looking into fixpoint combinator in rust

fix.rs

// https://docs.rs/rust2fun/latest/src/rust2fun/combinator.rs.html#399-417


trait Rec<T, R> {
    fn apply(&self, x: T) -> R;
}

impl<T, R, F> Rec<T, R> for F
where
    F: Fn(&dyn Rec<T, R>, T) -> R,
{
    fn apply(&self, x: T) -> R {
        self(self, x)
    }
}

pub fn fix<T, R, F>(f: F, x: T) -> R
where
    F: Fn(&dyn Fn(T) -> R, T) -> R,
{
    (|rec: &dyn Rec<T, R>, y| f(&|z| rec.apply(z), y)).apply(x)
}

const NONE : fn(())->() =  |()| fix::<_,(),_>(|none : &dyn Fn(())->() , ()| none(()), ());