
        pub fn wasm_opt_main(argc: c_int, argv: *const *const c_char) -> c_int;

As part of our build script we wrote a function that reads the wasm-opt.cpp source file, replaces int main with extern "C" int wasm_opt_main, then outputs the modified source to OUT_DIR. We then build our modified wasm-opt.cpp.

The full source of the Rust is simple, though ugly, just a bunch of raw FFI. It is small and interesting enough that I’ll just list it all here for commentary:

// Establish linking with wasm_opt_sys, which contains no Rust code.
extern crate wasm_opt_sys;

mod c {
    use libc::{c_char, c_int};

    extern "C" {
        pub fn wasm_opt_main(argc: c_int, argv: *const *const c_char) -> c_int;

fn main() -> anyhow::Result<()> {
    use libc::{c_char, c_int};
    use std::ffi::OsString;
    use std::os::unix::ffi::OsStrExt;

    let args: Vec<OsString> = std::env::args_os().collect();

    let c_args: Result<Vec<std::ffi::CString>, _> = args
        .map(|s| std::ffi::CString::new(s.as_bytes()))

    let c_args: Result<Vec<std::ffi::CString>, _> = args
        .map(|s| std::ffi::CString::new(s.to_str().expect("utf8").as_bytes()))

    let c_args = c_args?;
    let c_ptrs: Vec<*const c_char> = c_args.iter().map(|s| s.as_ptr() as *const c_char).collect();

    let argc = c_ptrs.len() as c_int;
    let argv = c_ptrs.as_ptr();

    let c_return;
    unsafe {
        c_return = c::wasm_opt_main(argc, argv);



The main Rust function’s only responsibilities are to translate Rust’s command-line arguments to C-compatible command-line arguments, call the C++ wasm_opt_main, and report its error code.

Its basic strategy is to collect the arguments into a Vec<OsString>, convert those into a Vec of the type the C++ code wants, then collect another Vec of pointers to those, then pass the length and pointer to that Vec’s buffer to the C++ wasm_opt_main.

The string handling is a mess here because of the differences between strings on Unix and Windows, and it is still wrong: this code is passing a buffer of bytes to wasm_opt_main on Windows, and that is what the Binaryen code is asking for, but Binaryen is then not treating those bytes as Unicode, at least when interpreting them as paths.

The result of this is that paths with extended Unicode characters do not work with wasm-opt (either the bindings or the original CLI program) on Windows.

What probably (based on my brief research of modern C++) should happen here is that on Windows, the Rust code should be calling OsStrExt::encode_wide to get (potentially ill-formed) UTF-16, and passing that to Binaryen; Binaryen then would need to be adapted to accept and process “wide” (16-bit) chars on Windows, perhaps leveraging the C++ std::filesystem::path type. We left this large upstream task to future work.

The two calls to drop in this function are functionally useless; they are just a written reminder that we want to lose access to the raw pointers before losing access to the things they point to.

cxx and Binaryen

We dedicated the wasm-opt-cxx-sys crate to creating bindings with cxx. We were happy with the decision to use cxx. It worked as advertised: we didn’t have to think much about memory-safety across the FFI as cxx guided us to the safe solutions.

The cxx docs are pretty great, with good API docs, and a book. The table showing the correspondence between C++ and Rust types is invaluable.

We used cxx version 1.0.79. There may be new versions by now with new features.

The purpose of cxx is to make it possible to communicate between C++ and Rust using safe Rust. As such, cxx is very opinionated about what the code on both sides of the FFI look like, and supports a relatively small set of types.

This is great for projects that are in control of both the C++ and Rust APIs, and it probably will help the C++ side be disciplined about ownership and const-correctness.

Binaryen was not designed for compatibility with with cxx, so we ended up creating a full C++ shim layer to adapt between Binaryen’s APIs and APIs that were suitable for binding through cxx. Fortunately Binaryen’s API surface is reasonable and modern and easy to understand, so our shims are simple.

cxx is capable of creating bindings in both directions of the FFI: to let Rust call C++, but also to let C++ call Rust. The former are done in macros containing extern "C++" blocks; the latter extern "Rust" blocks. We only used the former, calling from Rust to C++.

The next sections discuss our experience creating cxx bindings, some of the obstacles we encountered, and what our C++ shim layer looks like.

Defining cxx bindings

In cxx bindings are defined in a dedicated module annotated with #[cxx::bridge]. Within that module are any number of extern "C++" (or extern "Rust") blocks.

The pattern we followed used many unsafe extern "C++" blocks, one for each C++ class we wanted bindings for. Each of those blocks then declared a single type, and one or more constructor functions and methods.

