`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. There are two approaches to choose from:

If you are targeting a feature set no smaller than the W3C WebAssembly Core 1.0 recommendation -- which is equivalent to the WebAssembly MVP plus the mutable-globals feature -- and you are building no_std, then you can simply use the wasm32v1-none target instead of wasm32-unknown-unknown, which uses only those minimal features and includes a core and alloc library built with only those minimal features.
Otherwise -- if you need std, or if you need to target the ultra-minimal "MVP" feature set, excluding mutable-globals -- you will need to manually specify -Ctarget-cpu=mvp and also rebuild the stdlib using that target to ensure no features are used in the stdlib. This in turn requires use of a nightly compiler.

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 on either this target or wasm32v1-none, pass arguments of the form -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.

Broken `extern "C"` ABI

This target has what is considered a broken extern "C" ABI implementation at this time. Notably the same signature in Rust and C will compile to different WebAssembly functions and be incompatible. This is considered a bug and it will be fixed in a future version of Rust.

For example this Rust code:

#[repr(C)]
struct MyPair {
    a: u32,
    b: u32,
}

extern "C" {
    fn take_my_pair(pair: MyPair) -> u32;
}

#[no_mangle]
pub unsafe extern "C" fn call_c() -> u32 {
    take_my_pair(MyPair { a: 1, b: 2 })
}

compiles to a WebAssembly module that looks like:

(module
  (import "env" "take_my_pair" (func $take_my_pair (param i32 i32) (result i32)))
  (func $call_c
    i32.const 1
    i32.const 2
    call $take_my_pair
  )
)

The function when defined in C, however, looks like

struct my_pair {
    unsigned a;
    unsigned b;
};

unsigned take_my_pair(struct my_pair pair) {
    return pair.a + pair.b;
}

(module
  (import "env" "__linear_memory" (memory 0))
  (func $take_my_pair (param i32) (result i32)
    local.get 0
    i32.load offset=4
    local.get 0
    i32.load
    i32.add
  )
)

Notice how Rust thinks take_my_pair takes two i32 parameters but C thinks it only takes one.

The correct definition of the extern "C" ABI for WebAssembly is located in the WebAssembly/tool-conventions repository. The wasm32-unknown-unknown target (and only this target, not other WebAssembly targets Rust support) does not correctly follow this document.

Example issues in the Rust repository about this bug are:

This current state of the wasm32-unknown-unknown backend is due to an unfortunate accident which got relied on. The wasm-bindgen project prior to 0.2.89 was incompatible with the "correct" definition of extern "C" and it was seen as not worth the tradeoff of breaking wasm-bindgen historically to fix this issue in the compiler.

Thanks to the heroic efforts of many involved in this, however, wasm-bindgen 0.2.89 and later are compatible with the correct definition of extern "C" and the nightly compiler currently supports a -Zwasm-c-abi implemented in #117919. This nightly-only flag can be used to indicate whether the spec-defined version of extern "C" should be used instead of the "legacy" version of whatever-the-Rust-target-originally-implemented. For example using the above code you can see (lightly edited for clarity):

$ rustc +nightly -Zwasm-c-abi=spec foo.rs --target wasm32-unknown-unknown --crate-type lib --emit obj -O
$ wasm-tools print foo.o
(module
  (import "env" "take_my_pair" (func $take_my_pair (param i32) (result i32)))
  (func $call_c (result i32)
    ;; ...
  )
  ;; ...
)

which shows that the C and Rust definitions of the same function now agree like they should.

The -Zwasm-c-abi compiler flag is tracked in #122532 and a lint was implemented in #117918 to help warn users about the transition if they're using wasm-bindgen 0.2.88 or prior. The current plan is to, in the future, switch -Zwasm-c-api=spec to being the default. Some time after that the -Zwasm-c-abi flag and the "legacy" implementation will all be removed. During this process users on a sufficiently updated version of wasm-bindgen should not experience breakage.

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