<3

stuff.rs

trait MyTrait<U, T> {
    fn signing_func(&self, challenger: &U, history: &[T]) -> Result<usize, &'static str>;
}

impl MyTrait<TypeA, TypeB>
    for HistoryInfo<TypeB>
{
    fn signing_func(
        &self,
        challenger: &TypeA,
        history: &[TypeB],
    ) -> Result<usize, &'static str> {
        should_sign_attestation(challenger, history).map_err(|_| "invalid attestation")
    }
}

impl MyTrait<TypeC, TypeD>
    for HistoryInfo<TypeD>
{
    fn signing_func(
        &self,
        challenger: &TypeC,
        history: &[TypeD],
    ) -> Result<usize, &'static str> {
        should_sign_block(challenger, history)
    }
}

#[derive(Debug)]
struct HistoryInfo<T: Encode + Decode + Clone> {
    filename: String,
    mutex: Arc<Mutex<Vec<T>>>,
}

impl<T: Encode + Decode + Clone> HistoryInfo<T> {
    fn should_sign<U>(&self, challenger: &U) -> Result<usize, &'static str> {
        let guard = self.mutex.lock();
        let history = &guard[..];
        self.signing_func(challenger, history)
    }
}