Here our our bindings’ declarations for Binaryen’s ModuleReader:

#[cxx::bridge(namespace = "wasm_shims")]
pub mod wasm {
    unsafe extern "C++" {
        type ModuleReader;

        fn newModuleReader() -> UniquePtr<ModuleReader>;

        fn setDebugInfo(self: Pin<&mut Self>, debug: bool);

        fn setDwarf(self: Pin<&mut Self>, dwarf: bool);

        fn readText(
            self: Pin<&mut Self>,
            filename: &CxxString,
            wasm: Pin<&mut Module>,
        ) -> Result<()>;

        fn readBinary(
            self: Pin<&mut Self>,
            filename: &CxxString,
            wasm: Pin<&mut Module>,
            sourceMapFilename: &CxxString,
        ) -> Result<()>;

        fn read(
            self: Pin<&mut Self>,
            filename: &CxxString,
            wasm: Pin<&mut Module>,
            sourceMapFilename: &CxxString,
        ) -> Result<()>;

This example includes the surrounding module, but omits many other extern "C++" blocks within that module.

This is a DSL interpreted by a proc macro. It looks a lot like Rust, but it is not quite Rust, and using cxx means learning what its syntax means.

Some things to notice here:

  • The unsafe keyword indicates that we have thought real hard about these functions and determine that calling them from Rust will preserve Rust’s memory safety promises; callers don’t need to use unsafe to call them. If we could not guarantee memory safety, then we could leave off unsafe, and it would be up to the caller to call these bindings safely within an unsafe block.
  • The unsafe keyword here is a declaration that using these bindings is safe. Think of it the same as the use of unsafe around an expression. It is possible to write cxx bindings that propagate unsafety as well by omitting unsafe.
  • The naming conventions are a mashup of Rust and C++: function names necessarily come from C++; the UniquePtr type is the Rust wrapper for the C++ std::unique_ptr type.
  • newModuleReader is a free function. It is not a C++ constructor. It doesn’t appear that cxx handles C++ constructors directly, so the C++ side must define extra constructor functions.
  • Non-primitive types need to be passed as pointers, mostly UniquePtr or references.
  • Functions with an initial argument named self, and with the Self type, are interpreted as methods of the single type declared in the block. With this interpretation of Self it is cleanest to put each type in its own extern block, though it is possible to put all types into a single extern block by naming the self-type explicitly (e.g. Pin<&mut ModuleReader>).
  • The Result type is a typedef of std::Result where the error type is cxx::Exception. cxx will by default catch at the boundary any exception that implements std::exception and return it as an error, and we further augment this to catch some Binaryen exceptions that do not implement std::exception.
  • &CxxString is a const reference to a C++ std::string.

The final thing to note is that the self type of these methods is Pin<&mut Self>. What this means is that the underlying method is non-const: it is declared such that it may mutate its fields.

Access to mutable C++ types from Rust is always through Pin, to prevent the ability to move their underlying value. Move semantics in Rust and C++ are different.

Binaryen’s methods are not const-correct”: they are all declared non-const, even when they do no mutation. This could be worth fixing in Binaryen, but for our purposes we could easily hide this in higher-layers of our API. Our next layer up, the base API, hides all the interaction with Pin, though it still exposes incorrect mutability, and high-still layers of API hide the base API from users, exposing only APIs with correct mut declarations.

Papering over the missing const-correctness was easy in our case, but more complex or stateful APIs would cause bigger problems that might require fixing the underlying declarations.

We haven’t tried it yet, but cxx author dtolnay suggests a preferred shim pattern for dealing with non-const-correct methods, described later in this post.

Naming these types correctly is not exactly easy, especially the difficult-to-understand Pin type, but cxx gives a lot of help, and in our experience did not let us write a bindings declaration that differed from the actual C++ declaration.

When you define a Rust binding to C++ code, cxx will emit static checks that the types of the Rust declarations match the types of the C++ declarations. This is a great feature, and gives me confidence about the maintainability of the bindings.

The errors that are emitted when there is a declaration mismatch are emitted by the C++ compiler, and are quite challenging to understand, but at least they stop you from running the code with mismatched types.

Here’s an example of the error emitted when the above read method is declared to take an incorrect &Self self-parameter (slightly reformatted for slightly-better readability):

  In function ‘rust::cxxbridge1::{anonymous}::repr::PtrLen wasm_shims::wasm_shims$cxxbridge1$ModuleReader$read(const ModuleReader&, const string&, wasm_shims::Module&, const string&)’:

  error: cannot convert ‘void (wasm_shims::ModuleReader::*)(const string&, wasm_shims::Module&, const string&)’ {aka ‘void (wasm_shims::ModuleReader::*)(const std::__cxx11::basic_string<char>&, wasm::Module&, const std::__cxx11::basic_string<char>&)’} to ‘void (wasm_shims::ModuleReader::*)(const string&, wasm_shims::Module&, const string&) const’ {aka ‘void (wasm_shims::ModuleReader::*)(const std::__cxx11::basic_string<char>&, wasm::Module&, const std::__cxx11::basic_string<char>&) const’} in initialization

  cargo:warning=  145 |   void (::wasm_shims::ModuleReader::*read$)(const ::std::string &, ::wasm_shims::Module &, const ::std::string &) const = &::wasm_shims::ModuleReader::read;

I was able to figure these out at least. Mostly not from the text of the errors, but just by thinking about what I might have done wrong. Making only small incremental changes before recompiling kept the errors from getting overwhelming.

