use self::ChainState::{Buffered, Linked};

use std::vec;

#[derive(Clone)]
pub(crate) enum ChainState<'a> {
    Linked {
        next: Option<&'a (dyn StdError + 'static)>,
    },
    Buffered {
        rest: vec::IntoIter<&'a (dyn StdError + 'static)>,
    },
}

        let next = self.next?;
        self.next = next.source();
        Some(next)

        match &mut self.state {
            Linked { next } => {
                let error = (*next)?;
                *next = error.source();
                Some(error)
            }
            Buffered { rest } => rest.next(),
        }

impl DoubleEndedIterator for Chain<'_> {
    fn next_back(&mut self) -> Option<Self::Item> {
        match &mut self.state {
            Linked { mut next } => {
                let mut rest = Vec::new();
                while let Some(cause) = next {
                    next = cause.source();
                    rest.push(cause);
                }
                let mut rest = rest.into_iter();
                let last = rest.next_back();
                self.state = Buffered { rest };
                last
            }
            Buffered { rest } => rest.next_back(),
        }
    }
}

use crate::chain::ChainState;

            next: Some(self.error()),

            state: ChainState::Linked {
                next: Some(self.error()),
            },

use crate::chain::ChainState;

use std::error::Error as StdError;

    next: Option<&'a (dyn StdError + 'static)>,

    state: ChainState<'a>,