WASM Projects Broken: Target-cpu=mvp & Wasm-bindgen 2.104

Uh oh, Rustaceans! It seems like a recent update to wasm-bindgen (version 2.104) has introduced a rather disruptive issue. If you're working with WebAssembly (WASM) projects and using the target-cpu=mvp flag, you might have encountered a frustrating roadblock. Let's dive into the details of this problem, understand why it's happening, and explore potential workarounds.

The Issue: target-cpu=mvp Projects No Longer Working

The core problem, guys, is that projects compiled with the target-cpu=mvp flag are no longer playing nicely with wasm-bindgen 2.104. This flag, as highlighted in the official Rust blog post (https://blog.rust-lang.org/2024/09/24/webassembly-targets-change-in-default-target-features/#disabling-on-by-default-webassembly-proposals), is used to disable the default WebAssembly features. While this might seem like a niche use case, it's crucial for scenarios like WASM hot-patching, where ABI stability is paramount.

Why target-cpu=mvp Matters

So, why do we even need target-cpu=mvp in the first place? The Rust team made a change to how WebAssembly target features are handled, specifically related to proposals. By default, certain WebAssembly proposals are enabled. However, in some cases, you might want to explicitly disable these features. This is where target-cpu=mvp comes in handy. It allows you to target the Minimum Viable Product (MVP) feature set of WebAssembly, ensuring a more predictable and stable environment.

The Downstream Effects

The breakage caused by this issue isn't just a minor inconvenience. It has significant downstream effects, most notably breaking WASM hot-patching functionality. Hot-patching relies on a stable ABI (Application Binary Interface). When default WebAssembly features are enabled, the ABI can change between builds, making hot-patching unreliable. By using target-cpu=mvp, developers can guard against these ABI changes.

However, the problem extends beyond just hot-patching. Any project relying on the MVP target feature is now affected, making this a fairly widespread issue for those employing this specific configuration.

The Culprit: A Recent wasm-bindgen Change

The likely cause of this issue is a recent commit in the wasm-bindgen repository (https://github.com/wasm-bindgen/wasm-bindgen/commit/d450d5aa7ef5d36fc62d2affcdf1c145410b434b). While the exact details of the commit's impact require further investigation, it's clear that it has introduced incompatibility with projects using target-cpu=mvp.

Reproducing the Error

To illustrate the problem, let's walk through a simple reproduction case. This will help you confirm if you're experiencing the same issue and provide a clear example for bug reports and discussions.

Steps to Reproduce

1. Create a new Rust project:

   cargo new --bin project
   cd project
   

2. Add wasm-bindgen as a dependency: You can do this either directly or through a higher-level framework like dioxus. For simplicity, let's add it directly to the Cargo.toml:

   [dependencies]
   wasm-bindgen = "0.2.104"
   

3. Set the RUSTFLAGS environment variable: This is crucial for enabling the target-cpu=mvp flag.

   export RUSTFLAGS="-Ctarget-cpu=mvp"
   

4. Build the project for the wasm32-unknown-unknown target:

   cargo build --target wasm32-unknown-unknown
   

5. Run wasm-bindgen: This is where the error will manifest.

   wasm-bindgen-0.2.104 --target web --out-dir pkg ./target/wasm32-unknown-unknown/debug/<your_project_name>.wasm
   

   Replace <your_project_name> with the actual name of your project's WASM file.

The Error Message

If you're encountering the issue, you'll likely see an error message similar to this:

error: failed to find the `__wbindgen_externref_table_alloc` function

This error indicates that wasm-bindgen is unable to locate a specific function (__wbindgen_externref_table_alloc) that is expected to be present when using the default CPU features. However, when target-cpu=mvp is enabled, this function might not be included, leading to the error.

A Key Observation

It's important to note that wasm-bindgen works perfectly fine when the automatic CPU features are enabled. This further reinforces the idea that the issue is specifically related to the interaction between wasm-bindgen and the target-cpu=mvp flag.