Our C++ shim layer

To give the Binaryen C++ API a shape that fit the cxx model, we created a “shim” C++ layer the lightly wraps everything we want to call from Rust. These shims aren’t strictly necessary in all instances, but following a consistent pattern is valuable for maintainability, so all our library calls go through shims.h, which lives in the wasm-opt-cxx-sys crate with the cxx bindings.

The structure of shims.h mostly mirrors the structure of our cxx bindings, with the major exception that C++ requires items that reference each other to be ordered such that items being referred to lexically proceed items doing the referring, so some of our C++ shims are ordered differently than our Rust declarations.

Binaryen mostly puts its definitions in the C++ wasm namespace, and we put our shims in their own wasm_shims namespace.

Like the previously-described cxx bindings, our shims are organized such that within a single block we define a C++ struct containing an inner Binaryen type, methods on that struct presenting an interface cxx can work with, and which transform their arguments to the arguments expected by the underlying Binaryen APIs, and free-standing constructor functions as required by cxx (since cxx can’t call constructors).

Here is the shim declaration that corresponds to the previous ModuleReader cxx bindings:

namespace wasm_shims {
  struct ModuleReader {
    wasm::ModuleReader inner;

    void setDebugInfo(bool debug) {

    void setDwarf(bool dwarf) {

    void readText(const std::string& filename, Module& wasm) {
      inner.readText(std::string(filename), wasm);

    void readBinary(const std::string& filename,
                    Module& wasm,
                    const std::string& sourceMapFilename) {

    void read(const std::string& filename,
              Module& wasm,
              const std::string& sourceMapFilename) {,

  std::unique_ptr<ModuleReader> newModuleReader() {
    return std::make_unique<ModuleReader>();

Some things to notice about these shims:

  • Many of the Binaryen APIs take std::string by value. It is not though possible to pass std::string by value across the FFI boundary. These shims instead accept a const reference to std::string, then make a full copy of the string to pass to the inner method. For our purposes this is fine, others might want to avoid the copy, the pattern for which is described in the next section.
  • There is no exception handling. cxx does that for us.
  • newModuleReader is a free function that constructs a std::unique_ptr by deferring to std::make_unique, which eventually calls the actual constructor.

Moving owned strings across the FFI with cxx

Above I said that it isn’t possible to pass std::string by value from Rust to C++, and that because of this our bindings pass a reference and then make a copy.

But even though it isn’t possible to pass std::string by value, it is possible to move them across the FFI.

We didn’t do this because by the time we learned about it, we had already published the crate and didn’t want to break compatibility for it.

Understanding how to move values from Rust across the FFI to C++ requires a basic understanding of the move semantics of both Rust and C++, and the semantics of Rust’s Pin, and I don’t fully understand all three of these, but I’ll show how it’s done anyway.

Recall our C++ readText method:

    void readText(const std::string& filename, Module& wasm) {
      inner.readText(std::string(filename), wasm);

And its corresponding cxx binding:

        fn readText(
            self: Pin<&mut Self>,
            filename: &CxxString,
            wasm: Pin<&mut Module>,
        ) -> Result<()>;

This is saying that Rust is letting C++ look at the string filename, but it can’t mutate it, and it can’t move it, because it is a const reference.

In C++, unlike Rust, moves happen through non-const references, so if we instead define readText without the const, we can also call std::move on filename.

    void readText(std::string& filename, Module& wasm) {
      inner.readText(std::move(filename), wasm);

C++ move semantics are very different from Rust. Moving in C++ invokes a move constructor, something that Rust doesn’t know how to do. When an object is moved in C++, the original object still exists and is still accessible, and is still in some kind of valid state, but everything in that object has been transferred into the receiving object.

Coming from Rust it seems weird and error-prone.

The corresponding Rust bindings are now

        fn readText(
            self: Pin<&mut Self>,
            filename: Pin<& mut CxxString>,
            wasm: Pin<&mut Module>,
        ) -> Result<()>;

This means that Rust is allowing the string to be mutated from C++, and so it can call the move constructor.

It suggests that there must be some cleverness going on in cxx to make this compatible with Rust. If C++ moves the value out, mutating the string, what is left of the string when the call to readText returns?

I tested this by adjusting our call to readText:

    pub fn read_text(&mut self, path: &Path, wasm: &mut Module) -> Result<(), cxx::Exception> {
        let path = convert_path_to_u8(path)?;
        let_cxx_string!(path = path);

        println!("before: {}", path);

        let this = self.0.pin_mut();
        this.readText(path, wasm.0.pin_mut())?

        println!("after: {}", path);


let_cxx_string! is a cxx macro for creating C++ compatible strings. It returns a Pin<&mut CxxString>.

