Module rustc_target::spec::wasm32_wasi

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The wasm32-wasi target is a new and still (as of April 2019) an experimental target. The definition in this file is likely to be tweaked over time and shouldn’t be relied on too much.

The wasi target is a proposal to define a standardized set of syscalls that WebAssembly files can interoperate with. This set of syscalls is intended to empower WebAssembly binaries with native capabilities such as filesystem access, network access, etc.

You can see more about the proposal at https://wasi.dev.

The Rust target definition here is interesting in a few ways. We want to serve two use cases here with this target:

  • First, we want Rust usage of the target to be as hassle-free as possible, ideally avoiding the need to configure and install a local wasm32-wasi toolchain.

  • Second, one of the primary use cases of LLVM’s new wasm backend and the wasm support in LLD is that any compiled language can interoperate with any other. To that the wasm32-wasi target is the first with a viable C standard library and sysroot common definition, so we want Rust and C/C++ code to interoperate when compiled to wasm32-unknown-unknown.

You’ll note, however, that the two goals above are somewhat at odds with one another. To attempt to solve both use cases in one go we define a target that (ab)uses the crt-static target feature to indicate which one you’re in.

No interop with C required

By default the crt-static target feature is enabled, and when enabled this means that the bundled version of libc.a found in liblibc.rlib is used. This isn’t intended really for interoperation with a C because it may be the case that Rust’s bundled C library is incompatible with a foreign-compiled C library. In this use case, though, we use rust-lld and some copied crt startup object files to ensure that you can download the wasi target for Rust and you’re off to the races, no further configuration necessary.

All in all, by default, no external dependencies are required. You can compile wasm32-wasi binaries straight out of the box. You can’t, however, reliably interoperate with C code in this mode (yet).

Interop with C required

For the second goal we repurpose the target-feature flag, meaning that you’ll need to do a few things to have C/Rust code interoperate.

  1. All Rust code needs to be compiled with -C target-feature=-crt-static, indicating that the bundled C standard library in the Rust sysroot will not be used.

  2. If you’re using rustc to build a linked artifact then you’ll need to specify -C linker to a clang binary that supports wasm32-wasi and is configured with the wasm32-wasi sysroot. This will cause Rust code to be linked against the libc.a that the specified clang provides.

  3. If you’re building a staticlib and integrating Rust code elsewhere, then compiling with -C target-feature=-crt-static is all you need to do.

You can configure the linker via Cargo using the CARGO_TARGET_WASM32_WASI_LINKER env var. Be sure to also set CC_wasm32-wasi if any crates in the dependency graph are using the cc crate.

Remember, this is all in flux

The wasi target is very new in its specification. It’s likely going to be a long effort to get it standardized and stable. We’ll be following it as best we can with this target. Don’t start relying on too much here unless you know what you’re getting in to!

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