async-std's attribute macros are not IDE(rust-analyzer) friendly

[Lukas Wirth]

In short this is a problem with how syn and attributes using syn implement failure, as these crates tend to just discard the entire attributed item on a parsing failure, replacing it with a compile_error! invocation. This has the side effect that when typing inside an item, a user may momentarily create invalid syntax causing the item to be fully discarded resulting in loss of IDE features as the item now effectively does not exist in the file until this syntax error has been fixed.

This problem can be observed here:

#[async_std::test]
async fn buggy_autocomplete() {
    let x = 0;
    x.
    //^ cursor right after the dot, then try and request completions (ctrl + space in VSCode)
    // No completions will be suggested, while without the attribute there will
}

A simple solution to this problem is to change the attributes such that when they error, they re-emit the original item with the compile_error! invocation, that way IDEs will still see the item even if the attribute fails causing IDE features to continue to function.

Sidenote: This will fix the general problem, but if the attribute introduces new usable items inside the item they will of course not always be visible in completions with just this fix. An ideal fix would be to make the attribute do parsing with recovery in such cases if required.

Relevant r-a issues rust-lang/rust-analyzer#10468, rust-lang/rust-analyzer#10788 (comment)

Example PR that tokio received as a fix tokio-rs/tokio#4162

[Lukas Wirth]

Closing this, as we most likely will change our plans which will hopefully no longer require these workarounds rust-lang/rust-analyzer#11014