Trying to run this fails:

error[E0382]: borrow of moved value: `path`
  --> components/wasm-opt/src/
42 |         let_cxx_string!(path = path);
   |         ---------------------------- move occurs because `path` has type `Pin<&mut CxxString>`, which does not implement the `Copy` trait
47 |         this.readText2(path, wasm.0.pin_mut())?;
   |                        ---- value moved here
48 |
49 |         println!("after: {}", path);
   |                               ^^^^ value borrowed here after move
   = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0382`.
error: could not compile `wasm-opt` due to previous error

It’s obvious in retrospect: we have a Pin<&mut CxxString>, we pass it by value to the readText method, so it can pass it by mutable reference to C++; and now the Rust side of the FFI no longer has access to that Pin<&mut CxxString>.

So whatever C++ did to move the string, it is unobservable to Rust.

I was expecting we would be stuck with an error-prone empty string, but cxx protected us from that.

Pretty clever.

A better shim pattern for non-const methods

I mentioned previously that Binaryen’s methods are not const-correct: it has methods that do not mutate the receiver (the this pointer), but are also not declared const.

Taking again our readText example, from our shims:

    void readText(const std::string& filename, Module& wasm) {
      inner.readText(std::string(filename), wasm);

The inner method is a method on ModuleReader that does not mutate ModuleReader, and this is transitively true for the shim method shown.

To make this const-correct we want it to be declared:

    void readText(const std::string& filename, Module& wasm) const {
      inner.readText(std::string(filename), wasm);

Note the const on the right side of the method signature.

In the absence of fixing the underlying API, Rust bindings need to work around this problem to avoid making the Rust APIs incorrectly mutable.

Within wasm-opt this was easy because we don’t expose any of the underlying Binaryen types, even indirectly, to the user-facing API: users call OptimizationOptions::run and that method does all the interaction with the C++ code.

dtolnay suggested handling the mutability conversion within the C++ shim layer, by relying on std::unique_ptr’s operator ->(), which produces a non-const reference.

Adjusting our readText bindings to do this looks like

  void ModuleReader_readText(
    const std::unique_ptr<ModuleReader> &reader,
    const std::string& filename,
    Module& wasm
  ) {
    reader->inner.readText(std::string(filename), wasm);

And Rust:

        fn ModuleReader_readText(
            reader: &UniquePtr<ModuleReader>,
            filename: &CxxString,
            wasm: Pin<&mut Module>,
        ) -> Result<()>;

Note that this is now a free function and not a method (hence the ugly naming choice).

This binding can now be used from Rust without exposing unneeded mutable references.

I don’t fully understand the consequences of this pattern. To my eyes it looks scary: on the Rust side &UniquePtr is promising no mutation, on the C++ side we’re explicitly taking a mutable reference, but we also know that actually C++ is not going to mutate. Or at least we think we know that — we have no guarantees.

This looks like a situation where you need to be familiar with the C++ code and confident there is no mutation.

This is why cxx makes you put unsafe annotations around its safe bindings. As much as cxx helps, you still need to understand what is happening on the C++ side.

Lifetimes in cxx

cxx is also able to express methods that return types containing lifetimes, adding extra safety that the original C++ types can’t express.

In our case, the Binaryen PassRunner — the type responsible for running optimization passes to transform a wasm Module — holds a pointer to a Module and mutates the value it points to.

In the C++ code this is expressed with raw pointers. Here are the shims:

namespace wasm_shims {
  struct PassRunner {
    wasm::PassRunner inner;

    PassRunner(Module* wasm) : inner(wasm::PassRunner(wasm)) {}


    void run() {;

  std::unique_ptr<PassRunner> newPassRunner(Module& wasm) {
    return std::make_unique<PassRunner>(&wasm);

In the Rust bindings we add some lifetime annotations, and this ensures that no other code can touch the contained Module as long as the PassRunner is live:

    unsafe extern "C++" {
        type PassRunner<'wasm>;

        fn newPassRunner<'wasm>(wasm: Pin<&'wasm mut Module>) -> UniquePtr<PassRunner<'wasm>>;


        fn run(self: Pin<&mut Self>);

(Custom) exception handling with cxx

Any FFI bindings to C++ have to consider what happens if the C++ throws an exception: it is undefined behavior to unwind the stack from C++ into Rust.

The cxx crate provides a lot of help with this by automatically converting a C++ std::exception to a Rust cxx::Exception. All that one needs to do is declare a C++ function as returning Result<T, cxx::Exception>. And cxx bridge modules automatically import a type alias that makes Result<T> a Result<T, cxx::Exception>.

