How do I conditionally compile for WebAssembly in Rust?

Question

How can I make a config flag where I conditionally choose the wasm32-unknown-unkown target?

I printed the current environment using the following build.rs:

use std::env;

fn main() {
    for (key, value) in env::vars() {
        if key.starts_with("CARGO_CFG_") {
            println!("{}: {:?}", key, value);
        }
    }
    panic!("stop and dump stdout");
}

Which prints:

CARGO_CFG_DEBUG_ASSERTIONS: ""
CARGO_CFG_TARGET_ARCH: "wasm32"
CARGO_CFG_TARGET_ENDIAN: "little"
CARGO_CFG_TARGET_ENV: ""
CARGO_CFG_TARGET_HAS_ATOMIC: "16,32,8,ptr"
CARGO_CFG_TARGET_OS: "unknown"
CARGO_CFG_TARGET_POINTER_WIDTH: "32"
CARGO_CFG_TARGET_VENDOR: "unknown"

Normally I would do #[cfg(target_os = "linux")] but that probably doesn't work in this case because #[cfg(target_os = "unknown")] probably matches more than wasm32-unknown-unknown. Do I have to use a combination of target_arch and target_os for this to work properly or maybe just target_arch?

Answer 1

This is how stdweb is doing it:

#[cfg(all(target_arch = "wasm32", target_os = "unknown"))]

I tested it out and it looks like something simple like this works just fine:

#[cfg(target_arch = "wasm32")]
fn add_seven(x: i32) -> i32 {
    x + 7
}

#[cfg(not(target_arch = "wasm32"))]
fn add_seven(x: i32) -> i32 {
    x + 6
}

fn main() {
    let eight = add_seven(1);
    println!("{}", eight);
}

Conditional compilation in Rust allows for a great amount of granularity in that you can specify OS, architecture, etc. If you do not need that granularity then you do not have to use it.

There are unknown and emscripten OS targets for wasm32, so it would be best to differentiate the two if your code needs to be different for the two platforms.

Stdweb has chosen to use the more granular approach. If I were doing it I would follow what they are doing, but it seems like it would work either way.