use std::mem;

trait Alteration {
    fn method(&self);
}

enum State {
    Foo {
        var: Box<Alteration>
    },
    Bar {
        var: Box<Alteration>
    },
    Invalid,
}

struct Uploader {
    state: State
}

impl Uploader {
    fn problem(&mut self) {
        let old_state = mem::replace(&mut self.state, State::Invalid);
        let next_state = match old_state {
            State::Foo { var } => State::Bar { var },
            _ => return,
        };
        self.state = next_state;
        if let State::Invalid = self.state {
            panic!();
        }
    }

}

fn main() {

}