For example, the Binaryen ModuleReader has a readText method that might throw, and we declare it as:

    unsafe extern "C++" {
        type ModuleReader;

        fn readText(
            self: Pin<&mut Self>,
            filename: &CxxString,
            wasm: Pin<&mut Module>,
        ) -> Result<()>;

In C++ readText has a void return type, but by declaring it as Result<()>, cxx will generate bindings that catch any std::exception and convert it to a Rust error.

This is awesome … if your exceptions inherit from std::exception. If not then the process will probably abort if the exception is raised.

Binaryen has two custom exception types that readText might throw, ParseException and MapParseException, but that do not inherit from std::exception.

cxx includes a mechanism for customizing which exceptions are caught by its bindings. It requires defining a template function, rust::behavior::trycatch, and this is what the default implementation looks like:

namespace rust::behavior {
  template <typename Try, typename Fail>
  static void trycatch(Try &&func, Fail &&fail) noexcept try {
  } catch (const std::exception &e) {

Exactly how this works I can only guess. My C++ knowledge is not advanced enough (why are Try and Fail double-indirected?!).

But here is our implementation (that cxx author dtolnay wrote for us!):

namespace rust::behavior {
  template <typename Try, typename Fail>
  static void trycatch(Try&& func, Fail&& fail) noexcept try {
  } catch (const std::exception& e) {
  } catch (const wasm::ParseException& e) {
    std::ostringstream buf;
  } catch (const wasm::MapParseException& e) {
    std::ostringstream buf;

It just adds some extra arms for the Binaryen exception types, pulls out an explanatory string, and calls fail. (The Colors::setEnabled(false) calls are telling Binaryen not to emit ANSI terminal color escape sequences).

Sharing C++ headers between crates with cxx_build

We split our native build across two crates, wasm-opt-sys, and wasm-opt-cxx-sys, with the Binaryen build being performed in the former, and the cxx bindings in the latter.

This arrangement requires wasm-opt-sys to access all of the Binaryen .cpp and .h files, and wasm-opt-cxx-sys to access all of the .h files. Our initial solution to this was to simply package the entire Binaryen source code twice, as part of each package.

But cxx has a solution to this problem in the cxx_build crate.

The cxx_build crate adds a layer atop the cc crate: when calling cxx_build::bridge, cxx does all of its code generation, creating a bunch of files in $TARGET/cxxbridge, then it returns a regular cc::Build.

One of the things this code generation step can do is export C++ header paths from one crate to another. So our wasm-opt-sys build script can tell wasm-opt-cxx-sys the set of directories that contain Binaryen C++ headers that will be needed for the bindings.

To do this, we push include directories onto the exported_header_dirs Vec on the global CFG value.

Here’s approximately how it looks in the wasm-opt-sys build script:

    // Set up cxx's include path so that wasm-opt-cxx-sys's C++ header can
    // include from these same dirs.

    let mut builder = cxx_build::bridge("src/");



All the header files in src_dir, tools_dir, and output_dir are then available to instances of cxx_build in the wasm-opt-cxx-sys build script. The wasm-opt-cxx-sys crate doesn’t need to do anything further in its build script:

    let mut builder = cxx_build::bridge("src/");


I do not know what underlying mechanism cxx_build is using to make this possible, though I gave it a quick look. It’s pretty magical!

A Rusty API

We went into the project without a preconception of what the Rust API would be.

While creating the bindings we quickly recognized that the Binaryen API was not quite suitable for presenting to Rust wasm-opt users as-is. The API is clean, but doesn’t translate directly to idiomatic Rust, particularly with all its methods being mutable, and requiring a fair bit of boilerplate to set up in the way wasm-opt does.

We decided to hide the direct API from callers, and add a builder-style API on top.

With the builder, one sets up a declarative configuration, then calls a single run method that performs the work of reading the module, setting up the PassRunner, running the passes, and writing the module back to disk.

Configuring the builder is like passing the command line arguments to wasm-opt, and the run method contains essentially the same logic as the wasm-opt binary. The details of how to drive the underlying Binaryen APIs are hidden:

use wasm_opt::OptimizationOptions;

let infile = "hello_world.wasm";
let outfile = "hello_world_optimized.wasm";

    .run(infile, outfile)?;

The run method is essentially reproducing the logic of Binaryen’s wasm-opt.cpp. This was necessary because, while the core optimization functionality of Binaryen is factored into reusable types and optimization passes, the various drivers of those passes, of which wasm-opt is one, are written as CLI applications, and not suitable for reuse as libraries.

Duplicating so much logic in Rust necessitated more testing than we originally anticipated.

We factored the OptimizationOptions type across multiple modules for organizational purposes but reexported everything at the crate root. Rust allows a lot of organizational flexibility, and it is fun playing with new patterns.

