opt out of Send for wasm only

[ctaggart]

Hi @dtolnay 👋🏻 ,

I am trying to build Azure/azure-sdk-for-rust#228 for wasm, but I ran into a problem where wasm should not have the Send markers. This does not work, but what I roughly want to do is:

cfg_if::cfg_if! {
    if cfg(target_arch = "wasm32") {
        #[async_trait(?Send)]
    } else {
        #[async_trait]
    }
}

Any suggestions on how to solve this predicament?

[taiki-e]

I think cfg_attr would work. Something like the following:

#[cfg_attr(target_arch = "wasm32", async_trait(?Send))]
#[cfg_attr(not(target_arch = "wasm32"), async_trait)]