How to use with conditional compilation (cfg-macro)?

[DerZade]

I use the duplicate macro to generate a sync and a async version of a function. I would like to include the async version only if the feature async is enabled. Is there any way to achieve that?

The following code, does not work. I guess this is because the cfg macro has a "higher" priority as the duplicate_item macro and therefore it just says "code is inactive due to #[cfg] directives: cfg_filter is disabled"

#[duplicate_item(
    fn_name             cfg_filter        async;
    [my_function]       [all()]           [];
    [my_function_async] [feature="async"] [async];
)]
#[cfg(cfg_filter)]
async fn fn_name() {
    todo!()
}

Thanks a lot in advance!

[DerZade]

After playing around with it a bit more, I found a solution, but it is very verbose:

#[cfg_attr(feature = "async", 
    duplicate_item(
        fn_name             async;
        [my_function]       [];
        [my_function_async] [async];
    )
)]
#[cfg_attr(not(feature = "async"), 
    duplicate_item(
        fn_name             async;
        [my_function]       [];
    )
)]
async fn fn_name() {
    todo!()
}

Is there any better way?

[Emoun]

I tried your first example, that you said didn't work, and it seems to work for me when using one my feautures (pretty_errors):

With the feature disabled the expansion resulted in:

#[cfg(all())]
fn my_function() {
    ::core::panicking::panic("not yet implemented")
}

With the feature enabled result was:

#[cfg(all())]
fn my_function() {
    ::core::panicking::panic("not yet implemented")
}
#[cfg(feature = "pretty_errors")]
async fn my_function_async() {
    ::core::panicking::panic("not yet implemented")
}

However, I then tried to make another feature called "async", and this didn't work. I also tried a few more names, and they didn't work either. For some reason, only two of my feature names would work. This is extremely weird but also not a problem with duplicate but with #[cfg(feature = "...")] not working for some features.
I confirmed this by testing without using duplicate at all, and seeing that #[cfg(feature = "...")] would always be false for some features but not others.

I don't have time to look into this any more today, but if you can replicate what I've done perhaps this is a bug in the compiler and we should open an issue in rust-lang/rust.

[DerZade]

I don't have time to look into this any more today, but if you can replicate what I've done perhaps this is a bug in the compiler and we should open an issue in rust-lang/rust.

The following is my example from the OP with your feature (pretty_errors). Did I understand correctly that that worked for you, because it does not work for me 😅

#[duplicate_item(
    fn_name             cfg_filter                async;
    [my_function]       [all()]                   [];
    [my_function_async] [feature="pretty_errors"] [async];
)]
#[cfg(cfg_filter)]
async fn fn_name() {
    todo!()
}

fn shit() {
    my_function()
}

It does not expand to anything 🤔 and I get the following error

[Emoun]

Trying again, I got the following to work (regardless of feature name, so lets ignore that part of my last comment):

#[duplicate::duplicate_item(
    fn_name             cfg_filter                async;
    [my_function]       [cfg(all())]                   [];
    [my_function_async] [cfg(feature="async")] [async];
)]
#[cfg_filter]
async fn fn_name() {
    todo!()
}

fn shit() {
    my_function()
}

The key was moving cfg( into the substitution for both cases. Let me know if this works for you too.

Having to do this seems to me as a compiler error. If you look at the rust reference, their example shows that attributes should be expanded in descending order, which would imply duplicate should be able do what you originally tried. I'm going to open an issue about this in rustc and we'll see what they say

[Emoun]

@DerZade this issue seems to be known (rust-lang/rust#83331) but will probably not be fixed in the immediate future, so I would stick to moving the cfg( part into duplicate_item as mentioned earlier.

[DerZade]

Thank you very much! Moving the cfg( into the duplicate worked flawlessly 🙃