#![feature(try_from)]
#![feature(never_type)]

use ::std::collections::BTreeMap;
use ::std::convert::TryFrom;

#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
enum CborValue {
    Text(String),
    Map(BTreeMap<CborValue, CborValue>),
    Null,
}

impl TryFrom<CborValue> for String {
    type Error = ();

    fn try_from(cbor: CborValue) -> Result<String, ()> {
        if let CborValue::Text(string) = cbor {
            Ok(string)
        } else {
            Err(())
        }
    }
}

fn foo(cbor: CborValue) -> Result<BTreeMap<String, CborValue>, ()> {
    match cbor {
        CborValue::Map(map) => {
            map.into_iter()
               .map(|(k, v)| Ok((String::try_from(k)?, v)))
               .collect()
        },
        value => return Err(()),
    }
}

fn main() {}