src/librustc_target/spec/wasm32_unknown_unknown.rs

// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

// The wasm32-unknown-unknown target is currently an experimental version of a
// wasm-based target which does *not* use the Emscripten toolchain. Instead
// this toolchain is based purely on LLVM's own toolchain, using LLVM's native
// WebAssembly backend as well as LLD for a native linker.
//
// There's some trickery below on crate types supported and various defaults
// (aka panic=abort by default), but otherwise this is in general a relatively
// standard target.

use super::{LldFlavor, LinkerFlavor, Target, TargetOptions, PanicStrategy};

pub fn target() -> Result<Target, String> {
    let opts = TargetOptions {
        // we allow dynamic linking, but only cdylibs. Basically we allow a
        // final library artifact that exports some symbols (a wasm module) but
        // we don't allow intermediate `dylib` crate types
        dynamic_linking: true,
        only_cdylib: true,

        // This means we'll just embed a `start` function in the wasm module
        executables: true,

        // relatively self-explanatory!
        exe_suffix: ".wasm".to_string(),
        dll_prefix: "".to_string(),
        dll_suffix: ".wasm".to_string(),
        linker_is_gnu: false,

        // A bit of a lie, but "eh"
        max_atomic_width: Some(32),

        // Unwinding doesn't work right now, so the whole target unconditionally
        // defaults to panic=abort. Note that this is guaranteed to change in
        // the future once unwinding is implemented. Don't rely on this.
        panic_strategy: PanicStrategy::Abort,

        // Wasm doesn't have atomics yet, so tell LLVM that we're in a single
        // threaded model which will legalize atomics to normal operations.
        singlethread: true,

        // no dynamic linking, no need for default visibility!
        default_hidden_visibility: true,

        // we use the LLD shipped with the Rust toolchain by default
        linker: Some("rust-lld".to_owned()),
        lld_flavor: LldFlavor::Wasm,

        .. Default::default()
    };
    Ok(Target {
        llvm_target: "wasm32-unknown-unknown".to_string(),
        target_endian: "little".to_string(),
        target_pointer_width: "32".to_string(),
        target_c_int_width: "32".to_string(),
        // This is basically guaranteed to change in the future, don't rely on
        // this. Use `not(target_os = "emscripten")` for now.
        target_os: "unknown".to_string(),
        target_env: "".to_string(),
        target_vendor: "unknown".to_string(),
        data_layout: "e-m:e-p:32:32-i64:64-n32:64-S128".to_string(),
        arch: "wasm32".to_string(),
        linker_flavor: LinkerFlavor::Lld(LldFlavor::Wasm),
        options: opts,
    })
}