Is there a way to have enum branches that introduce their own type parameters?

weird.rs

trait Hey {
    fn ha(&self) -> u32;
}

enum Foo<A> {
    Ret(A),
    Bar<X: Hey>(Box<Fn(X) -> Foo<A>>)
}

fn test() -> Foo<u32> {
    Foo::Bar(Box::new(|x| {
        Foo::Bar(Box::new(|y| {
            Foo::Ret(ha(y) + ha(x))
        })))
}


// actual code

enum HttpClient<R, Req: FromHttpRequest> {
    Return(R),
    Send(Req, Box<Fn(String) -> HttpClient<R, Req>>),
    Parse(String, Box<Fn(ParseResult) -> HttpClient<R, Req>>)
}