Enable a Cargo feature by default when the target arch is WASM?

Question

Part of my Cargo.toml for my crate:

[features]
wasm = ["ed25519-dalek/nightly", "rand/wasm-bindgen", "js-sys"]

This works, when the crate is used in another project, and the "wasm" feature is explicitly chosen.

I want to automatically enable this feature only when the target arch is WASM. I tried adding this:

#[cfg(target_arch = "wasm32")]
default = ["wasm"]

When I compile for a target other than WASM, "wasm" is included as the default, how can I make "wasm" the default only when the target is WASM?

Have you defined your default set of features (`default` under `[features]`)? If not, `wasm` might be getting pulled in automatically. — Sébastien Renauld, Sep 17 '19 at 10:49
Actually, it is certainly getting pulled in. `default-features = off` or set a `default` set of features to avoid this. — Sébastien Renauld, Sep 17 '19 at 10:50
If I add default = [] before the cfg line, the toml won't parse due to a duplicate key. Is that what you were suggesting? — user964375, Sep 17 '19 at 11:06
Let me just write an answer, comments are terrible for this. You can let me know if it works :-) — Sébastien Renauld, Sep 17 '19 at 11:14

Boiethios · Accepted Answer · 2019-09-17T14:23:16.880

You can only have target-specific dependencies, not target-specific features. This is a known bug that is unfortunately opened since 2015.

People expects this syntax to be supported, but right now there is nothing scheduled to make this work:

[target.'cfg(target_arch = "wasm32")'.features]
default = ["ed25519-dalek/nightly", "rand/wasm-bindgen", "js-sys"]

As a ugly workaround, you can create another crate that depends on your crate and let the user use this new crate:

[target.'cfg(target_arch = "wasm32")'.dependencies.your_crate]
version = "1.0.0"
features = ["wasm"]

1 Answers1