Understanding the Root Cause

To get a deeper understanding of why this is happening, we need to delve into the inner workings of wasm-bindgen and how it interacts with the Rust compiler and the WebAssembly ecosystem.

The Role of wasm-bindgen

wasm-bindgen is a powerful tool that bridges the gap between Rust and JavaScript. It takes your Rust code, compiles it to WebAssembly, and generates the necessary JavaScript bindings to allow seamless interaction between the two languages. This involves tasks such as memory management, type conversions, and function calls.

The Impact of Target Features

WebAssembly is an evolving standard, with new features and proposals being added over time. The Rust compiler allows you to target specific feature sets through target features. These features control which WebAssembly instructions and capabilities are included in the compiled output. The target-cpu=mvp flag essentially tells the compiler to stick to the core, most widely supported WebAssembly features.

The Disconnect

The issue seems to stem from a disconnect between how wasm-bindgen expects certain functions to be available and how the Rust compiler generates code when target-cpu=mvp is enabled. The __wbindgen_externref_table_alloc function, for example, might be a part of a newer WebAssembly proposal that is not included in the MVP feature set. Consequently, when you compile with target-cpu=mvp, this function is not generated, leading to the error when wasm-bindgen tries to find it.

Potential Workarounds and Solutions

While a definitive fix will likely require an update to wasm-bindgen itself, there are a few potential workarounds you can try in the meantime.

1. Reverting to a Previous wasm-bindgen Version

The most immediate workaround is to downgrade to a previous version of wasm-bindgen that doesn't exhibit this issue. You can specify a specific version in your Cargo.toml file:

[dependencies]
wasm-bindgen = "0.2.103" # Or an earlier version

Then, run cargo update to update your dependencies. This should temporarily resolve the problem, allowing you to continue working on your project. However, keep in mind that you'll be missing out on any bug fixes or new features introduced in later versions of wasm-bindgen.

2. Conditional Compilation

If you only need target-cpu=mvp for specific parts of your project (e.g., for hot-patching), you might be able to use conditional compilation to isolate the affected code. This involves using Rust's #[cfg] attribute to conditionally include or exclude code based on the target features.

For example, you could wrap the code that interacts with wasm-bindgen in a module that is only compiled when target-cpu is not mvp:

#[cfg(not(target_feature = "mvp"))]
mod wasm_bindings {
    // Your wasm-bindgen related code here
}

#[cfg(target_feature = "mvp")]
mod wasm_bindings {
    // Alternative implementation or empty module
}

This approach allows you to use target-cpu=mvp for the rest of your project while avoiding the issue with wasm-bindgen in the wasm_bindings module.

3. Exploring Alternative Tooling

While wasm-bindgen is the most popular tool for Rust-to-WebAssembly interop, it's not the only option. Depending on your specific needs, you might be able to use alternative tools or approaches, such as:

• wasm-pack: A tool for building, testing, and publishing Rust-generated WebAssembly. It uses wasm-bindgen under the hood but provides a higher-level interface.
• Manually writing JavaScript bindings: For simple cases, you might be able to avoid wasm-bindgen altogether by manually writing the JavaScript code to interact with your WebAssembly module.

However, these alternatives might require more effort and might not provide the same level of convenience as wasm-bindgen.

4. Awaiting a Fix

The most reliable solution is to wait for an official fix from the wasm-bindgen team. The issue has been reported, and the maintainers are likely aware of it. Keep an eye on the wasm-bindgen repository for updates and releases. You can also subscribe to the issue on GitHub to receive notifications when there's any activity.

Conclusion

The target-cpu=mvp breakage in wasm-bindgen 2.104 is a significant issue for developers relying on this flag for WASM hot-patching and ABI stability. While there are temporary workarounds, the best long-term solution is to await an official fix. In the meantime, make sure to stay informed about the issue's progress and consider contributing to the discussion on the wasm-bindgen repository.

To stay updated on WebAssembly and Rust-related topics, check out the WebAssembly official website for more in-depth information: https://webassembly.org/