Rust not recognizing equivalent types

[bebber8]

Hi, I'm having trouble debugging my code that I think is correct, because Rust wouldn't recognize that the types of two data are actually same. I'm using Arkworks and Spartan (GitHub - arkworks-rs/spartan: Spartan on Arkworks) to write zkSNARKS.

type Fq = ark_bls12_381::Fq;
type JubJub = ark_ed_on_bw6_761::EdwardsProjective;
let inst = Instance::<Fq>::new(num_cons, num_vars, num_inputs, &a_flat, &b_flat, &c_flat).unwrap();
// Ignore other inputs to prove()
let proof = NIZK::<JubJub>::prove(&inst, instance_assignment, &witness_assignment, &gens, &mut prover_transcript);

prove() expects inst to be of type 'scalar field of JubJub (CurveGroup)'. The compiler says &inst is a wrong argument to prove():

expected reference `&libspartan::Instance<Fp<MontBackend<ark_ed_on_bw6_761::FrConfig, _>, _>>`    // this is JubJub
   found reference `&libspartan::Instance<Fp<MontBackend<ark_bls12_381::FqConfig, _>, _>>`      // this is Fq

So this error is correct, but technically, Fq and the scalar field of JubJub are both Fp384<MontBackend<FqConfig, 6>>. The are literally defined like this in the source code:

pub type Fq = Fp384<MontBackend<FqConfig, 6>>;      // Fq

pub struct EdwardsConfig;               // JubJub
impl CurveConfig for EdwardsConfig {
    type ScalarField = Fr;
}
pub type Fr = Fp384<MontBackend<FrConfig, 6>>;      

How can I use them interchangeably as the same type - which they are?

[ogeon]

I know absolutely nothing about what this is, but the error and the definition say that one is parameterized by FrConfig and the other by FqConfig. They aren't interchangeable if they are defined as e.g. separate structs/enums.