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This commit adds a new wasm32-based target distributed through rustup, supported in the standard library, and implemented in the compiler. The `wasm32-unknown-wasi` target is intended to be a WebAssembly target which matches the [WASI proposal recently announced.][LINK]. In summary the WASI target is an effort to define a standard set of syscalls for WebAssembly modules, allowing WebAssembly modules to not only be portable across architectures but also be portable across environments implementing this standard set of system calls. The wasi target in libstd is still somewhat bare bones. This PR does not fill out the filesystem, networking, threads, etc. Instead it only provides the most basic of integration with the wasi syscalls, enabling features like: * `Instant::now` and `SystemTime::now` work * `env::args` is hooked up * `env::vars` will look up environment variables * `println!` will print to standard out * `process::{exit, abort}` should be hooked up appropriately None of these APIs can work natively on the `wasm32-unknown-unknown` target, but with the assumption of the WASI set of syscalls we're able to provide implementations of these syscalls that engines can implement. Currently the primary engine implementing wasi is [wasmtime], but more will surely emerge! In terms of future development of libstd, I think this is something we'll probably want to discuss. The purpose of the WASI target is to provide a standardized set of syscalls, but it's *also* to provide a standard C sysroot for compiling C/C++ programs. This means it's intended that functions like `read` and `write` are implemented for this target with a relatively standard definition and implementation. It's unclear, therefore, how we want to expose file descriptors and how we'll want to implement system primitives. For example should `std::fs::File` have a libc-based file descriptor underneath it? The raw wasi file descriptor? We'll see! Currently these details are all intentionally hidden and things we can change over time. A `WasiFd` sample struct was added to the standard library as part of this commit, but it's not currently used. It shows how all the wasi syscalls could be ergonomically bound in Rust, and they offer a possible implementation of primitives like `std::fs::File` if we bind wasi file descriptors exactly. Apart from the standard library, there's also the matter of how this target is integrated with respect to its C standard library. The reference sysroot, for example, provides managment of standard unix file descriptors and also standard APIs like `open` (as opposed to the relative `openat` inspiration for the wasi ssycalls). Currently the standard library relies on the C sysroot symbols for operations such as environment management, process exit, and `read`/`write` of stdio fds. We want these operations in Rust to be interoperable with C if they're used in the same process. Put another way, if Rust and C are linked into the same WebAssembly binary they should work together, but that requires that the same C standard library is used. We also, however, want the `wasm32-unknown-wasi` target to be usable-by-default with the Rust compiler without requiring a separate toolchain to get downloaded and configured. With that in mind, there's two modes of operation for the `wasm32-unknown-wasi` target: 1. By default the C standard library is statically provided inside of `liblibc.rlib` distributed as part of the sysroot. This means that you can `rustc foo.wasm --target wasm32-unknown-unknown` and you're good to go, a fully workable wasi binary pops out. This is incompatible with linking in C code, however, which may be compiled against a different sysroot than the Rust code was previously compiled against. In this mode the default of `rust-lld` is used to link binaries. 2. For linking with C code, the `-C target-feature=-crt-static` flag needs to be passed. This takes inspiration from the musl target for this flag, but the idea is that you're no longer using the provided static C runtime, but rather one will be provided externally. This flag is intended to also get coupled with an external `clang` compiler configured with its own sysroot. Therefore you'll typically use this flag with `-C linker=/path/to/clang-script-wrapper`. Using this mode the Rust code will continue to reference standard C symbols, but the definition will be pulled in by the linker configured. Alright so that's all the current state of this PR. I suspect we'll definitely want to discuss this before landing of course! This PR is coupled with libc changes as well which I'll be posting shortly. [LINK]: [wasmtime]: |
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.. | ||
abi | ||
spec | ||
build.rs | ||
Cargo.toml | ||
lib.rs | ||
README.md |
librustc_target
contains some very low-level details that are
specific to different compilation targets and so forth.
For more information about how rustc works, see the rustc guide.