Derive macro MontConfig panics if any other attribute is applied

[aleasims]

When I'm trying to apply any additional attributes to structure with #[derive(MontConfig)], it results in a panic:

error: proc-macro derive panicked
 --> /root/proj/arkworks-curves/pallas/src/fields/fq.rs:4:10
  |
4 | #[derive(MontConfig)]
  |          ^^^^^^^^^^
  |
  = help: message: attribute modulus should be a string

E.g in case I want my config structure depend on some feature, I can't compile something like:

#[derive(MontConfig)]
#[modulus = "28948022309329048855892746252171976963363056481941560715954676764349967630337"]
#[generator = "5"]
#[cfg(feature = "some_feature_name")]
pub struct FqConfig;

As far as I understood, the problem is in ark_ff_macros::fetch_attr function, which rejects any meta except syn::Meta::NameValue. That's why attribute #[cfg(feature = "some_feature_name")] leads to a panic.

Is there any reason, why you disallow other attributes for MontConfig structures?

[mmagician]

You're right, this is a potentially desirable feature. I'm not aware of any way of doing directly what you're proposing.
I can offer a simple workaround though:

#[cfg(feature = "feature1")]
pub use fp::FqConfig;

#[cfg(feature = "feautre1")]
mod fp {
    use ark_ff::fields::{Fp384, MontBackend, MontConfig};

    #[derive(MontConfig)]
    #[modulus = "28948022309329048855892746252171976963363056481941560715954676764349967630337"]
    #[generator = "5"]
    pub struct FqConfig;
}

#[cfg(feature = "feature2")]
pub use fp::{Fq, FqConfig};

#[cfg(feature = "feature2")]
mod fp {...}

I hope this helps!

[aleasims]

@mmagician Thanks for your answer!

I've come up with a possible fix for this issue, including changes in ark-ff-macros. Hope that will close this issue.

• Fix fetching attributes in MontConfig macro #747