use std::fmt;
use std::fmt::{Debug, Formatter};
use std::mem::replace;

#[allow(dead_code)]
enum Dir<T> {
    None(T),
    A(T),
    B(T),
}

impl Debug for Dir<String> {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self {
            Dir::None(v) => write!(f, "None: {}", v),
            Dir::A(v) => write!(f, "A: {}", v),
            Dir::B(v) => write!(f, "B: {}", v),
        }
    }
}

impl<T> Dir<T>
where
    T: Default,
{
    pub fn a(&mut self) -> &Self {
        if let Dir::None(v) = self {
            *self = Dir::A(replace(v, T::default()));
        }
        self
    }
}

fn main() {
    let mut n = Dir::None("hello".to_string());
    let mut a = Dir::A("bye".to_string());
    std::mem::swap(&mut a, &mut n);
    a.a();
    dbg!(a, n);
}