Note: This section is about Rust target triples, not targets like node/web workers/browsers. More information on that coming soon!

The wasm-bindgen project is designed to target the wasm32-unknown-unknown target in Rust. This target is a "bare bones" target for Rust which emits WebAssembly as output. The standard library is largely inert as modules like std::fs and std::net will simply return errors.

Note that wasm-bindgen also aims to compile on all targets. This means that it should be safe, if you like, to use #[wasm_bindgen] even when compiling for Windows (for example). For example:

#[wasm_bindgen]
pub fn add(a: u32, b: u32) -> u32 {
    a + b
}

#[cfg(not(target_arch = "wasm32"))]
fn main() {
    println!("1 + 2 = {}", add(1, 2));
}

This program will compile and work on all platforms, not just wasm32-unknown-unknown. Note that imported functions with #[wasm_bindgen] will unconditionally panic on non-wasm targets. For example:

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(js_namespace = console)]
    fn log(s: &str);
}

fn main() {
    log("hello!");
}

This program will unconditionally panic on all platforms other than wasm32-unknown-unknown.

For better compile times, however, you likely want to only use #[wasm_bindgen] on the wasm32-unknown-unknown target. You can have a target-specific dependency like so:

[target.'cfg(target_arch = "wasm32")'.dependencies]
wasm-bindgen = "0.2"

And in your code you can use:

#![allow(unused_variables)]
fn main() {
#[cfg(target_arch = "wasm32")]
#[wasm_bindgen]
pub fn only_on_the_wasm_target() {
    // ...
}
}

The wasm-bindgen target does not support the wasm32-unknown-emscripten nor the asmjs-unknown-emscripten targets. There are currently no plans to support these targets either. All annotations work like other platforms on the targets, retaining exported functions and causing all imports to panic.