Code change:
<ins> # `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`](./wasm32-wasip1.md) or [`wasm32-wasip2`](./wasm32-wasip2.md) 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](https://github.com/rustwasm/wasm-bindgen) 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 since when this document was last updated 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`](./wasm32-wasip1.md) - 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: ```sh $ rustup target add wasm32-unknown-unknown ``` and then compiling with the target: ```sh $ 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: ```text #[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][proposals] 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. [proposals]: https://github.com/WebAssembly/proposals 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: ```sh $ 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: ```sh $ 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. [reference]: https://doc.rust-lang.org/reference/attributes/codegen.html#wasm32-or-wasm64 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: ```rust,ignore (not-always-compiled-to-wasm) #[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: ```rust,ignore (not-always-compiled-to-wasm) #[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. </ins>

Related discussion:
text: and WebAssembly book () In Rust some requirements for WebAssembly targets. This makes Wasm binaries compiled with Rust (at least with default options/out of the box std) incompatible with older runtime environments, in particular not-so-old iOS safari (). Here is for example the support for Safari iOS 15.5 simulator: ! I searched but did not find any place where these requirements are written down. I think that this is a problem.
In principle Rust can produce binaries that only target the core subset of wasm 1.0 just fine. You do need to set some additional options with and build your own standard libraries though. But point taken, is missing dedicated pages on the wasm target that could contain this information.
The 2nd sentence right after the one you quoted outlines exactly what you need to do to take advantage of rustc’s capability to emit outputs to the wasm 1.0 spec (or any superset of it:) Make sure to do this and report back if you're still seeing the unexpected instructions/types/other entities after rustc is told to produce wasm binaries the way you need them to be. AFAIK, not for WASM.
Sorry, I realized I misunderstood your answer and deleted my post while you were answering it. Do you know how to show which features are enabled by default for a given target? I searched and did not find. I'm trying to discover whether is enabled by default on nowadays. As it seems to be because I am seeing types in the emitted wasm, even though I disabled the feature by "" in an environment variable RUSTFLAGS. It would be surprising to me that is built with that feature by default on wasm32, but maybe it is.
Hm, I'm actually not sure why you might be seeing simd128 in your modules. As far as I can tell it is not enabled by default for any wasm target. In order to print out features used you should look for lines in: For what it is worth I’ve been using this exact target at my $DAYJOB with a pretty primitive runtime and it has been working largely OOB if my memory serves me right. So it indeed could have something to do with your dependencies possibly using something like ? Not sure!