Here’s how our modules are organized, as declared in

// Most of the API surface is exported here.
// Many public methods are defined in other non-pub modules.
pub use api::*;

// Returned by the `run` method.
pub use run::OptimizationError;

// The "base" API.
// This API hides the `cxx` types,
// but otherwise sticks closely to the Binaryen API.
// This is hidden because we don't need to commit to these low-level APIs,
// but want to keep testing them from the `tests` folder.
pub mod base;

// Types and constructors used in the API.
mod api;

// A builder interface for `OptimizationOptions`.
mod builder;

// The list of optimization passes.
mod passes;

// Definitions of -O1, -O2, etc.
mod profiles;

// The list of wasm features.
mod features;

// The `run` method that re-implements the logic from `wasm-opt.cpp`
// on top of `OptimizationOptions`.
mod run;

Toolchain integration considerations and a Command-based API

The builder might have been the final API, but as we started looking at integrating the wasm-opt crate into cargo-contract, and thinking about how to make the decision as easy as possible for cargo-contract’s maintainers, we came up with new considerations.

We were particularly concerned that, given these bindings were new and untested and likely to contain unknown bugs, and that the crate imposed new compile-time requirements (a C++17 compiler), client crates might need to quickly backtrack on their adoption of the crate, at least for a period of time while bugs are resolved.

So we gave ourselves some additional requirements:

  • It should be easy to use either the wasm-opt binary or the API.
  • Switching between the two should be doable at either compile-time or runtime.
  • It should be easy to completely revert the code that integrates the wasm-opt crate.

We did a survey of a few projects that use wasm-opt, and they all launched it via the standard Command API, so it was obvious that the way to make our crate most compatible with existing code was to be compatible with Command. This would mean parsing command-line arguments the way wasm-opt does. We also discovered that some prospective clients allowed passing arbitrary arguments to wasm-opt, which would mean parsing all command-line arguments the way wasm-opt does.

So we added a CLI-style argument parser, that accepted an existing Command, parsed its arguments, and called all the appropriate builder methods.

This felt a little “wrong” — now we were rewriting an ever larger amount of Binaryen code.

We documented it as “best-effort”, providing parsing necessary for integration, but not necessarily with full fidelity to whatever wasm-opt actually does.

With enough of the CLI parser implemented for cargo-contract’s needs, we produced a patch series against cargo-contract for discussion that was super-clean, showing how to progressively go through all of the options for integration:

  • The first patch just added a help suggestion to cargo install wasm-opt --locked.
  • The second patch used the Command API to allow the library and binary to coexist.
  • The third patch removed usage of the Command API completely and used just the builder API.

In the end the cargo-contract developers accepted the full patch series, not retaining any compatibility with the binary, and relying entirely on the library. So we didn’t actually need this big chunk of compatibility code for our main client.

But that was fine, because the most valuable purpose of this CLI parser ended up being testing.

Testing for maintainability

We entered this project expecting to make a thin layer of bindings atop Binaryen. We expected this to need minimal testing, and we only promised to deliver “smoke tests”. But what we actually did was use Binaryen’s ModuleReader and ModuleWriter plus its PassRunner to completely reimplement the logic of wasm-opt, as defined in wasm-opt.cpp, along with several data structures.

The Binaryen pieces we completely reimplemented, and their Rust counterparts:

C++ Rust
wasm-opt.cpp OptimizationOptions::run
InliningOptions InliningOptions
PassOptions PassOptions
pass name strings Pass enum
Feature enum Feature enum

With all these reimplementations, we must:

1) ensure they are identical to the original definitions, 2) continue to be identical as Binaryen changes in the future.

So we asked ourselves questions like:

  • “How can we guarantee the default value the Rust struct is the same as the C++ struct?”
  • “How can we detect if the field of a redefined C++ struct is added or removed?”
  • “How can we guarantee that our Pass enum variants include all C++ pass name strings?”
  • “How can we guarantee that our redefined C++ enum variants have the same numerical values as C++?”
  • “How can we test that our Rust APIs behave the same as the real wasm-opt CLI?”

And for each of these types of questions we figured out how to write a test case. Here are a few examples:

Checking that Rust enum variants match a set of C++ strings

In Binaryen, passes are represented as string names. In Rust we made them variants of the Pass enum:

pub enum Pass {
    /// Lower unaligned loads and stores to smaller aligned ones.
    /// Async/await style transform, allowing pausing and resuming.

Binaryen maintains a global registry of passes that is statically initialized, and has a function, PassRegistry::getRegisteredNames that returns the name of every pass.

If we can call that function from Rust, and if we can iterate over the variants of Pass, and convert those passes to a string, then we can create a set of all the C++ pass names, and all the Rust pass names.

So we did that.

Checking that a Rust struct matches a C++ struct definition

Binaryen’s InliningOptions is a simple plain-old-data struct, which we duplicated in Rust. We also duplicate the initialization of this struct in Rust, as an implementation of Default. We wanted to be sure that our default implementation matched Binaryen’s, and that if any fields were added or removed from this struct in the future we would know about them.

