`wasm32-unknown-unknown`

Tier: 2

The wasm32-unknown-unknown target is a WebAssembly compilation target which does not import any functions from the host for the standard library. This is the "minimal" WebAssembly in the sense of making the fewest assumptions about the host environment. This target is often used when compiling to the web or JavaScript environments as there is no standard for what functions can be imported on the web. This target can also be useful for creating minimal or bare-bones WebAssembly binaries.

The wasm32-unknown-unknown target has support for the Rust standard library but many parts of the standard library do not work and return errors. For example println! does nothing, std::fs always return errors, and std::thread::spawn will panic. There is no means by which this can be overridden. For a WebAssembly target that more fully supports the standard library see the wasm32-wasip1 or wasm32-wasip2 targets.

The wasm32-unknown-unknown target has full support for the core and alloc crates. It additionally supports the HashMap type in the std crate, although hash maps are not randomized like they are on other platforms.

One existing user of this target (please feel free to edit and expand this list too) is the wasm-bindgen project which facilitates Rust code interoperating with JavaScript code. Note, though, that not all uses of wasm32-unknown-unknown are using JavaScript and the web.

Target maintainers

When this target was added to the compiler, platform-specific documentation here was not maintained at that time. This means that the list below is not exhaustive, and there are more interested parties in this target. That being said, those interested in maintaining this target are:

Alex Crichton, https://github.com/alexcrichton

Requirements

This target is cross-compiled. The target includes support for std itself, but as mentioned above many pieces of functionality that require an operating system do not work and will return errors.

This target currently has no equivalent in C/C++. There is no C/C++ toolchain for this target. While interop is theoretically possible it's recommended to instead use one of:

wasm32-unknown-emscripten - for web-based use cases the Emscripten toolchain is typically chosen for running C/C++.
wasm32-wasip1 - the wasi-sdk toolchain is used to compile C/C++ on this target and can interop with Rust code. WASI works on the web so far as there's no blocker, but an implementation of WASI APIs must be either chosen or reimplemented.

This target has no build requirements beyond what's in-tree in the Rust repository. Linking binaries requires LLD to be enabled for the wasm-ld driver. This target uses the dlmalloc crate as the default global allocator.

Building the target

Building this target can be done by:

Configure the wasm32-unknown-unknown target to get built.
Configure LLD to be built.
Ensure the WebAssembly target backend is not disabled in LLVM.

These are all controlled through config.toml options. It should be possible to build this target on any platform.

Building Rust programs

Rust programs can be compiled by adding this target via rustup:

$ rustup target add wasm32-unknown-unknown

and then compiling with the target:

$ rustc foo.rs --target wasm32-unknown-unknown
$ file foo.wasm

Cross-compilation

This target can be cross-compiled from any host.

Testing

This target is not tested in CI for the rust-lang/rust repository. Many tests must be disabled to run on this target and failures are non-obvious because println! doesn't work in the standard library. It's recommended to test the wasm32-wasip1 target instead for WebAssembly compatibility.

Conditionally compiling code

It's recommended to conditionally compile code for this target with:

#[cfg(all(target_family = "wasm", target_os = "unknown"))]

Note that there is no way to tell via #[cfg] whether code will be running on the web or not.

Enabled WebAssembly features

WebAssembly is an evolving standard which adds new features such as new instructions over time. This target's default set of supported WebAssembly features will additionally change over time. The wasm32-unknown-unknown target inherits the default settings of LLVM which typically matches the default settings of Emscripten as well.

Changes to WebAssembly go through a proposals process but reaching the final stage (stage 5) does not automatically mean that the feature will be enabled in LLVM and Rust by default. At this time the general guidance is that features must be present in most engines for a "good chunk of time" before they're enabled in LLVM by default. There is currently no exact number of months or engines that are required to enable features by default.

As of the time of this writing the proposals that are enabled by default (the generic CPU in LLVM terminology) are:

multivalue
mutable-globals
reference-types
sign-ext

If you're compiling WebAssembly code for an engine that does not support a feature in LLVM's default feature set then the feature must be disabled at compile time. Note, though, that enabled features may be used in the standard library or precompiled libraries shipped via rustup. This means that not only does your own code need to be compiled with the correct set of flags but the Rust standard library additionally must be recompiled.

Compiling all code for the initial release of WebAssembly looks like:

$ export RUSTFLAGS=-Ctarget-cpu=mvp
$ cargo +nightly build -Zbuild-std=panic_abort,std --target wasm32-unknown-unknown

Here the mvp "cpu" is a placeholder in LLVM for disabling all supported features by default. Cargo's -Zbuild-std feature, a Nightly Rust feature, is then used to recompile the standard library in addition to your own code. This will produce a binary that uses only the original WebAssembly features by default and no proposals since its inception.

To enable individual features it can be done with -Ctarget-feature=+foo. Available features for Rust code itself are documented in the reference and can also be found through:

$ rustc -Ctarget-feature=help --target wasm32-unknown-unknown

You'll need to consult your WebAssembly engine's documentation to learn more about the supported WebAssembly features the engine has.

Note that it is still possible for Rust crates and libraries to enable WebAssembly features on a per-function level. This means that the build command above may not be sufficient to disable all WebAssembly features. If the final binary still has SIMD instructions, for example, the function in question will need to be found and the crate in question will likely contain something like:

#[target_feature(enable = "simd128")]
fn foo() {
    // ...
}

In this situation there is no compiler flag to disable emission of SIMD instructions and the crate must instead be modified to not include this function at compile time either by default or through a Cargo feature. For crate authors it's recommended to avoid #[target_feature(enable = "...")] except where necessary and instead use:

#[cfg(target_feature = "simd128")]
fn foo() {
    // ...
}

That is to say instead of enabling target features it's recommended to conditionally compile code instead. This is notably different to the way native platforms such as x86_64 work, and this is due to the fact that WebAssembly binaries must only contain code the engine understands. Native binaries work so long as the CPU doesn't execute unknown code dynamically at runtime.

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