diff --git a/src/doc/rustc/src/SUMMARY.md b/src/doc/rustc/src/SUMMARY.md index 08460af15d4c7..c5622a5e14e20 100644 --- a/src/doc/rustc/src/SUMMARY.md +++ b/src/doc/rustc/src/SUMMARY.md @@ -78,6 +78,7 @@ - [wasm32-wasip1](platform-support/wasm32-wasip1.md) - [wasm32-wasip1-threads](platform-support/wasm32-wasip1-threads.md) - [wasm32-wasip2](platform-support/wasm32-wasip2.md) + - [wasm32-unknown-unknown](platform-support/wasm32-unknown-unknown.md) - [wasm64-unknown-unknown](platform-support/wasm64-unknown-unknown.md) - [\*-win7-windows-msvc](platform-support/win7-windows-msvc.md) - [x86_64-fortanix-unknown-sgx](platform-support/x86_64-fortanix-unknown-sgx.md) diff --git a/src/doc/rustc/src/platform-support.md b/src/doc/rustc/src/platform-support.md index cbb338f481172..1fb7d2d150385 100644 --- a/src/doc/rustc/src/platform-support.md +++ b/src/doc/rustc/src/platform-support.md @@ -190,7 +190,7 @@ target | std | notes [`thumbv8m.main-none-eabi`](platform-support/thumbv8m.main-none-eabi.md) | * | Bare Armv8-M Mainline [`thumbv8m.main-none-eabihf`](platform-support/thumbv8m.main-none-eabi.md) | * | Bare Armv8-M Mainline, hardfloat `wasm32-unknown-emscripten` | ✓ | WebAssembly via Emscripten -`wasm32-unknown-unknown` | ✓ | WebAssembly +[`wasm32-unknown-unknown`](platform-support/wasm32-unknown-unknown.md) | ✓ | WebAssembly `wasm32-wasi` | ✓ | WebAssembly with WASI (undergoing a [rename to `wasm32-wasip1`][wasi-rename]) [`wasm32-wasip1`](platform-support/wasm32-wasip1.md) | ✓ | WebAssembly with WASI [`wasm32-wasip1-threads`](platform-support/wasm32-wasip1-threads.md) | ✓ | WebAssembly with WASI Preview 1 and threads diff --git a/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md b/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md new file mode 100644 index 0000000000000..ee37e5e90cfb4 --- /dev/null +++ b/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md @@ -0,0 +1,198 @@ +# `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. diff --git a/src/doc/rustc/src/platform-support/wasm32-wasip1-threads.md b/src/doc/rustc/src/platform-support/wasm32-wasip1-threads.md index c3eda26ca8ef9..994c0f4bbb3e9 100644 --- a/src/doc/rustc/src/platform-support/wasm32-wasip1-threads.md +++ b/src/doc/rustc/src/platform-support/wasm32-wasip1-threads.md @@ -162,3 +162,17 @@ It's recommended to conditionally compile code for this target with: Prior to Rust 1.80 the `target_env = "p1"` key was not set. Currently the `target_feature = "atomics"` is Nightly-only. Note that the precise `#[cfg]` necessary to detect this target may change as the target becomes more stable. + +## Enabled WebAssembly features + +The default set of WebAssembly features enabled for compilation includes two +more features in addition to that which +[`wasm32-unknown-unknown`](./wasm32-unknown-unknown.md) enables: + +* `bulk-memory` +* `atomics` + +For more information about features see the documentation for +[`wasm32-unknown-unknown`](./wasm32-unknown-unknown.md), but note that the +`mvp` CPU in LLVM does not support this target as it's required that +`bulk-memory`, `atomics`, and `mutable-globals` are all enabled. diff --git a/src/doc/rustc/src/platform-support/wasm32-wasip1.md b/src/doc/rustc/src/platform-support/wasm32-wasip1.md index fb70bbdc2b403..0e4def6768dc4 100644 --- a/src/doc/rustc/src/platform-support/wasm32-wasip1.md +++ b/src/doc/rustc/src/platform-support/wasm32-wasip1.md @@ -132,3 +132,9 @@ It's recommended to conditionally compile code for this target with: Note that the `target_env = "p1"` condition first appeared in Rust 1.80. Prior to Rust 1.80 the `target_env` condition was not set. + +## Enabled WebAssembly features + +The default set of WebAssembly features enabled for compilation is currently the +same as [`wasm32-unknown-unknown`](./wasm32-unknown-unknown.md). See the +documentation there for more information. diff --git a/src/doc/rustc/src/platform-support/wasm32-wasip2.md b/src/doc/rustc/src/platform-support/wasm32-wasip2.md index 1e53fbc178e2e..bb2348b201e39 100644 --- a/src/doc/rustc/src/platform-support/wasm32-wasip2.md +++ b/src/doc/rustc/src/platform-support/wasm32-wasip2.md @@ -61,3 +61,9 @@ It's recommended to conditionally compile code for this target with: ```text #[cfg(all(target_os = "wasi", target_env = "p2"))] ``` + +## Enabled WebAssembly features + +The default set of WebAssembly features enabled for compilation is currently the +same as [`wasm32-unknown-unknown`](./wasm32-unknown-unknown.md). See the +documentation there for more information.