To do this we construct the Rust version of InliningOptions, and pass that to a hand-written C++ function, checkInliningOptionsDefaults, that constructs the C++ type and compares all the fields.

It also checks that the C++ type has a known hard-coded size: if that size ever changes, it probably indicates a field has changed.

Ensuring that the Rust API’s run method behaves like wasm-opt

The most important testing we did was to esure that our API behaved the same as Binaryen’s CLI.

We leveraged our Command-based API for this to create a set of confarmance tests that:

  • build the Binaryen wasm-opt
  • build the Rust wasm-opt

then, the test suite creates command line arguments for exercising wasm-opt under various scenarios, under both binaries and the API, and checks that for all three implementations:

  • wasm-opt either succeeded or failed
  • the output files were exactly the same

These tests caught many bugs in our implementation.

Outcome and future plans

This was a great bite-sized project: perfectly scoped to complete quickly and successfully, with many clear and distinct tasks to split between myself and my partner. Most hacking isn’t so clearcut, so I am grateful we found this one.

It has already been integrated into the master branch of cargo-contract, and somebody other than us took the initiative to integrate it into Substrate’s wasm-builder.

The final crate has a few caveats for prospective integrators to consider:

  • The wasm-opt-sys crate takes a non-negligible amount of time to build. It also does not do any incremental recompilation, so if the build is invalidated it will rebuild the C++ code from scratch. The lack of incremental recompilation is a limitation self-imposed by not using cmake or other external build system.
  • wasm-opt on Windows does not support extended Unicode paths (probably anything non-ASCII). This is a limitation of binaryen and not a regression of the bindings. It may or may not be fixed in the future. The APIs will return an error if this occurs.
  • cargo tarpaulin (code coverage) segfaults running any wasm-opt crates, reason unknown. This behavior could infect other crates that link to wasm-opt. If you use tarpaulin, you might verify it continues to work.

These are mentioned in the project readme.

This crate will need light maintenance over time to keep up with Binaryen releases, which happen a few times a year. The Web3 Foundation, which funded this project, has another process for “maintenance grants”, and we plan to apply for one. As part of that maintenance we would also upgrade crate dependencies, and submit wasm-opt updates to projects the W3F cares about, including Ink!’s cargo-contract and Substrate’s wasm-builder. Assuming that is accepted, prospective integrators can be confident this crate will be maintained in the future.

We may also propose fixing Binaryen’s Unicode support on Windows, if Binaryen maintainers want that. It would probably be a bunch of churn for Binaryen, but I am not too comfortable having a Rust crate that breaks in surprising ways on Windows.

We are also interested in writing a pure-Rust wasm minifier, with much reduced scope compared to wasm-opt. Being pure Rust would fix all the aforementioned caveats about this crate as it stands today, and there may be techniques Binaryen doesn’t implement that we could pursue. Also, it would just be fun to write. But it’s a big project, and justifying it in a grant is harder than this project.

Appendix: The W3F grant experience

This project was funded by a grant from the Web3 Foundation. We are thankful for the support of the organization and the individuals responsible for helping us secure the grant.

The process for this particular grant went as smoothly as possible, for a number of reasons. I would not expect most grants to be as easy, but still I thought I would outline what we did for the sake of other prospective proposal writers.

Some factors that made this project successful include:

  • The scope of this project and what success would look like was well-defined, and small: bind wasm-opt; use it in cargo-contract.
  • The path to implementing the project technically had a few risks and unknowns.
  • The cost was modest: 30,000 USD. More about the cost below.

It also helps that I have had professional contact for years with both Parity and W3F employees: networking and brand-building have a compounding beneficial effect over one’s career. I am grateful to know many people in the industry, and that many of them remain willing to work with me.

In 2020 I wrote a series of blog posts in which I explored Ink!, the Rust DSL for smart contracts on Substrate. I had some chats with one of the Ink! maintainers at the time. In 2021 I wrote another. So the Ink! maintainers were aware of me and my interest in their project. This year (2022) I participated in, but did not complete, a Substrate hackathon. I intended to blog about it again, but did not achieve enough for it to be worthwhile.

I did note this time though that the experience of running cargo-contract, Ink!’s build tool, was not as smooth as I would expect, one issue being that wasm-opt was not trivial to install. A solution to this little inconvenience was obvious to me, and it seemed a great candidate for a grant.

So I pinged one of the Ink! maintainers whom by now I already knew and told them about my experience, and my idea for creating wasm-opt Rust bindings. They agreed with the idea and indicated that it would be an easy grant to approve; and though the Ink! maintainers are not the ones responsible for approving a grant, it surely can only help to have the support of the maintainers of the project one is looking to improve when seeking funding for that project.

Since this project was to create a binding to the 3rd-party Binaryen project, I also pinged the author of that project, who I also knew previously. I was looking to see whether they approved or disapproved of the idea (they liked it), whether they knew of any obvious obstacles, and whether there were any prior efforts to bind Binaryen that I was not considering (there was).

When building consensus for a proposal of any kind, having a sense ahead of time how that proposal is likely to proceed is helpful. Maintainers don’t generally like to be surprised with big changes, even if the ideas are great. In this case I had been laying the groundwork to make a good proposal of this nature for years, mostly by exploring my own interests and meeting people.

Proposing a W3F grant requires filling out a template and submitting a pull request. The process is completely open on GitHub. For an open source hacker, this is awesome: I love working on GitHub; I love working in the open.

Our proposal was pretty simple. I tried to make the deliverables precise and measurable.

W3F grants are based on deliverables at milestones, each milestone receiving an agreed payout if the deliverables are completed as specified in the approved proposal. The W3F has three funding tiers that require progressively more approvals. The first tier is for proposals less than 10,000 USD. This is a very small amount of compensation, and only suitable for tiny projects, or perhaps students looking to start their careers. The second tier is the sweet spot for a small project, providing funding for up to 30,000 USD, and only requiring a modest number of approvals. The third tier requires many approvals.

I was a little concerned about completing the work for a budget of 30,000 USD, but thought we could likely do it without going too far overbudget and undervaluing ourselves. In any case, even if we did go overbudget, I suspected we wouldn’t go too far over budget, and having a solid success with the W3F would be valuable in the future.

So we asked for 30,000 USD. I didn’t attempt to do any estimation of how many hours it would take besides looking at the deliverables we were proposing and thinking hard about whether we could do it within budget at our typical hourly rates. I don’t have a lot of faith in software estimation.

The W3F wants a grant to be divided into milestones, with payment at each, and has a list of specific items they want at each milestone. I didn’t really think this project was large enough for more than one milestone, but for the sake of getting paid at a reasonable interval, I split the project into two milestones, the first (M1) was to “prove the concept”, in which we promised to get the bindings working end to end, with a rough sketch of a high-level API; the second (M2) was to do everything else.

Fortunately, there were no high-risk setbacks discovered in the implementation of this project. In our M1 deliverable we made clear a few things we hadn’t known or considered during the proposal:

  • The crate would require a Rust 1.48+ compiler
  • The crate would require a c++17 compiler
  • Binaryen did not provide suitable error handling in some cases, and this would require an unexpected but doable upstream fix.
  • We discovered that Binaryen did not handle Unicode correctly on Windows.
  • wasm-opt had fuzzing capabilities that we had not mentioned in the proposal, and we did not intend to expose them to the Rust API.

We communicated every negative we knew about the project as clearly as we could.

We were asked to amend the original proposal to indicate that fuzzing was out of scope, so that its omission could be evaluated in the final M2 delivery, and we did so.

We also published a preview build of the crate so the API and its docs could be evaluated. Although we put a lot of thought into the design of the Rust API, nobody at any stage of the process actually commented on it one way or the other. And in practice, any one wasm-opt integrator is only going to use a small part of the total API surface. They reasonably just care that it works like wasm-opt. I am proud though of the API and its docs.

With M1 done, and the API already in shape (if not actually functioning), our plan was to file an issue against cargo-contract early, explaining our plan, laying out the options for integrating the crate, and soliciting feedback. As with our work prior to submitting the proposal, we wanted to lay the groundwork early for the end of the project.

The second milestone (M2) stretched out as we spent a lot of time identifying areas in need of polish and perfection. As we were nearing completion, we were delighted when someone we did not know produced a pull request to Substrate’s wasm-builder integrating our bindings. Though they used the preview build of the crate which was broken in various ways, this gave us a lot of encouragement that other people were interested in using our work.

Throughout the project we used the GitHub issue tracker to track work items, and assigned issues to GitHub milestones, either M1, M2, or none. This project was thankfully amenable to subdivision into small tasks, which my junior partner could often complete on her own without my oversight.

We went over the 30,000 USD budget. By the time we were about 99% done with the project we were right on budget. I had performed about 20,000 USD worth of work, and my partner, whose time we charge less for, but whose time we did not track precisely, nevertheless probably put more hours into the project than I. So we were on budget. What put us overbudget was this blog post and the rework it lead to.

One of the deliverables W3F asks for every milestone is a blog post. I declined to offer a milestone (M1) blog post, but promised to deliver one for M2 — this one! I kept notes throughout the project, and snippets of text to inform the blog, so that when it came time to finish it I would remember what was important.

This blog post was definitely the single largest work item of the entire project. After passing the draft to David Tolnay to review, he came back with many good ideas, some of which we postponed to the future, but some of which we needed to take action on, if only to reflect the mistakes we made with cxx in this blog post, and how to correct them. I think I worked on the blog for about 30 hours, stretching on for weeks after the project was otherwise complete. I don’t mind that at all though — I’m happy with how it turned out, lots of solid technical content, a strong addition to my website.

And now that the blog post is published we can submit the M2 deliverables. How that turns out is yet to be determined.